Plants are greatly influenced by temperature, because the complex chemical processes which are taking place in a plant both day and night are dependent on the amount of heat available. Plants vary very considerably in their heat requirements and here again their native habitat is usually a good guide. Alpine plants, for example, have very different needs to those of tropical plants. All plants have their maximum and minimum temperature requirements. The maximum temperatures are not often given in horticultural books, and this is not an aspect with which we need to be concerned as the plant does not usually suffer if the temperature is too high, but merely ceases to grow. The minimum temperature requirement is a different matter, because usually it represents the minimum temperature at which the plant will survive without suffering damage.
There is much inaccuracy in this field and the figures given in many publications are far too high; consequently one should seek these figures in several different sources, and the lowest figure given will usually be the true temperature the plant can survive.
The growth of a plant is also affected by the humidity of the air surrounding it. Fortunately many flowering pot plants are quite tolerant in this respect and will survive a warm dry atmosphere during the period they are in flower without suffering irreparable damage. On the other hand there are a few species which will not survive if the surrounding atmosphere is too hot and dry. A perfect example of this is the cyclamen which requires a moist atmosphere and ideally a temperature range of 50-60DF(10- 15DC). Put a healthy cyclamen plant, in full bloom, into a centrally-heated room, averaging say 70°F (21 °C) and the high temperature coupled with the dry atmosphere will usually prove detrimental to the plant in a matter of days.
The humidity of air is measured by an instrument called a hygrometer, which shows the relative humidity of the air on a percentage scale; 0 per cent is completely dry air and 100 per cent is saturated air. As the temperature of air rises, its water absorption capability increases; consequently, as the temperature rises the drier it becomes, unless the water content is being increased at an appropriate rate. To clarify this, if a given quantity of air at 35°F (2°C) is saturated when it contains 2 oz (57 g) of water, it would require 8 oz (228 g) of water to remain saturated at 70°F (21 °C).
During daylight hours the stomata on the undersides of the leaves are open and air is flowing into the interior of the leaves and circulating round the loosely-packed cells, before passing out of the leaves. The manufacturing cells in a leaf have a layer of moisture on the outside walls, which absorbs minute quantities of carbon dioxide from the air; this passes into the cell and is used in conjunction with the hydrogen and oxygen of water (H,0) to produce sugar (a carbohydrate). The water or, more accurately, the dilute chemical solution, which the plant takes up from the soil, by means of its roots, is used by the plant to fill the cells and vessels of the plant, and a tiny fraction of the total quantity of water is used in the manufacture of sugar in the cell. Much larger quantities of water are drawn up into the leaves than the plant requires and this excess water evaporates from the moist layer on the outside walls of the cells, being absorbed by the air, as it circulates, throughout the interior of the leaf. This loss of water vapour from the leaves is called transpiration.
It can be appreciated that, on a hot summer's day, when a strong sun is shining on the surface of a leaf, the rate of transpiration is very high and, when the point is reached that the plant is losing water at a greater rate than its intake through the roots, the sausage-shaped guard cells forming the stomata lose their rigidity and become soft and flaccid, thus closing the pores and reducing or stopping the flow of air.
The ideal atmosphere for most pot plants is air with a relative humidity of 45-70 per cent which, incidentally, is the ideal humidity range for human beings. When pot plants are surrounded by warm dry air, such as the atmosphere of a centrally-heated living room, the rate of transpiration is far too high and consequently adversely affects some of the processes which are taking place in the plant. In some plants the tips of the leaves die and go brown, buds drop off, flowers wither quickly, leaves wilt and the edges turn yellow.
There arc, of course, quite a few plants which will grow satisfactorily in warm dry air, because these plants have been accustomed to growing in these conditions in their natural habitat. Cacti and succulents are examples, but in general it is found that thick leathery-leaved plants will tolerate this type of atmosphere whereas thin-leaved plants require an increased humidity.
There are several ways of providing humidity, and these are described in detail on pp.29-30.
Despite all that has been written about the subject of growing plants in pots, it is probably still true to say that more pot plants die from being overwatered than from any other single cause. Possibly the reason for this is the fact that it is impossible to lay down any hard and fast rules for watering, and, although advice can be given, until you are experienced it is only too easy to make the mistake of overwatering.
There are many factors which have a bearing on the frequency of watering and these have all to be taken into consideration. Plants require more water when they are making good growth in warm sunny weather than when the weather is cooler and growth is slower. Plants growing in pots in the greenhouse on a warm sunny day in early summer would usually require watering morning and evening, whereas in dull weather in early summer would only need to be watered every other day at the most. Similarly a plant growing in a centrally-heated living room in winter might need watering every day, whereas the same plant, if placed in a virtually unhealed room, might only require water once every 5-7 days.
The frequency of watering is also very dependent on the proportion of root to compost in the pot, as well as the size of the pot. Al! things being equal, plants growing in 3'/a-in (9-cm) pots require more frequent watering than plants growing in 5-in (I3-cm) pots, but if there is only a small amount of root growth in the 3Va-in (9-cm) pot, and the 5-in (13-cm) pot is full of roots, you would find the reverse to be the case, and it would be necessary to water the plant in the 5*in (13-cm) pot more frequently.
Another factor to be taken into consideration is the plant itself. Some plants require more water than others and, as a rule, thick leathery or fleshy-leaved plants do not require watering as frequently as soft thin-leaved plants. In any case the former will tolerate a drier compost without distress whereas thin-leaved plants usually wilt and droop as soon as the water supply is inadequate. Unfortunately a drooping plant is not always a sign that it requires water because plants also droop when they have been overwatered and it is this which makes it so confusing for the beginner.
Imagine that you buy a plant, bring it home and put it on a window-sill. The following day you think the surface soil feels dry, so you give it a good watering and, as the plant pot has been put in a decorative pot holder, the excess water drains into the holder, with the result that the plant is standing in water. This causes the compost in the lowest part of the pot to become waterlogged. Strange as it may seem, healthy soil or compost contains entrapped air, and without this air being present roots cease to function and die. When the compost becomes waterlogged it loses its air; consequently when pot plants are left standing in water the roots cease to function and begin to die. After a while the leaves begin to droop, because the roots are not working, and the leaves are not receiving any water to replace the water lost by transpiration. As soon as you see the plant beginning to wilt you think it must be short of water so you water it again with disastrous results.
In a case like this, when a plant begins to wilt a day or two after you have watered it, you can sometimes save the plant, if you realise it has been overwatered, by putting it in a warm position and allowing it to dry out completely. When the compost has dried out new roots will grow, if the plant has not been irreparably damaged. During this drying-out process, the plant must not be watered at all, and if the leaves droop badly they should be sprayed with water using a fine spray.
Successful watering of pot plants consists of being able to detect when the plant needs watering and applying the appropriate quantity of water. It is, of course, better to water a plant before it shows obvious signs of requiring water, and when you become really experienced you can often observe that a plant needs watering just by the general appearance of the whole plant.
It is difficult to describe, but when you are familiar with a plant, having watched it grow for weeks on end, you can see a difference when it is iust beginning to run short of water: it loses that robust, healthy look.
The main point, however, is to learn from your mistakes. Never water when the surface of the compost is damp. Do not always water when it feels dry, hut be guided by other factors such as those already mentioned. Is the plant growing strongly? Is the pot full of roots? Is it a thick- or a thin-leaved plant? And soon.
Not many growers still use clay pots, but if the plant happens to be in a clay pot you can ascertain whether it is short of water by tapping the side of the pot with a piece of hardwood. If this produces a ringing sound, the plant needs water, but if it produces a dull thud it does not.
The best way to test 3 plant growing in a plastic pot is to grasp the rim of the pot between the thumb and the forefinger and 'feel' the weight, by lifting the pot up and down. This will not convey much to you at first, but if you make a regular practice of feeling the weight of pots, in this way, both before watering and after watering, you will, in the course of time, find it an excellent guide, but it does require a considerable experience.
Watering in the summer is not very difficult, and the time to water is usually when the surface of the soil feels dry. The plant will no doubt be growing strongly, the rate of transpiration will be high if it is an average summer temperature, and providing the plant is not left standing in water there is no chance of it being overwatered, by a correct application of water. It is, however, during the late autumn, and winter in particular, that the correct watering of plants becomes difficult. During this period there is another point to bear in mind and this is connected with the cycle of a plant's growth.
Many plants have a short resting period after they have flowered and during this period their water requirement is very small. In general most plants tend to grow most strongly when approaching their flowering time and it is during this period that they should not be allowed to become short of water.
Plants which bloom in the autumn and winter will consequently require watering more frequently, during this period, than plants which flower in the summer, as these are usually growing very slowly, if at all, during the autumn and winter months. Much has been said and written about the correct way to water a pot plant. Should it be watered from the top or the bottom? Either way is satisfactory but if you water from the bottom, by standing the pot in 1 - 2 in (2Vi-5 cm) of water for a long enough period for it to penetrate the whole of the compost, you then have to allow the excess water to drain. When this excess water drains out of the compost it takes with it some of the soluble salts, which are always present in a good compost, and which the plant needs for its growth. If you make a regular practice of watering by this immersion method you will, in time, leach most of the soluble salts out of the soil and thus deprive the plant of its essential chemicals. This is the big disadvantage of the method of watering plants from the bottom.
Watering pot plants from the top by means of a watering can, with a long thin spout, is the easiest and the most satisfactory way of watering pot plants in the house. A thin spout is desirable because this delivers the water to the compost in a fine jet which can easily be controlled. This is important when watering plants such as cyclamen, Primula malacoides and saintpaulias, as the jet of water must be directed away from the crowns or centres of these plants. They have a tendency to rot if the crowns are allowed to become and remain wet. Cyclamen are particularly prone to rot, in the centre of the corm, when it is planted below or just level with the surface of the compost, and the leaf and flower stems are very numerous and crowded. In this case it is almost essential to water by the immersion method in which case the loss of soluble salts should be kept to a minimum by standing the pot in a shallow container holding a minimum quantity of water. Leave the pot standing in the water for two or three hours, as it takes much longer for the compost to absorb the water in these conditions. If you have gauged the amount of water correctly there should be very little excess to drain away.
Much more could be written about the subject of watering but the foregoing will give you some idea of the complexity of this aspect of pot plant cultivation.
It will be appreciated that certain facts, as already detailed, have to be taken into account when deciding whether a plant needs watering. Let us take a typical example. You have, say, a Regal pelargonium, in flower, growing in 3 4'/2-in (I L5-cm) pot. The plant, being in flower, in this size of pot would usually be about 10- 12 months old, and the pot would be full of roots. You look at the surface of the compost and it is dry. You look at the plant as a whole and it looks healthy and robust. The flowers are beautiful and the petals are not showing any signs of flagging. You 'weigh' the plant in your hand in the manner described and it does not feel light. The plant does not need watering even if the compost does feel dry to the touch. If in doubt leave the plant until the end of the day and if it is still looking happy leave it until the following day.
In the case of a regal pelargonium, the first signs of water shortage are the flower petals beginning to droop. In other plants it may be the leaves which droop first. As you grow the various plants you will learn their different characteristics.
Finally, there is the actual watering. Plants which have been potted correctly have the surface of the compost about V* in (2 cm) below the rim of the pot, and when watering with a watering can cover the surface of the compost with water to a depth of about V; in (1.3 cm) or just below the rim of the pot. If the plant needs watering, it is not likely that there would be any surplus water running out of the drainage holes in the bottom of the pot, unless the compost had become too dry, in which case the water just runs through the compost without really wetting it. When this happens it pours out through the drainage holes much quicker than surplus water from a properly watered compost. This is more likely to happen with a soilless compost which shrinks when it dries, leaving a space between the compost and the side of the pot. In cases like this it is necessary to stand the pot in water, as deep as the compost in the pot, and leave it to soak for a while until the water has permeated the whole of the compost, then allow the surplus water to drain away. Of course, plants should never be allowed to dry out 3S much as this, but it does sometimes happen with plants purchased from a florist or a supermarket, particularly with azaleas.
For healthy growth, plants need a steady supply of food, in the form of chemicals which, of course, must be in solution otherwise they could not be assimilated by the plant. The main elements required, apart from carbon, hydrogen and oxygen, which as we know the plant obtains from air and water, are nitrogen, phosphorus and potassium. In addition the plants also require very small quantities of certain other elements, mainly manganese, magnesium, iron, molybdenum, boron, zinc and copper. Because only minute quantities are required of these elements they are referred to as trace elements.
Vegetation in general does not usually suffer any shortage of trace elements because these are usually present in the soil in sufficient quantities. Also the commercial chemicals used in the manufacture of fertilisers very often contain trace elements as impurities; consequently when these fertilisers are used the necessary trace elements are supplied at the same time. Certainly when making up composts it is never necessary specifically to add any trace elements, and the pot plant grower need not be concerned with this aspect of the plant's requirements, as it will be well catered for in the course of feeding the plant with the main elements it requires.
The feeding of plants is similar to the watering of plants in that you have to take certain factors into consideration in order to know when to feed a plant and the type of fertiliser to use. Plants which have been potted in a good soil-based compost such as JI No.2 will not usually require feeding for at least three months. If a plant is potted on into a 5-in(13-cm)pot, say, from a 4-in (10-cm) pot, or from a 4'6-in (11.5-cm) pot into a 6-in (15-cm) pot, it will not require feeding for a further six to eight weeks.
On the other hand a soilless compost contains much less plant food, and, in this type of compost, plants will usually need feeding about six to eight weeks after potting.
The modern gardener has a very wide choice of fertilisers, and it is therefore all the more necessary to have some idea of the purpose served by all these different mixtures. Nitrogen (N) is the element required for the growth of the green leaves and stems. Phosphorus (P) is the element required for healthy root growth. Potassium (K) is the element required for the production of flowers and fruits. It also promotes ripening of green stems giving strength to the plant structure. Too much nitrogen and too little potassium produces soft lush growth and stems with insufficient strength to support the leafy growth and keep the plant erect.
The best fertilisers to use for pot culture are liquid fertilisers or water soluble crystals, powders or granules. The relative amounts of the three main elements are always shown on the label of a fertiliser, and the important point to note is that the elements are always shown in the same order: nitrogen, phosphorus and potassium.
The relative quantities are calculated as the amount of nitrogen (N), phosphorus pentoxide (P205) and potassium oxide (K,0) which are present in the mixture. Sometimes the full NPK analysis is shown, including the trace elements. Sometimes it merely gives numbers such as 'Formula 25:15:15', which means 25 per cent N, 15 per cent Pj05, 15 per cent K,0.
John Innes base, which is the mixture of chemicals added to the soil, peat and sand to make up a John Innes compost, has the following content: 5.1 per cent N, 8.2 per cent P ,Ov 10 per cent K,0.
John Innes potting compost is made up in three different strengths (sec p. 13); therefore it depends to some extent on which strength has been used as to when the feeding should commence. No useful purpose is served in feeding a plant unnecessarily and more harm than good will be done by overfeeding, especially if this is done by applying the fertiliser in too concentrated a solution. When making up solutions of fertilisers always ensure that the quantities used are in accordance with the manufacturers' instructions. For the most part the type of fertiliser to use is a general purpose feed which has a balanced NPK content. Other than this the one to use will depend on the purpose for which it is required. If you wish to stimulate leaf growth use a high nitrogen mixture such as 25:15:15; and if this is being applied to a flowering plant you should change to a high potassium mix, such as 10:10:27, when the buds are well formed and beginning to show colour. The question of how often to feed cannot be determined without taking all factors into consideration.
A perennial-flowering pot plant, which has been potted at the beginning of its growing season, will only require feeding about five or sis times, about every seven to ten days, commencing usually when the flower buds are beginning to form and ceasing two or three weeks after the flowering is over.
On the other hand, a permanent house plant, which has not been repotted or potted on, will require feeding about once a fortnight during the whole of its growing season, i.e. when it is growing strongly. It should not be fed when growth is very slow, usually during the few weeks before and after the winter solstice.
Some house plant growers prefer to feed their plants every time they water them, using a half-strength feed, and this is quite an effective way of meeting the plant's requirements. It should be pointed out that if a plant has been allowed to become too dry, due to an oversight, to the extent that it is showing signs of distress, it is not considered advisable to feed it in this condition. On occasions such as this it should be watered first and fed when it is next due to be watered again.
Finally, the important point to remember is that, as in the case of watering plants, it is far better to underfeed than to overfeed. Plants which are grown in a good compost such as compost E6 need very little extra food during a season's growth and many plants will give excellent results without any extra nutrients. It is usually only plants which make a lot of growth in a short period of time which really benefit, because they need the extra nutrients to keep pace with their growth.
There are two ways of propagating plants; one is by seed (sexual) and the other is by vegetative (asexual) methods. The latter vary, but the most common method involves cuttings. Plants which have been propagated from seed may be very similar to their parent plant or plants but they are never identical, whereas plants which are propagated by vegetative methods are identical to their parent plant in every way.
There is something very satisfying in raising new-plants from seed, and although one's efforts are not always successful this is not necessarily the grower's fault. Consequently, do not be discouraged if, on occasions, seeds fail to germinate despite the fact that you have done all the right things. Before now I have sown two packets of seeds, being different varieties of the same plant, in the same seed pan; half the seed pan to each packet. The seed pan was enclosed in a plastic bag and placed in a heated propagator. One variety germinated very well whereas only one seed of the other variety germinated. As these seeds were varieties of the same species and were subjected to identical conditions, one can only conclude that one of the packets contained sterile seed, for one reason or another, not necessarily the fault of the seed growers. The point is that, although the seed growers make every effort to provide gardeners with viable seeds, there are occasions, and these are more frequent than you might think, when poor germination or complete failure is not the gardener's fault.
On the other hand by far the majority of failures are due to incorrect sowing methods. Perhaps the main cause of germination failure is making the seed compost too wet, which causes the seeds to rot, especially if the seed pan is enclosed in a plastic bag, which effectively prevents the escape of any moisture.
The sowing compost should be moist enough to obviate the necessity of watering again before germination is complete, but on the other hand it must be free from any excess moisture. For most sowings a 4Vj-in(l 1.5-cm) plastic half-pot is a suitable container. Fill this with the sowing compost, lightly compressing it, to make the compost firm, leaving the surface of the compost about J/« in (2 cm) below the rim of the pot. It should then be placed in water about 2 in (5 cm) deep until the water appears on the surface of the compost. Allow the excess water to drain from the pot, then put it in your heated propagator for 24 hours, after which the compost should be in the right condition for you to sow the seed. A useful gadget for firming the compost in pots and seed trays can very easily be made from 1-in (2.5 cm) thick wooden board. Cut a piece measuring 3'/ix2 in (9x5 cm) and then cut one of the corners off, cutting about Vi in (1.3 cm) from the corner. Hammer a I'A-in (4-cm) nail into the middle of one side of the wood to serve as a handle. This can be used for firming the compost in any size of container from a 2l/i-in (6-cm) pot upwards, and also for seed trays, by using the flat side or the edges, according to the container.
A good guide to follow, when sowing seed, is the size of the seed, as this gives you some idea of the depth at which it should be sown. Seed should be sown at a depth which is about three times the diameter of the seed. A seed measuring l/s in (33 mm) should he sown V» in (I cm) deep. Very fine seed should be sown on (he surface and not covered with any compost. A good method with fine seed is to spray the surface of the compost with a mist spray, after sowing, which helps to settle the seed into the surface of the compost.
Seeds vary in their temperature requirements, and these are always shown on the seed packet or in the seed catalogue. After sowing, place the pot in a propagator, at the correct temperature, either enclosed in a plastic bag or covered with a sheet of glass, to prevent the surface of the compost drying out. From time to time examine the pot for germination, and as soon as the seedlings begin to appear remove the covering and ensure that the seedlings are exposed to a good light. When germination appears to be complete remove the pot to a cooler temperature, say about 10°F (5.5°C) less than the germination temperature, to ensure sturdy growth, as loo high a temperature will cause the seedlings to grow too 'leggy*. Some seeds germinate within a few days whereas others take several weeks. When sown at the correct temperature, coleus seeds
Fig, 1 A household sieve it most effective for providing a fine level surface of »owing compost, necessary when sowing very Mnall iceds.
will germinate in some six to eight days whereas cyclamen seed takes four to six weeks at the earliest, some of the seeds taking as long as eight weeks and more.
Seeds which arc large enough to handle can be sown by placing each individual seed by hand on the surface of the compost, adequately spaced out, then covered with the appropriate amount of compost, preferably by riddling the compost through a fine sieve (a cook's household sieve is ideal for this purpose). When sowing seed which is too small to handle, it is a good idea to prepare a level surface by sieving a layer of fine compost on to the compost in the pot and very* lightly firming this before sowing. This helps to keep the seed at an even depth.
There are various methods of sowing fine, to very fine, seed and all of them have their advantages and disadvantages. Perhaps the easiest is to sow straight from the packet. Cut the top offthe packet, squeeze it to form a shallow trough, and while holding it at an angle keep tapping it with your finger to allow the seed to trickle out as slowly as possible. At the same time keep moving the packet backwards and forwards across the surface of the compost.
Whichever method you use it is inevitable, unless you are a genius, that when germination takes place you will get some overcrowding of the seedlings. These should be thinned as soon as possible, using a pair of tweezers, which enables you to pick up seedlings when they are very small. Even if germination has only been average, you will usually have many times more seedlings than you require, consequently you can thin the seedlings quite drastically to ensure that those that are left have adequate room for their development.
Fig.2 A sicm cumng showing ¡a) the node which occurs just below the junction of the leafstalk with the stem and is often apparent as i slight swelling on the stem; fb) the axil which is [he upper angle between [he leaf and the stem from which it is growing; (c) the stipule which is the leafy outgrowth at the outer base of [he leafstalk.
There are different opinions on the subject of transplanting seedlings. Some gardeners advocate transplanting the seedlings as soon as they are large enough to handle, whereas others are of the opinion that seedlings should not be moved until the first true leaves have formed. The first leaves which appear when a seed germinates are the seed leaves, after which the shoot continues to grow and the first true leaves then appear. The method I always use is to transplant as soon as the seedlings are large enough to handle. Using a plastic plant label, or a table fork, lift a portion of the compost, complete with seedlings, and separate the seedlings from the compost and from each other, being careful to avoid breaking the thin tapering roots. Handle the seedlings by their leaves only, and plant in 2'/:-in (6-cm) pots, or prick out into a seed tray, depending on your requirements. It is quite satisfactory, in fact it is advantageous, to plant the seedlings, even at this stage in their development, in compost Eft or John Innes potting compost No.2 or whatever compost they will finally be growing in. The seedlings should be planted to a similar depth to that at which they were already growing, never any-higher but sometimes a little lower, especially if they look a bit drawn or leggy, through having been grown in too warm an atmosphere or too poor a light.
After transplanting, water the pots or seed trays, and keep them in a suitable place where they are receiving sufficient heat and light, but in all instances protected from direct sunlight, which would be detrimental at this stage,
When a gardener talks about a cutting he or she is usually referring to a stem cutting, and, in particular, a tip cutting, so callcd because it consists of the top 2-4 in (5- 10 cm) of the stem, which includes the growing tip. It is very simple to take a stem cutting. All you have to do is to select a healthy stem and make a clean cut, through the stem, below the third node from the tip. (A node is a joint on the stem from which the leaf grows.) Remove all the leaves except the top two and also remove any stipules as these tend to rot and this can spread throughout the whole cutting, (A stipule is a leaf-like growth which is found on the stem at the nodes often growing round the base of the leaf stalk. These are very prominent on the stems of geraniums.) When the leaves have fallen off a stem the nodes can still be detected as a slight swelling on the stem.
It is essential that sufficient leaf area is left on the cutting to enable it to grow; therefore in some cases it may be necessary to allow more than two leaves to remain, if they are only small ones. It is really a question of striking a balance. The more leaf area there is on a cutting the more transpiration there will be, and as in the first instance the cutting does not have any roots the flow of water up the stem to the leaves is very slow, and too much leaf area will cause the cutting to wilt badly. This slows down, or may even prevent, the necessary chemical processes taking place; thus the cutting does not grow. On the other hand too little leaf area will have more or less the same effect.
To reduce transpiration, new cuttings should always be kept in a moist atmosphere. The easiest and most convenient method is to use rigid plastic domes, which can be obtained to fit various sizes of seed trays from half trays upwards, and there are also domes for 3lA-in (9-cm) and 4Vi-in (11.5 cm) plant pots. A cheap but efficient way of covering a single cutting is to place an empty glass jar over it, the rim of the jar resting on the top of the compost. A Vh-\n (9-cm) pot is the ideal size to use for this purpose.
Although it is not essential, particularly with cuttings which root easily, it helps to speed up the rooting process if you dip the base of the stem in a hormonerootingpowder.
For success with cuttings the compost is an important factor. Ideally you require a compost which will remain moist, but not damp, as this would cause the cuttings to rot, firm enough to hold the cuttings in an upright position, even when they are being watered, and one which will encourage root growth and provide some food for the rooted cuttings. Many different cutting mixtures are recommended and, of all these, the one I have found the most satisfactory is a mixture of 3 parts of soilless cutting compost and 2 parts of fine vermiculite, by volume, well mixed together. There are numerous proprietary brands of soilless sowing and cutting composts and they all contain sufficient plant food, particularly phosphates. Vermiculite is expanded mica, and is an inert substance which will absorb and retain a considerable quantity of water. It has a twofold purpose in that it not only absorbs any surplus water from the soilless compost but also gives the compost a gritty texture. It has been shown that when roots meet with some resistance and have to force their way through the compost they develop more strength and become much sturdier. The vermiculite provides this resistance, and you find that cuttings grown in this mixture are appreciably bushier than cuttings which have been grown in a straight soilless compost.
Most cuttings will root satisfactorily in a minimum temperature of65°F(18°C) day and night.
The light requirements for cuttings is the same as for the mature plants from which they have been taken and it is essential to ensure that the cuttings are placed in as bright a light as possible, but shaded, of course, from direct sunlight.
As soon as possible the plastic covers should be taken offthe cuttings, which will improve the light intensity. It is difficult to know when to remove the covers. Softwood cuttings, such as geraniums, fuchsias and begonias, should generally remain
covered for about seven to ten days. After this period, remove the cover and observe whether the leaves remain turgid after several hours' exposure; if not, replace the cover for a day or two until further tests show that they do. Using the cutting compost described, it will not usually be necessary to water the cuttings during the period they are covered by the plastic dome, but as soon as this has been removed the compost should be inspected daily to ensure that it does not become too dry.
One of the advantages of using this type of cutting compost is that the cuttings can be left growing in it for some six or seven weeks, and even longer, until they have developed a strong bushy root system. When the cuttings are well rooted, pot in 2Vi- or 3'A-in (6- or 9-cm) pots depending on the size of the cutting, using the recommended compost for the plant being grown.
By far the majority of plants provide cuttings which root below the node but there are a few which provide internodal cuttings, a notable example being rhododendrons (which includes azaleas) and cuttings from these plants are prepared by cutting the stem halfway between two nodes.
Stem cuttings need not always be tip cuttings, and many plants such as dracaenas and dieffenbachias can be propagated by taking a section of the main stem, say 2 or 3 in (5-7.5 cm) long, containing at least two nodes, and treating it in the same way as a tip cutting. Providing there is a dormant bud to develop into a shoot, this method will provide you with a new plant; but it is a much more difficult way of raising plants than by tip cuttings, and the failure rate is very much higher.
There are other methods of vegetative propagation, such as root division, offsets, and plantlets, but these are peculiar to certain types of plants and are described, where applicable, in the plant section of the text.
Plants grown from seed or from cuttings are initially potted in 2Vi-in (6-cm) or 3Vi-in (9-cm) pots but, as they continue to grow, the time comes when it is necessary to pot on into a larger size. By the time a plant needs potting on it has usually reached the point when its roots fill the pot, and it has used up most of the plant food in the compost. It is not difficult to know when a plant is ready for moving into a larger pot. The most obvious sign is when the plant begins to look too large for its pot. Another sign is when the roots begin to grow through the holes in the bottom of the pot, which usually, but not always, indicates that the pot is full of roots. Again, when a plant needs watering much more frequently, this also indicates a pot full of roots.
When this stage is reached, remove the plant from its pot and if there is a network of roots all round the outside of the root-ball, the plant is ready for potting on, but on the other hand, if the roots only look sparse at the bottom of the root-ball, keep the plant in the same pot for a while longer. If you find a mass of roots which have grown round the bottom of the root-ball, it means that the plant has been left in the pot too long and it is said to be 'pot-bound1.
One of the easiest ways of removing a plant from its pot is to place one hand over the surface of the compost, with the main stem between the fingers, grasp the outside of the pot with the other hand, turn it upside down and gently tap the rim of the pot on the edge of the potting bench, or any solid surface. Do not attempt to remove a plant from its pot when the compost is dry, because firstly it tends to stick to the sides of the pot, and secondly if the plant is not ready for potting on the root-ball is liable to collapse and you will lose half the compost onto the bench, so if your plant is on the dry side water it a few hours before removing it from its pot.
Having established that the plant needs potting on, you have to decide which size of pot to use. To some extent this will depend on the plant you are growing. If it is a permanent foliage type of house plant the transfer should be confined to the smallest suitable size, which would be a pot 1 in (2.5 cm) larger in diameter, e.g. from a 3l/*-in (9-cm) pot to a 4Vi-in (11.5 cm) pot (potting from a 3'/i-in (9-cm) pot into a 4-in [10-cml pot is not only difficult but it is quite unsatisfactory). The same remarks apply to slow-growing plants, whether foliage or (lowering, as a pot 1 in (2.5 cm) larger will usually be sufficient for the rest of the growing season and further potting should not be necessary.
Some flowering pot plants and fast-growing plants, which make a lot of growth in a season, are better potted on into 5-in (12.5-cm) pots from 3V2-in (9-cm) and if necessary their next move is into a 6-in(15-cm) pot.
As a general rule, however, select a pot 1 in (2.5 cm) larger in diameter when potting on.
Up to now we have been concerned with potting on, but another aspect to consider is repotting. A plant will grow in the same container, quite satisfactorily, for a long period of time, providing it is regularly fed, but there comes a time when, for various reasons, it ceases to thrive and begins to deteriorate. This could be caused by various factors but, whatever the cause, the only cure is to repot the plant. This is done by knocking the plant out of its pot and then, using a sharp pointed thin stick or preferably a steel knitting needle, loosen the compost from the roots. If you prod the root-ball all over then gently squeeze it between your hands, give a few more prods with the needle and then lightly shake it, you will find it possible to remove most of the compost from the roots. It does not matter if some of the roots are lost at the same time, providing a good root system still remains on the plant. You can then pot the plant into the same container using fresh compost when you will soon find a remarkable improvement in its growth. After you have repotted in this way the plant can then be potted on into a larger pot in due course if required.
A perennial plant which has been growing in the same pot for a whole season's growth should always be treated in this way, and not just potted on, if you wish to obtain the best results.
Thirty years ago everybody used clay pots. Nowadays all the professional growers supply their plants in plastic pots. Whichever type of pot you use it is quite unnecessary to put broken pot, gravel or any other kind of drainage material at the bottom of the pot, as recommended in so many gardening books and articles; in fact, when using watering trays or capillary matting, this can be detrimental. If you use the composts recommended, or soilless composts, these will drain quite satisfactorily. When using clay pots put a plug of fibreglass in the hole at the bottom of the pot; this will prevent any of the compost being lost and will also form a link between the watering tray and the compost.
Pots should preferably be washed before being reused, unless you arc repotting into the same pot.
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