An understanding of the physiology of the vegetable or plant material being stored is necessary to achieve the best possible results. Root vegetables are normally biennial and naturally prepared to be overwintered, whether in a store or outside (see p119). Annual vegetables are actively respiring at the time of picking (see p118), but with the correct temperature and humidity conditions the useful life can be extended considerably. Great care must be taken with all produce to be stored as any bruising or physical damage can become progressive in the store. Dormant vegetables can be cold stored, but care must be taken to prevent drying out. For this reason different types of store are used depending on the crop; ambient air cooling is used for most hard vegetables and refrigeration for perishable crops gives a fast pull-down of temperature and field heat (see p119).
There are many differences between vegetable and fruit growing, most of which are related to how long the crop is in the ground before replanting. Whereas most vegetables are in the soil for less than a year, fruit is in for much longer; typically strawberries last for two to three years, raspberries for eight to ten years and top fruit for some 15 to 20 years or more. Fruit plants should not be replanted in the same place (see p278).
There are many production methods and the choice is mainly related to the space available, aftercare (such as pest and disease control) and the method of harvesting; taking fruits from large trees presents difficulties and making it easy for the public in 'pick your own' (PYO) situations is essential. Several methods lend themselves to smaller gardens, growing against walls or as hedges. These considerations greatly influence the selection of cultivar and rootstocks.
Top fruit can be grown in a natural or 'unrestricted' way in which case the size of the tree depends on the cultivar and whether it is grown as a standard, half standard or bush. Restricted forms include cordons, espalier, fan and columns (see Figure 1.4). Rootstocks play an important part in determining the size of top fruit trees, e.g. by grafting a cultivar with good fruiting qualities on to the roots of one with suitable dwarfing characteristics (see p177). Excess vigour, which can lead to vegetative growth (leafiness) at the expense of fruit, may be reduced by restricting nutrient and water uptake by growing in grass (see competition p46), ringing the bark (see p95) or, more rarely, root pruning. Soft and cane fruits are usually grown on their own unrestricted roots.
Figure 1.4 Fruittreeforms
Training and pruning plays an important part of the husbandry of fruit growing. The shape of trees and bushes is established in the early years ('formative pruning'). Suitable frameworks and wiring systems are set up for many of the growing systems (see Figure 1.4) and the new growth has to be tied in at appropriate times. Pruning plays a major part in maximizing flowering and fruiting, as does the bending down of branches (see p158). The shape created and maintained has a significant effect on pest and disease control; the aim is usually to have an open centre which reduces humidity around the foliage (see p159) and lets the sunlight into the centre of the tree to give a good fruit colour. Pruning is also undertaken to remove weak and diseased growth (see p159).
Fertilization of flowers is required before fruits are formed (see p137). In order for this to be successful pollination needs to take place (see p134).
Most top fruit is not self fertile. Therefore, another plant is needed to supply pollen and insects are required to carry it. Since successful pollination will only take place when both plants are in flower the choice of cultivars becomes limited; later flowering cultivars do not pollinate early flowering ones. Apple cultivars are placed in seven groups to help make this choice whereby selection is made from the same group (ideally) or an adjoining one. However, choice is further limited because some cultivars are incompatible with each other (p146). In particular, triploid cultivars, such as Bramley's Seedling, are unable to pollinate any other (see p146). Similar considerations apply to pears, but some plums, cherries and peaches are self fertile.
Propagation of top fruit is by grafting (see p176), raspberries by suckers (see p174), blackberries by tip layering and strawberries by runners.
Pest and disease control methods are discussed in Chapter 16. Note that Certification Schemes and Plant Passports are particularly important for plants that are propagated by vegetative means where viruses can be a significant problem. This is especially the case where they are grown for many years before renewal (see also p294).
Harvesting fruit for immediate sale or consumption must be undertaken at maturity to present the full flavour of the variety. Techniques involved in handling fruit to prevent bruising and subsequent rotting require an understanding of fruit physiology. Stone fruits, e.g. plums and cherries, are picked directly into the market container being graded at the same time because these fruits often have a very attractive bloom which is lost if handled too often. Soft fruits will not tolerate washing or excessive handling and grading is done at picking. With strawberries the stalk is not left attached, only the calyx, to prevent it sticking into an adjoining fruit and causing a rot. Machine harvesting of raspberries for the processing industry is less important now as most fruit is grown for the dessert market and is often protected during harvest by temporary, polythene covered structures known as 'Spanish Tunnels' or 'Rain Sheds'.
Storage of fruit crops requires considerable skill and technique. Pip fruits, e.g. apples and pears, must be at an exact stage of maturity for satisfactory storage. If storage is to be for a long time, e.g. the following spring, then controlled atmosphere storage is used, where the levels of CO2 and O2 are controlled as well as temperature and humidity.
Soft fruit crops are harvested during the summer when the ambient air temperature is high and the fruit will continue to ripen after it has been picked. It is therefore essential to lower the temperature of the fruit quickly, known as removing 'field heat'. Refrigerated storage is used, but excessively low temperatures will cause the fruit to respire even more quickly when removed from store (see p119). This causes punnets (fruit containers) to mist up and the fruit to rot more quickly. The maintenance of the fruit at a cool temperature from grower to consumer is referred to as 'cool chain marketing'.
Protection for plants can be in the form of simple coverings such as floating mulches, cloches or cold frames and more complex structures such as polytunnels or glasshouses.
The advantage of protection by these various methods is that to a greater or lesser extent they modify weather conditions, particularly wind, and so keep the environment around the plants warmer. This factor enables plants to be grown over a longer season, which is advantageous where continuity of supply, or earlier or later produce commands a premium. In leisure horticulture, the protection offered enables a wider range of plants to be kept, propagated and displayed.
The changed environment in protected cropping necessitates a careful management approach to watering (p350) and ventilation. Any plants requiring insect pollination have to be catered for (p137). Pests, diseases and weeds can also benefit from the warmer conditions and tropical species assume more importance.
Glasshouses, or conservatories, enable tender plants (see p156) to be grown all year round, especially if a source of heat is also available. Half hardy plants can be 'brought on' earlier and similarly plants can be grown from seed and planted out when conditions are suitable after a period of 'hardening off' (p156).
The closed environment makes it possible to maximize crop growth by using supplementary lighting, shade, and raising carbon dioxide levels (see p113).
Day length can be modified by the use of night lighting and blackouts to encourage flowering out of season (see p161). A wider range of biological control is possible within an enclosed zone (see p271). Greenhouses also allow work to continue even when the weather is unsuitable outside.
There are many designs of greenhouses, some of which are illustrated in Figure 1.5. Others are much more ornamental rather than purely functional. They range from the grand, as seen in the Botanic Gardens, to the modest in the smaller garden. Although the structures can be clear glass to the ground, there are many situations where brick is used up to bench level e.g. Alpine Houses. Many older 'vinery' style houses were substantially underground to conserve heat.
Structural materials used for glasshouses depend again on their intended purpose, but most are either aluminium and steel construction or wood (usually Western Red Cedar). Those which are for commercial production tend to be made of aluminium and steel with an emphasis on maximizing light (see p113) by increasing the height of the gutter and using larger panes of glass. Aluminium is lightweight and very suitable as glazing bars for glasshouse roofs, it is also virtually maintenance free, but does transmit heat away more than alternatives such as wood. Where more attractive structures are preferred, wood is often chosen although
such structures are less efficient in light transmission and require more maintenance.
Cladding materials are usually glass or plastic although there are many types of plastic available. Glass has superior light transmission and heat retention. Plastics tend to be cheaper but are less durable. They have poorer light transmission when new and most deteriorate more rapidly than glass. Polycarbonate is often used in garden centres where the danger of glass overhead is considered to be too great in public areas. The biodomes at the Eden Project in Cornwall are made up of hexagonal panels made of thermoplastic ETFE cushions (see Figure 1.6).
Orientation of the glasshouse depends on the intended purpose. For many commercial glasshouses the need for winter light is the most
significant consideration, this is achieved with an east-west orientation. However, the most even light distribution occurs when the house is orientated north-south which may also be the choice if several houses are in a block. For many decorative structures the orientation is subservient to other considerations.
The siting should ensure an open position to maximize light, but with shelter from wind. Frost pockets need to be avoided (see p36) and there should be good access which meets the needs of the intended use. Water is needed for irrigation and normally an electricity supply needs to be available.
Light availability is emphasized in the selection of structure, cladding and siting, as this is fundamental to the growth of plants (see photosynthesis p110). Supplementary lighting in the greenhouse is advantageous in order to add to incoming light when this is too low (see p114). More rarely, total lighting can be used when plants are grown with no natural light such as in growth cabinets for experimental purposes. Low level lighting to adjust day length is used to initiate flowering out of season, e.g. year round chrysanthemums, poinsettia for the Christmas market (see photoperiodism p160).
Careful water management is essential in the glasshouse where plants are excluded from rainfall. A suitable supply of water, free from toxins and pathogens (see p351), is a major consideration especially with increasing emphasis on water conservation (see p351). For many, water is supplied by hoses or watering cans with spray controlled with the use of a lance or rose. There are many systems which lend themselves to reduced manual input, and on both small and large scale automatic watering is preferred, using one or other of the following:
Water is not only used to supply plant needs directly, but also to help cool greenhouses. 'Damping down' is the practice of hosing water on to the floor, usually in the morning, so that the evaporation that follows takes heat out of the air (see p37). This increases the humidity in the environment (see p39) which can advantageously create a good environment for plant growth. On the other hand, if done at the wrong time it can encourage some pests and diseases (see p267). Water can also be used to apply nutrients through a dilutor, either as a one-off event or at each watering occasion; this is known as 'fertigation' and enables the grower to provide the exact nutritional requirement for the plant at particular stages of its development.
Heating can be supplied by a variety of methods including paraffin, electricity, methane (mains gas), propane (bottled gas) and, less commonly now, solid fuel. Some commercial growers are installing biomass boilers and some are in a position to use waste heat from other processes. Fuel costs and environmental considerations have put increasing emphasis on reducing the need for heat (choice of plants, use of thermal screens, etc.) and reducing heat losses with insulation materials such as bubble wrap (with consequent reduction in light transmission).
Ventilation is essential in order to help control temperature and humidity (see p39). Air is effectively circulated by having hinged panes set in the roof and the sides (these are often louvre panes). The movement of air is often further enhanced by the use of fans.
Shading is used to reduce the incoming radiation (see p113). Although much emphasis is put on ensuring good light transmission, particularly for winter production, the high radiation levels in summer can lead to temperatures which are too high even with efficient ventilation. Traditionally, shading was achieved by applying a lime wash. This has been superseded by modern materials which are easier to remove and some even become less opaque when wet to maintain good light levels when it is raining. Most modern production units have mechanized blinds which can also help retain heat overnight. Many ornamental houses will have attractive alternatives such as external shades in natural materials.
Growing media options in protected culture are very extensive, but the choice depends on whether the plants are grown in soil, in containers on the ground or in containers on benching. Border soils have been used over the years, but they have many disadvantages, especially with regard to pest and disease problems and the expense of controlling this (see soil sterilization p265). A range of composts is available for those who choose to grow in containers (see p390). However, a significant proportion of commercial glasshouse production uses one of the hydroponics systems (see p394).
Pest and disease control has special considerations because the improved conditions for plants can also lead to major pest and disease outbreaks which develop quickly. If the atmosphere becomes wet, too humid or too dry even more problems can be expected. Furthermore this environment supports organisms not commonly found outdoors such as two-spotted red spider mites (see p224). Besides a range of cultural and
chemical methods, the enclosed space makes it possible to use a wider range of biological controls than is possible outside (see p275).
Automatic systems to control temperature, ventilation and lighting have developed over the years to reduce the manual input (and the unsociable hours) required to manage conditions through the growing season. Some of the most exciting developments have occurred as computerized systems have been introduced to integrate the control of light, temperature and humidity. In order to control the conditions indoors the systems are usually linked to weather stations (see p39) to provide the required information about the current wind strength and direction, rain and light levels (see Figure 1.7). The use of the computer has made it possible for the whole environment of the glasshouse and the ancillary equipment to be fully integrated and controlled to provide the optimum growing conditions in the most efficient manner. It has also enabled more sophisticated growing regimes to be introduced.
Polytunnels provide a cheaper means of providing an enclosed protected area. They are usually constructed of steel hoops set in the ground and clad with polythene, but in some cases, such as for nursery stock, a net cover is more appropriate (see Figure 1.8). They are not usually considered to be attractive enough for consideration outside commercial production although they are often seen in garden centres.
Figure 1.8 Nettunnel
Walk-in tunnels offer many of the features of a greenhouse, but there are considerable drawbacks besides looks; they tend to have limited ventilation and, despite use of ultra violet inhibitors, the cladding is short lived (3-6 years). Nevertheless there have been steady improvements in design and there are many hybrids available between the basic polytunnel and the true traditional greenhouse, utilizing polycarbonate either as double or triple glazing.
Low tunnels (with wire hoops 30 to 50 cm high) are commonly used to protect rows of vegetables. These are put in place after sowing or planting; access and ventilation is gained thereafter by pulling up the sides.
Cold frames are mainly used to raise plants from seed and to harden off plants from the greenhouse ready to be planted outdoors. The simple 'light' (a pane of glass or plastic in a frame) is hinged on the base of wood or brick and propped up to provide ventilation and exposure to outdoor temperatures. The degree to which plants are exposed to the outdoor conditions is steadily increased as the time for planting out approaches. A frameyard is a collection of cold frames.
Cloches were originally glass cases put over individual plants for protection (cloche comes from the name of the cover used in old clocks). They are now more usually sheets of glass or plastic clipped together over individual plants, or rows of them can cover a line of vegetables (mostly superseded today by low tunnels in commercial production).
Floating mulches are lightweight coverings laid loosely over a row or bed of plants (see Figure 1.9) and held in place by stones or earth at intervals. They provide some protection against frost, speed up germination and early growth and provide a barrier against some pests.
In contrast to the production of plants for food and flowers, those in service horticulture (embracing the many facets of landscaping, professional gardening and turf culture) are engaged in plant selection, establishment and maintenance. This will mainly involve:
Many will be involved in aspects of garden planning such as surveying and design.
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