Plant propagation

Most greenhouse operators in Canada grow their own transplants. This is a desirable practice because it reduces the possibility of importing diseases and insects. However, transplant raising in other countries has been practiced successfully by specialized nurseries that ensure a reliable supply of low-cost high-quality transplants to local growers through the application of modern technology. Plant propagation is a vitally important stage in greenhouse vegetable production. The success of a crop depends largely on the attention paid to detail and the care taken during plant raising. Moreover, with early spring crops, propagation must take place in the winter, when natural light is limited. To make the best use of available light, other factors such as spacing, temperature, irrigation, and nutrition must be subject to close and accurate control. Artificial light is now used widely to enhance transplant growth when natural radiation is limited, with the result that the performance of early-planted spring crops has improved significantly.

Propagation schedules

In deciding when to seed, the desired harvesttime should be considered. It usually takes 5 months from seed to first pick in a normal spring crop but only 4 months in a normal fall crop. A spring crop that comes into production in the beginning of April requires seeding to take place in the latter part of November. In recent years, an increasing number of growers are planting a late spring crop in plastic houses. In that case, seeding takes place in January and planting in-house in March. Harvest is in May, June, July, and later. The late spring crop is easier and less expensive to grow but comes into production when prices are relatively low.

Seed sowing

Each gram contains about 300 seeds. Assuming a planting density of 25 000 plants per hectare, a germination rate of 80%, and a safety margin of an additional 10%, seed should be sown at approximately 120 g/ha.

The most common approach is as follows. Fill a plastic tray (55 x 27 cm) with a soilless mix, such as a commercial peat mix, and strike it off level. Press the medium down evenly with a wooden board to about 1.5 cm from the top. Broadcast the seed or sow it in rows, as evenly as possible, at a rate of 500-600 seeds per tray. Cover the seed with 0.5 cm of fine-grade growth medium to assist the prompt shedding of the seed coat, thus reducing the risk of transmitting tomato mosaic virus and of distorting the seed leaves. After sowing, cover seed trays with glass or paper, which conserves moisture; no further watering is needed before germination. Place the seed trays in a small greenhouse or special propagation room (no light is needed at this stage) at a day and night temperature of 24°C until daily inspection shows seedlings to be breaking the surface of the growth medium; the higher the air temperature of the propagation room during germination the faster and more uniform the germination will be. However, seedling growth is fast at high temperatures, which makes the use of a high germination temperature risky because a delay of a few hours in removing the seed tray cover can result in excessive elongation of the seedling stems and carbohydrate depletion. Once seedling emergence is well under way, remove the seed tray covers, reduce the day and night air temperature to 20 °C, and supply as much light as possible. Maintain these conditions for

Free The Plant Ftp
Fig. 11 Typical stages of seedling growth frequently used as markers of the ideal time for initiating (plant A) and terminating (plant B) the cold treatment.

2-3 days to allow all seedlings to emerge and become photosynthetically active and to prevent excessive elongation.

Cold treatment

Under a cold treatment regimen, place young tomato seedlings in a day and night air temperature of 10-13°C for approximately 2 weeks, while providing as much light as possible for 9-12 h. Seedlings should be subjected to cold treatment just after the seed leaves (cotyledons 1 unfold and the first true leaves start to appear (Fig. 11). Shoots kept at low temperatures at this stage of growth produce a small number of leaves below the first cluster and therefore flower earlier; roots kept at low temperatures cause branched clusters, i.e., many flowers in the first and possibly the second cluster. Cold temperatures during both day and night are effective. The cold treatment increases the number of flowers but does not influence the setting of fruit. If later conditions for fruit setting are right, a greater number of flowers will set fruit because of the increased number of blossoms. If, however, the temperature for fruit set remains less than ideal, the pollen does not germinate and grow normally resulting in poor fruit set and cat-faced fruit. When the cold treatment is used, seed 10-14 days earlier than usual to compensate for the slow growth rate during the cold treatment. The growth medium in the seedling trays must be sterile, because when plants are grown at relatively low temperature the danger of damping-off is increased.

Transplanting into pots (pricking out)

The best time to prick out tomato seedlings is at cotyledon expansion, just after the cold treatment. Seedlings are too hard to handle before this time and if pricking out is delayed further, the transplanting shock will be greater because more roots are broken. Transplants are grown in 7.5cm or plastic pots or in soil blocks. In addition to good topsoil, peat mixes are used extensively as growth media but always after proper sterilization. Growers should avoid modifications to recommended mixtures, as the results could be disastrous. A worldwide trend toward peat-based mixtures is replacing those containing loam, because loam of desirable specifications is difficult to obtain. However, greenhouse soil that has good texture and structure is a valuable asset as a growth medium for transplant raising, provided it is sterilized effectively before use. Heavy leaching following soil sterilization is also highly recommended. This treatment ensures the removal of excess salt, which can be harmful to young seedlings and results in low nutrient levels, especially nitrogen, in the growth medium. Low nutrient levels allow for better control of plant growth through the manipulation of liquid feeding.

Do not change frequently the substrate used for raising transplants because seedlings respond differently to different substrates, and the experience gained over the years on one substrate is not entirely transferable to other substrates.

Although larger pots (10 cm) appear to increase the cost of producing plants, they are, in fact, the best choice because they allow growers to hold their plants longer in the propagation house, which is much less expensive to heat than the entire greenhouse. Furthermore, an extended propagation time results in greater use of artificial light whenever available. Finally, the use of large pots for transplant raising has frequently been associated with increased early yields. Pots can, of course, be used again the following season, but they should first be washed and soaked in a bleach (10%) solution, or any other approved disinfectant.

Watering and nutrition

Immediately after pricking out the seedlings, water them thoroughly to bring the growth medium to field capacity and to settle it around the roots. Careful watering is necessary during the propagation period. Keep the young plants well supplied with water without depleting the growth medium of its oxygen by overwatering. Because it is difficult to judge the moisture content of growth media in plastic pots, pull out two or three plants regularly and make sure that the medium at the bottom of the pots is kept moist but not too wet. Transplants raised in 10-cm pots require watering daily in good weather; in very bright weather, more than one watering a day may be necessary; in dull winter weather, watering as infrequently as once every 3 days may be adequate. The use of smaller pots requires more frequent watering. A deliberately short water supply in the propagation pots restricts growth and helps produce hard, stocky, dark green plants. However, this type of growth control invariably results in excessive hardening of the transplants because of the difficulty of regulating the water supply, resulting in yield losses early in the production season when prices are best. In recent years, research has identified more dependable means of growth control. Fertilizer is now fed continuously at every watering, with the fertilizer concentration in the solution used as an osmoticurn in regulating water availability to the plants (Table 2). The recommended fertilizer concentration in the irrigation water, measured as its electrical conductivity (EC), varies according to the environmental conditions. For transplants raised during the winter, complete nutrient solutions of an EC that ranges between 3000 and 6000 |iS/cm have been used with good results (Table 3). Higher conductivities now appear safer when the potassium-to-nitrogen ratio in the nutrient solution is higher than 4:1 (Plate 1).

Also, the supply of artificial light allows the use of higher ECs than normal, but not when artificial light results in overheating the transplants and in drier conditions in the greenhouse.

The individual nutrient concentration in the final solution varies according to EC, but in the standard case, where 1 L of each stock solution is diluted in 100 L of water (1:100 mixing ratio), the concentration of nutrients is as described in Table 4.

Transplants of similarly good quality can also be raised when commercial mixes of fertilizer such as those commonly known as starters are used at appropriate rates. A simple solution of starter fertilizer (3 g of 10-52-10 per litre of water) with an approximate EC of 4000 |iS/cm used in continuous feeding produces transplants acceptable to most growers in a simple and safe way Alternatively, commercial fertilizer mixes that contain all nutrients except calcium and that offer a potassium-to-nitrogen ratio of about 5:1 can be used safely with acceptable results at ECs up to 5000 nS/cm.

Table 2 Stock solutions required for the preparation of complete fertilizer solutions with a varying potassium-to-nitrogen ratio

Potassium-to-nitrogen ratio Fertilizers -

Stock A, 1000 L Calcium nitrate Potassium nitrate

Stock B, 1000 L Potassium sulfate

Stock C, 1000 L Monopotassium phosphate Magnesium sulfate Iron chelate (13% iron) Micronutrient mix (STEM)

  1. 0 70.0 70.0
  2. 0 9.5 9.5
  3. 5 92.5 150.0
  4. 5 22.0 22.0
  5. 0 50.0 50.0
  6. 6 0.6 0.6
  7. 8 0.8 0.8

• The stock solutions described can be used, as shown in Table 3, to produce fertilize1

solutions of various ECs for raising transplants.

• A typical micronutrient mix, e.g. Peters soluble trace element mix (STEM), contains

1.45% boron, 3.2% copper, 7.5% iron, 8.15% manganese, 0.046% molybdenum and 4.5% zinc.

Plate 1 Effect on cultivar Ohio CR-6 of the potassium-to-nitrogen ratio and EC (|lS/cm) of the fertilizer solution applied on tomato transplants. Note the excellent quality of transplants produced with nutrient solutions of moderately high EC (i.e., 3000-6000|lS) when the potassium-to-nitrogen ratio is also high (i.e., 4:1 or 6:1).

Plate 1 Effect on cultivar Ohio CR-6 of the potassium-to-nitrogen ratio and EC (|lS/cm) of the fertilizer solution applied on tomato transplants. Note the excellent quality of transplants produced with nutrient solutions of moderately high EC (i.e., 3000-6000|lS) when the potassium-to-nitrogen ratio is also high (i.e., 4:1 or 6:1).

Table 3 Amount of each stock solution (in litres) required to produce 100 L of final nutrient solution with varying EC

(|j.S/cm)

Potassium-to-nitrogen 2:1 4:1

ratio 6:1

1000 3000 6000

0.375 1.450 3.350

0.375 1.450 3.150

0.275 0.875 2.400

Table 4 Nutrient concentration

in final nutrient

solution*

Potassium-to-nitrogen

ratio

Nutrients

2:1

4:1

6:1

ppm

Nitrogen, nitrate (NO 3")

193.00

113.00

113.00

Nitrogen, ammonia (NHp

7.00

7.00

7.00

Phosphorus

50.00

50.00

50.00

Potassium

400.00

480.00

720.00

Calcium

127.00

133.00

127.00

Magnesium

50.00

50.00

50.00

Iron

8.00

8.00

8.00

Zinc

0.07

0.07

0.07

Copper

0.07

0.07

0.07

Boron

0.30

0.30

0.30

Manganese

2.00

2.00

2.00

Molybdenum

0.05

0.05

0.05

  • Stocks described in Table 2 are diluted with a fertilizer injector having a 1:100 mixing ratio.
  • Stocks described in Table 2 are diluted with a fertilizer injector having a 1:100 mixing ratio.

Artificial light

Artificial light, as mentioned earlier, is first used immediately after germination. A relatively small installation is needed at this stage, and high light intensity is economically feasible. Both fluorescent (ideally in mixture with some incandescent) and high pressure sodium (HPS) lamps are acceptable and are widely used to generate a minimum light intensity of 100 p.mol/s per square metre (equivalent to 20 W/m2 or 8000 Lux or 760 fc) in growing rooms. The fluorescent lamps produce slightly shorter plants with a deeper green bluish color than HPS lamps, but the latter are the most economical to install and operate. During the first few days after pricking off, when the pots can be arranged close together, it is still economical to maintain a high light intensity (75-100 (imol/s per square metre) for approximately 16-18 hours daily. However, as plants grow they are spaced progressively to avoid crowding and becoming spindly, making the use of high light intensity less and less cost effective. For the rest of the time, while the plants are in the propagation house, provide supplemental light (artificial light in addition to natural light) at a light intensity of about 50 |imol/s per square metre. Obviously, whenever cost is not a factor, the highest light intensity available should be provided for a maximum of 18 h daily, as this treatment results in shorter propagation time and heavier, stronger, sturdier transplants. There is no advantage in using low-intensity incandescent light on tomato plants in midwinter to extend the daylight period.

Temperature control

Recommended temperatures for transplant raising, along with those mentioned earlier for seed germination and cold treatment, are summarized in Table 5.

Table 5 Recommended temperatures for tomato transplant raising

Air temperature

in °C

Growth stage

Light conditions

Day

Night

Seed germination

Not critical

24

24

For 2 weeks after cotyledon expansion (i.e., cold treatment)

Maximum available light intensity for 9-12 h daily

10-13

10-13

After pricking out (i.e., while in pots)

Good light conditions

21*

18

After pricking out (i.e., while in pots)

Poor light conditions

19

17"

  • When growing cold-tolerant cultivars such as Vendor, air temperatures can be 1-2°C lower than those indicated. However, when growing vigorous cultivars such as Ohio CR-6, a similar reduction in air temperature results in poor-quality, cat-faced fruit.
  • When growing cold-tolerant cultivars such as Vendor, air temperatures can be 1-2°C lower than those indicated. However, when growing vigorous cultivars such as Ohio CR-6, a similar reduction in air temperature results in poor-quality, cat-faced fruit.
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