| | Most of the vegetable transplants are on coco coir bags or rockwool and the main task is to establish a strong root system. Maintaining a constant temperature of 19 to 20°C in the root zone is important for good root development. Root development can be slowed considerably when root zone temperature is 15 to 16°C or even if the temperature fluctuates between 16°C and 19°C. The only way to maintain optimum root zone temperature is to supply heat to the root zone; this is where the pipe and rail systems offer an advantage. Growers with pipe and rail systems tend to have faster establishment of the seedlings on the bag, with rapid root development and a uniform crop. Without a source of heat at the floor, a strategy is to ensure that the air temperature is 21 to 22°C at the floor and the irrigation water is warm (20 to 22°C). It is difficult to maintain heat to the root zone with air temperature, as the ground is a strong heat sink, drawing heat out of the bags faster than the bags draw heat from the air. Maintaining air temperature at 22°C at the floor will often mean that temperatures near the roof could be near 27°C. A lot of gas is required and this can be costly. However, if there is poor root development, the crop will not yield to its full potential -- which is also costly.
It is also critical to avoid high relative humidity. The relative humidity should not be allowed to go over 80%, not only will the plants slow down, the potential for disease development increases dramatically. A good target for relative humidity is 70 to 75%. In terms of vapour pressure deficits (VPD), the optimum target is 3 to 7 grams/m3 or 8 to 10 millibars. Some greenhouses experience low relative humidity, 65% or lower, undue transpiration stress is placed on the plants at this level. Misting systems can be used to optimize the relative humidity/VPD in those greenhouses that are equipped with them.
With the root zone temperature optimized, it is also important to optimize the air temperature, running a fairly high 24 hour average of 20 to 22°C until the plant is well established on the bag. Depending on conditions, the environment must be kept steady for about 2 to 3 weeks for the transplants to establish. This period is sometimes extended if the crop is exposed to extended cloudy periods. In the absence of desired light levels, higher temperatures should be used to drive the plants. When the plants are well established and look ready to set and hold fruit, drop the night time temperature, but the optimum 24 hour average temperature target is maintained for each crop (about 19 to 19.5°C for tomatoes, 20°C for cucumbers and 18 to 19°C for peppers).
During transplant establishment, the feed E.C. should start at about 3 mmhos and raised slowly as the plants root in. How high the feed E.C. is allowed to go depends on the crop; tomatoes and peppers can go as high as 4 mmhos, cucumbers up to 3.4 to 3.7 mmhos for cultivars like Flamingo, not as high for others. Feed E.C. is managed according to the root zone E.C level and how the plant responds. The rule of thumb is not to let the root zone E.C. go higher than 1 mmho above the feed E.C. These E.C. targets are also modified depending on the relative humidity in the greenhouse. Lower relative humidity places a higher transpiration stress on the plant. Higher root zone E.C. makes it more difficult for the plant to access water from the bag. A combination of low relative humidity and high root zone E.C. can place too much stress on the plants. Growers must watch the plants closely to avoid placing excessive stress on them.
A discussion of feed E.C. is not complete without discussing watering. Varying the amount of water delivered to the plants can be used to manage the E.C. level in the root zone without changing the feed E.C. Generally, with less water, the root zone E.C. increases as the plant draws the water it requires from the bag, leaving a higher proportion of the fertilizer salts behind. However, the strategy of watering does not usually have E.C. management as its primary focus. Water is used to manage the growth and development of the plant directly.
During transplant establishment, water management is used to develop a strong root system. Making water too available, too plentiful, can encourage the plants to put down a minimum of roots and develop more top growth. With typical southern Alberta summer light levels, development of top growth without a strong root system spells disaster; the roots will not be able to keep up with the plant's demand for water. The result is wilting and loss of yield as the plants try to put more roots out to meet the demand of the increased transpiration stress. Even if the plants appear to recover -- the damage is done -- yield expectations drop considerably.
The goal of watering during crop establishment is to provide the plants with adequate water, but to supply it in such a way that the plant looks for the water by putting out roots. Different growers have their own strategies for accomplishing this but the same general principles apply. For example, using a target of 5% overdrain, with the expectation that the overdrain occurs primarily at mid afternoon, the plants are watered 1 to 3 times a day. The plants are not allowed to wilt between waterings and are watered once they achieve the dark blue-green color that indicates they are under transpiration stress and will soon need water. At this point, expect the plants to be looking for water by sending out roots.
It is also important that the plants display the light green "halo" in the morning . This indicates that they have recovered overnight and are moving into the day fully charged and not yet under transpiration stress. If the plants are a blue-green color in the morning, the amount of water delivered over the course of the day is increased (to meet the overdrain target). Add more water with each watering or add an extra watering during the day to achieve the overdrain target.
Successful growers are often successful waterers -- at no time during the crop year is watering as important as it is at crop establishment.
J. Calpas, CDCS, Brooks |
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