Controlling Transplant Height in Tomatoes

Janice LeBoeuf, OMAFRA Vegetable Crop Specialist, Ridgetown

With wet fields and not the most promising forecast, transplant growers will have the challenge of keeping transplants in good condition for their eventual trip through the transplanter.  They have to keep the plants from stretching, and avoid stressing the plants so much that they will perform poorly in the field.

There are several techniques available to control transplant growth.  Some come with more risks of side-effects than others.

Fertilizer

Reducing phosphorus fertilizer rates to zero will restrict tomato growth, unless the growing media contains a supply of phosphorus that the plant can draw on.  Ideally, if this method is used, a high-phosphorus fertilizer (eg. 10-52-10) should be applied through the irrigation system 5 days before the plants go to the field.  In reality, under wet weather conditions it may be impossible to know 5 days in advance when the plants are going to the field, but the shot of phosphorus is important to ensure the plants will be ready to take off once they are transplanted.  The field grower should also consider using starter fertilizer for plants that have been held back by phosphorus restriction.

Nitrogen should be restricted to ¼ to ½ of the normal rate when trying to reduce tomato transplant growth.

Overdoing the fertilizer restrictions can result in weak plants that do not recover their original yield potential.

Temperature

Keep the average temperature in the greenhouse as low as possible without damaging the plants.  Tomato plants can get chilling injury from extended periods below 10°C.

DIF refers to the difference between the daytime and nighttime temperature.  A negative DIF (nighttime temperature higher than daytime) will result in shorter internodes (less stem elongation) in tomatoes and pepper seedlings.  In tomato, a low-temperature pulse during the first two hours of daylight can be effective.  The final hours of darkness and the first hours of light are when the plant is most responsive to the temperature differential.  This technique requires some nighttime heating (with the key time period being after 1 am) in order that the temperature in early daylight can be cooler than the overnight temperature (by about 5-8°C).  Do not cool the greenhouse below 10°C.

Water

Water restriction also slows plant growth, but must be used very carefully.  The interval between waterings can be increased, but as soon as you see a slight wilting response (slight curling) in the upper leaves, water immediately.  You may also see the leaves changing from a brilliant green to a dull green.  As wilting progresses beyond this point, the plant is at risk, so this technique must be used with extreme caution and close observation.

Tray Spacing

Small-scale transplant growers may also have the option of spacing out the trays to increase airflow.  Increasing the movement of air around the plants can help to reduce stem elongation.  Unfortunately, this is impractical in a large-scale operation.