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Monday, November 19, 2007

Steering Tomatoes

This article should, preferably, be read after reading the article on General irrigation principles for greenhouse crops grown in media

In the wild, tomatoes tend to grow vegetatively (growing stems and leaves) when they are young plants in spring but when they have grown to a reasonable size going into summer, change to generative growing (most effort put into flowers, buds and fruit).

Modern tomato growers, however, want to grow the plants for 10 or more months, right into winter when prices are high. To do this they need to steer the plants away from its natural tendency to grow too generatively in summer and also to avoid it becoming too vegetative in cool, low light conditions as winter is approached. Growers call this balancing the plant and spend considerable time and energy achieving this as it has a major effect on quality, disease control and yield. In addition, optimizing the watering regimen minimizes wastage of water and fertilizer and reduces harm to the environment.

Warm, high light, low humidity, summer conditions steer the plant toward generative growth and cooler, lower light conditions toward more vegetative growth. This means that it is difficult to keep the plants growing with some vegetative tendency in summer and to stop them becoming too vegetative in cooler, dull conditions. At these times, manipulating the root zone conditions in conjunction with environmental conditions can be particularly advantageous.

Conditions that encourage vegetative growth include:-

Environmental
Less intense light
Cooler temperatures
Large day-night temperature difference

Root-zone
Wetter root conditions
Many small irrigations
Starting watering earlier and stopping later in the day
Lower EC in root zone
Less dry down over night (4% to 6%)
High nitrogen fertilizer mix

Conditions that favour generative growth include:-

Environmental
More intense light
Warmer temperatures
Small day-night temperature difference (but if too small, flower set may fail)

Root-zone
Dryer root conditions
Fewer large irrigations
Higher EC in root zone
More dry down over night (8% to 12%)
Starting watering later and stopping earlier in the day

Things are not as simple as that though, as excesses of some of these influences can create other problems and sensible limits must be applied. In fact, many of the changes that can steer toward vegetative can also cause fruit split or cause disease problems if overdone especially if implemented after conditions have allowed to become too generative for any period of time.

Much steering can be done by adjusting the irrigation EC (increase in low light conditions and lower in bright conditions), by adjusting the watering start and stop times and by changing the irrigation duration and frequency.

For night dry down, it is normally considered that a dry down of 4 to 6% will be generative and 8 to 12% will be generative. Lower than 4% and you might be looking at poor root development and subsequent disease problems. Too dry and it will be difficult to wet up again the next day. In general, changes to the media moisture content should be made slowly at say 1% change per day.

The watering start time must be close to sunrise. Too much before sunrise and there is a risk of splitting the fruit. Too far after sun rise, may make it difficult to wet up for the day as the crop will already be taking water from the media before watering commences.

The watering stop time is frequently manipulated to steer the plant. Stopping earlier will cause greater dry down and steer toward generative. The average time to stop is when it is estimated that there will be a further 200 to 300 Joules/cm2 of light energy received before dusk. Obviously, this has to be an estimation based on expected weather and previous days experience, watching carefully to see what dry down was actually achieved the night(s) before. The exact setting will also depend on the growers observations ie is the crop.

To determine if the crop is tending toward veg or gen, observations and measurements are made on the crop as described below. Bear in mind that exact dimensions and observations will depend on the specific cultivar.

Crop observations:-

Measure length between highest flower truss and growing tip (long = veg; short = gen)
Measure diameter of growing head (large and open = veg; small and tight = gen)
Colour of leaves and flowers (light = veg; dark = gen)
Diameter of stem (thin = veg; thick = gen)
Truss curled indicates generative growth

For some common tomato varieties, the flower truss to tip is typically about 14cm and the stem diameter is 11mm for a “balanced” plant.
These observations are then used to alter the growing conditions to steer the crop back to a more balanced position.

Example
Lets assume that we have had a period of very hot, dry weather and observations indicate that the crop is becoming overly generative. Here are some of the things that can be done.

To steer back to a more vegetative position:-
Try to provide extra cooling and humidification in the greenhouse. This can be achieved by fogging during the day and/or providing increased shading.
On the irrigation side look carefully at the run-off EC. It is likely that this is on the high side. Normally the run-off EC will start the day high and drop back to the desired EC soon after mid-day. If it is staying high and has a big differential as compared with the irrigation EC then you need to increase watering during the day. This can be done by either lowering the solar integration trigger (so it waters more often) or by increasing the irrigation duration so that more water is applied when it irrigates. From what we have said earlier, it is the solar integration trigger that will have the biggest effect on steering toward veg. Also, check that run-off is adequate eg 30% during the afternoon and that it starts early enough in the day (by about 10:30am for example)
If, on the other hand the EC is high but not excessively different from the irrigation water, then lowering the irrigation EC a bit will help. A typical summer setting may be irrigation EC of 2.0 expecting a run-off EC of 3.0 to 3.5.

The other correction to make is to check the irrigation start time is not too late and also adjust the stop time to ensure there is not excessive dry down overnight. Normally, stop time is set to about 2hours (or 250J/cm2) before sunset hoping to achieve a dry down of between 2% to 4% by sunset.
In very exceptionally dry cases it may be necessary to have a scheduled small irrigation during the night but normally just adjusting the stop time will solve this problem.

Whilst this article may provide the grower with a better understanding of some of the general principles of crop steering by root-zone control, it is important to get advice from a horticultural consultant that is familiar with the particular climate and crop variety.

Jeff Broad
Autogrow Systems Ltd

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