Why are my tomato plants not fruiting?


From a NSW hobby grower. Why are my tomato plants not fruiting? 

My tomato plants look healthy, but the flowers are not forming any fruit. There are plenty of flowers and the first two or three trusses produce fruit, but there is no fruit forming on the later trusses.

I grow the plants on reused coco slabs using a recycling drip irrigation system, feeding in an EC of 2.8 mS/cm. I use a general purpose two-part ‘Grow’ nutrient available from my local hydroponic store. I grow outdoors in a protected sunny spot and run the plants up a single string. There is no evidence of pests or diseases. Plant stems are thick and the leaves are dense and dark green in colour.

What is my problem?

Thanks for your detailed information, although there are a few areas where I need more. However, you have given me enough that I am confident that I can suggest the probable cause of your problem.

Reused media
Firstly—what about the cocopeat? There are risks attached to reusing any medium, especially an organic one such as cocopeat. The two major risks are: the retention of disease, especially to attack vulnerable young plants; and possible breakdown of its structure leading to inadequate aeration in the medium, which will in turn lead to stressed and probably diseased plants.

Does this apply in our case? No! You highlight that the plants are healthy, so this is a very unlikely influence.

Vegetative crop
Your description of thick stems and dense dark leaves is the classic description of a strongly vegetative plant. It is OK for a young plant to be like this, as it establishes a strong plant for extending and increasing your fruit yield longer term.

However, you want to produce fruit. This requires that the plant later becomes much more ‘generative’. Maintaining a reasonable balance between ‘vegetative’ and ‘generative’ is essential to have your plant produce well.

What is happening is that the continuing strong leaf growth is overpowering the fruiting capacity of the plant. It has some fruit, but struggles to make any more. I would expect that the last truss to fruit would have had smaller and/or fewer fruit than the earlier trusses.

Generative influences
What can you do to make the plant more generative and produce more fruit? To avoid your problem this should have been done from the onset of fruiting.

Firstly, your plant probably has far too many leaves. Commercial tomato growers regularly leaf prune to retain a standard number of leaves. How many are kept depends upon the variety and the plant vegetative/generative balance they are trying to achieve. Typically, it is around 25 leaves for a single stem plant. So that is my recommendation—leaf prune. I guess that many hobby growers are reluctant to leaf prune, probably because they aren’t aware of this aspect, but also because the plant looks quite attractive and they don’t want to ‘despoil’ it.

You mention a single string. Presumably you are removing the laterals (side shoots), but if not the plant may well be spreading too far and getting too leafy. Depending on how close together are your plants (plant density), you may be able to keep one extra lateral on a separate string. A plant stem density of 3 to 4 per square metre is OK in summer. If you have only one or two plants with plenty of light, plant density is irrelevant.

EC (Electrical Conductivity)
The strength (indicated by EC) of the solution that you have around your roots has an influence on plant balance. The higher the EC around the roots, the more generative is its influence.

Depending upon how you recycle your nutrient solution and how often, you could run at a higher EC. It is essential that you measure the EC of the solution running off before you recycle it. This is your best indication of what is around the plant roots. I suggest that you aim to have a run-off EC of above 4 and no higher than 5. Depending upon several factors, such as the weather and especially the proportion that you run off, this may require a feed EC between about 3.0 and 3.5. Take care here because the run-off EC could get much higher if not checked daily, leading to problems such as blossom end rot and even plant death.

An important warning to other growers—only try this if you are conscientious about measuring your run-off solution. Otherwise, keep to a feed EC of around 2 and a run-off no higher than 3. Also, tomato is a plant which is very tolerant of high EC. Do not go this high with most other crops—they are safer with a feed of about 1.5 and a run-off no higher than 2.0.

(The root zone solution is the one that needs to be managed. Many growers give all their attention to the feed, but this is only important as to what influence it has on the root zone solution, which will be different in terms of EC and especially nutrient balance. Remember that its EC gives no indication of the nutrient balance of a solution.)

Nutrient balance
One difficulty that any grower faces with a recirculating system is keeping the nutrient balance of the root zone solution within a reasonable range. The run-off balance will always be different to the feed balance because of the different relative rates of uptake of individual nutrients. Each time you return the run-off the nutrient balance of the root zone solution will continue to change until it can finally go too far and begin to impact on the plant through individual nutrient deficiencies and/or toxicities.

In your case, I suggest that you change to a ‘bloom’ formulation soon after fruiting starts. It is important to realise that ‘grow’ and ‘bloom’ formulations do not force the plant, they are followers of the changes that the plant is demanding. With a crop such as tomato, it goes from vegetative through flowering to fruiting. The result is a steady change of different uptake nutrient balances, not just two distinct formulations. To keep the root zone solution from getting too far out of balance you need to occasionally discard the daily run-off. Dilute it and use it to fertilise your garden.

PH&G May 2013 – Issue #131