My tomato plants are flowering but they’re not producing fruit. What’s the problem?

There can be several reasons for tomato plants flowering but then failing to produce fruit. One of the main ones with indoor or greenhouse growing is a lack of pollination. Tomatoes are self-fertile, however, they need some assistance for the pollen to be released inside the flower. On a small scale this can be done by gently flicking or tapping the back of the stem behind each flower truss, this causes the pollen to be released for pollination and subsequent fruit set. This process needs to be carried out every day to make sure all flowers are pollinated as they open. Often bees and insects may visit a greenhouse if they have easy access and carry out some pollination, but tomatoes, like many other commercial greenhouse crops, have flowers that are not very favorable to bees.

Another common reason for a lack of fruit or flower/fruit drop is low light levels and/or high temperatures. High air temperatures, particularly high night temperatures and if combined with high humidity (which restricts the plant’s ability to cool itself via transpiration), result in pollen becoming non-viable, so even if pollen transfer occurs, fruit may not set and develop. Similarly, cool temperatures (below 61°F) can also result in a lack of pollen viability and fruit set. Low light can be an issue with indoor cropping or during winter in greenhouses and often results in weak flowering and a lack of fruit set. With indoor growing, a combination of low light and warm growing conditions is particularly damaging for fruiting as the plant directs assimilate into new leaf and shoot growth rather than flowers and fruit. Tomatoes are a high-light crop requiring a daily integral of at least 22-24 mol m-2 d-1. A day length of 18 hours and a suitably high level of light can be used to obtain this, however, plant density also plays a role with how much light each plant intercepts. Tomatoes should be limited to two to three plants per square meter to make sure each receives sufficient light.

Overcrowding of large, tall plants (like tomatoes) is common where space is limited and is another major cause of a lack of fruit or undersized fruit development. Under low light such as in greenhouses in winter, fruit set can often be improved with use of CO₂ enrichment up to 1,000ppm to help the plant produce enough assimilate for strong flowering and fruit development. Other less likely causes of a lack of fruit development after flowering are water stress, low EC, and nutrient imbalances — with hydroponics these are not often the cause of a lack of fruiting where such factors are easily controlled by the grower. EC levels for flowering tomatoes should be run at 28–35 and pH at 5.8. Oversaturation of the root zone (which leads to oxygen starvation of the roots) can sometimes lead to flower drop in tomatoes, so keeping a check on the irrigation program and adjusting nutrient levels to plant size and growing conditions helps prevent this issue.

Kind Regards,
Lynette Morgan
Suntec International Hydroponic Consultants