Why Tomatoes Don't Set Fruit

by Tim Baker, University Extension

Occasionally, I will receive a call from a home gardener who has beautiful tomato plants, but little if any fruit.  They have spent a lot of time and money and have gorgeous, lush plants.  But no fruit.  What went wrong?

There are several possibilities that can lead to this problem.  One of the more common problems is excess nitrogen.  When you see a large, beautiful tomato plant, with little fruit, this is the likely cause.  The tomato is putting all its energy into vine, and little into fruit. I remember visiting a gardener once that had put lots of compost into his garden site, and then fertilizer on top of that.  He had no tomatoes.  In this case, it was too much of a good thing.  Compost is great, but don’t overdo it.

Temperature affects tomato fruit set as well.  If it’s too hot or too cold, tomatoes will not set fruit.  If night time temperatures are below 55 degrees, tomatoes may not set fruit.  They are, after all, a warm season plant.

But too much heat can be just as bad, even though tomatoes like warm weather.  When daytime highs reach 95 degrees or above, many tomatoes will not set fruit.  Flowers will still form, but no fruit will set.  Just as critical is the night time temperature.   If the temperatures remain above 70 degrees at night, this can create problems for many tomato varieties.

So generally, tomatoes like to see night time temperatures between 59 and 68 degrees, and daytime temperatures less than 95 degrees.  “But wait a minute,” you say, “I live in Missouri and want to grow tomatoes.”  If you are having problems with tomato fruit set due to high temperatures, there is hope.  Plant breeders have bred tomato plants which are more tolerant of high heat, and will set fruit better under those conditions.  Just pick one of those varieties.

A couple of other weather-related factors can inhibit tomato fruit set.  One is low humidity.  In some instances, low humidity, especially during periods of high temperatures, will cause poor fruit set.  I would hazard a guess that low humidity won’t be a problem for most of Missouri.  But low moisture can certainly be a problem and contribute to low fruit set.  During periods of drought, stressed tomato plants will not set fruit well.  So be sure to water your tomatoes regularly, but not excessively.

It has also been found that tomatoes under continuous light do not set fruit well.  The key here is to avoid garden sites that are under lights that remain on all night, such as security lights or street lights.

Finally, are your tomatoes getting enough light?  Tomatoes like full sunlight, and while they can tolerate some shade, if there is too much shade, they many not set fruit well.

Ok, so you’ve done everything right, and your tomatoes are still not setting fruit.  What can you do?  There are sprays based on plant hormones that will encourage fruit set.  Just go to your local garden center and ask for them.