Why Tomatoes May Flower ... But Not Set Fruit

My co-worker asked me today if I had any idea why her heirloom tomato plant was flowering and had been for a few weeks, but she had not yet seen fruit start to form.

I wasn't sure, but encouraged her to feed the plant if she hadn't (it's growing in a container, which makes it even more important to fertilize) as a starting point.

I did some research tonight on tomatoes and their fruit-setting ways and found two things, both highlighted in this article from Bonnie Plants, but also noted by other authors:


1. Heat or stress can cause the tomato plant to flower, but not set fruit. Given that we have recently seen nearly 100-degree temps here in Colorado, I suppose I shouldn't be surprised that I too have noticed one of my three tomato plants has only set one fruit despite the fact that there are several flowers on that particular plant. (For the record, my other two plants - the Christmas Grape and the San Marzano - are doing just fine. Apparently some tomato plants are more heat-resistant than others.)

The article linked above suggests you just make sure the plant is as healthy as possible, getting the right amount of water and fertilizing, so that when the temps or stress drop off, the plant will return to producing.

2. Tomato plants are self-pollinators, meaning they have male and female flowers. That also means that the pollen from one has to get on the other for the plant to make baby tomatoes. Wind and insects help with this, but there's not a lot the gardener can do.

Other causes include too much or too little nitrogen, according to this article, which means feeding the plant may indeed help. Stress from insect damage or diseases, as well as lack of water may also cause it. But from what I've seen around the Web, the most common cause is one we can't really do much about - the heat and lack of pollination.