Tomatoes flower but don't set fruit? Here's help

If tomato plants aren't setting, you can assist pollination of the flowers. (Photo: Kathy Morrison)

Ways to nudge your vines to a better harvest


By Debbie Arrington

Are you seeing lots of flowers on your tomato plants but hardly any fruit? You may need to give nature a hand.

If your tomatoes refuse to set, the most likely issue is the weather. Tomato pollen dries out when exposed to temperatures above 95 degrees – and we’ve seen plenty of hot days lately. Lack of humidity also plays havoc with pollen.

Tomatoes are self-fertile; each flower can pollinate itself. But it needs a little help moving the pollen from the male anthers to the female stigma. Pollination can be tricky with tomatoes. Topped by the stigma, the female pistil is encased in a circle of male stamens that form a little tube with the anthers on the top. The pollen has to shake down inside that tube.

Tomatoes usually are wind pollinated; they like a little breeze to help distribute their pollen. When there’s no or little breeze or if the plant is cut off from any wind (such as in a greenhouse or in a grow tent), pollination can be problematic.

The solution: Give your plants a nudge – or a gentle shake. Lightly bump or shake the tomato cage, stakes or trellis to get the pollen moving.

Bees can help tomato pollination, too. Not honeybees, but bumblebees; the vibration of their wings shakes loose lots of pollen.

But bumblebees don’t like the heat. Or there may not be any bumblebees in your garden; they’re easily killed off by pesticides. That could be another reason for lack of pollination.

One trick to mimic bumblebees: Use an electric toothbrush. Place the back of the brush to the back of the flower clusters. A few seconds of gentle vibration will shake the pollen up. This method works best in late morning or early afternoon, when the flowers are most receptive.

Another cause of failure to set fruit: Not enough potassium. That macro-nutrient (the third listed on fertilizer packages) triggers fruit formation and development. Organic sources of potassium include compost (especially when laced with banana peels), kelp meal or liquid kelp, potash, wood ash or greensand. Give vines a side dressing and deep watering.

Avoid adding more high-nitrogen fertilizer; that produces lots of vine but few flowers – or tomatoes.