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

Yellow tomato flower
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

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.


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Garden Checklist for week of July 7

Take care of garden chores early in the morning, concentrating on watering. We’re still in survival mode until this heat wave breaks.

* Keep your vegetable garden watered, mulched and weeded. Water before 8 a.m. to conserve moisture.

* Prevent sunburn; provide temporary shade for tomatoes, peppers, eggplant, melons, squash and other crops with “sensitive” skin.

* Hold off on feeding plants until temperatures cool back down to “normal” range. That means daytime highs in the low to mid 90s.

* Don’t let tomatoes wilt or dry out completely. Give tomatoes a deep watering two to three times a week. Harvest vegetables promptly to encourage plants to produce more.

* Squash especially tends to grow rapidly in hot weather. Keep an eye on zucchini.

* Some weeds thrive in hot weather. Whack them before they go to seed.

* Pinch back chrysanthemums for bushy plants and more flowers in September.

* Harvest tomatoes, squash, peppers and eggplant. Prompt picking will help keep plants producing.

* Remove spent flowers from roses, daylilies and other bloomers as they finish flowering.

* Pinch off blooms from basil so the plant will grow more leaves.

* Cut back lavender after flowering to promote a second bloom.

* One good thing about hot days: Most lawns stop growing when temperatures top 95 degrees. Keep mower blades set on high.

* Once the weather cools down a little, it’s not too late to add a splash of color. Plant petunias, snapdragons, zinnias and marigolds.

* After the heat wave, plant corn, pumpkins, radishes, winter squash and sunflowers. Make sure the seeds stay hydrated.

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