Bean plants not making beans

Question:

I'm growing red runner beans and they are not setting fruit. The flowers bloom, then fall off - no beans. We enriched the soil before planting by adding compost, humate, and gypsum, and I'm still having same problem as last year. There are bees in the yard, so I don't think it's pollination. Is there anything I can do before it's too late?

I also have very tall tomato plants with very little fruit. I've given them special tomato food and I still have only a few undersized tomatoes. What do they want?

There are several potential answers to your problem. Perhaps one applies to your garden and another answer will solve someone else's problem. We'll discuss several common problems resulting in failure of beans to set fruit even when flowers are present.

Pollination failure is indeed one potential problem. Some members of the bean family are self-pollinated, but some beans do require a pollinator (honey bees or bumble bees). When there are plenty of other flowers nearby, the pollinators may avoid the beans because their nectar is lower in sugar content than flowers of other plants. As the other flowers cease flowering, the pollinators may be more attracted to the beans.

Night temperatures can also prevent fruit set. Even when pollen is transferred from one flower to another, the generative nucleus in the pollen must fertilize the ovule inside the ovary. When night temperatures are high (as they were earlier this summer when you sent your e-mail), this process fails and the seeds do not form and the pods drop.

Uneven moisture can also cause the pods to drop. If the soil dries too much between irrigations or between rains, there will be no bean pods formed. Mulch can help, but hot, windy days can dry the plants even when the soil is moist. Wind protection by taller crops upwind may be a good addition to mulch. Traditional Native American gardens solved this problem by growing beans (and other crops under corn plants). The corn shaded the beans, protected from wind, and maintained higher humidity around the bean plants. Try combining mulch and wind protection in your garden next year.

Finally, the clue you provided when you discussed your tomatoes suggests that you were too kind this spring when you prepared your soil. If the soil is too fertile, tomatoes and beans will grow vigorously but fail to set fruit. This is what you have described. I suspect that some of the things you added to your soil were unnecessary and that you may have over-applied some nitrogen containing materials (compost, manure, etc.) Collect a soil sample to send to the NMSU Soil and Water Testing Laboratory or another soil testing laboratory of your choice. You can do this now so you will have the information you need next spring as you prepare for next year's garden. Your local NMSU Cooperative Extension Service office can give you information telling how to collect the sample and can help you interpret the results.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

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