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9/6/2003

Squash, Beans and Deadheading Flowers II

(Read Part I)

If I did not pick squash and beans frequently, not only would I get larger, less tender, and less saleable produce, I would probably get less produce altogether.  As squash grows to baseball bat size, or as bean seeds fill their pods and begin to dry, the plant produces fewer flowers, or stops flowering altogether.  Fewer new squash are produced while the plant puts its resources into maturing a large squash.

Botanists hypothesize that the fruit or seeds produce a hormone that prevents further flowering in plants with indeterminate bloom.  As more flowers are pollinated and fruits develop, the hormone accumulates.  To my knowledge, the hormone has never been identified, but hypothesizing that it exists explains the way these plants react when they flower and set fruit.

To keep my bachelor's buttons and cosmos blooming, I deadhead the flowers about once a week.    Once a little girl asked if the flowers in my mini vases were going to die because they had been picked.  Cut flowers upset her.  I tried to explain that cut flowers are not really alive like a person or an animal.  You don't kill the plant when you cut a flower.  Cutting flowers is more like cutting hair.  When you cut flowers, like hair, they grow back so you can cut again.

Some crops, however, require mature fruit.  If one grows dry beans, for example, one leaves the pods to mature.  As pods mature, fewer new pods develop.  The dry bean crop can be harvested once, when all the seeds are ready.  Similarly, melons are harvested when the seeds are mature.  Melons take much longer to mature to harvestable size than do baby squash.  Melon plants put most of their resources into maturing just a few fruits.

Alfalfa is grown for seed in Idaho's Treasure Valley, and like dry beans, alfalfa seed requires maturing a small seedpod, called a curl.  Seed growers use large populations of leafcutting bees to pollinate the alfalfa flowers.  The wooden shelters that look like a bus stop in the middle of alfalfa fields around the Valley provide nests for as many as 100,000 female bees pollinating 10 acres of blooming alfalfa.  Southern Idaho is one of the few places in the country where these shelters are common in the landscape.

At the peak of alfalfa bloom in mid July, bee activity in the shelters bustles like an over-crowded third world city, or a high-rise tenement project.  The over crowded conditions attract predators, parasites and disease, which take their toll on the leafcutting bee population.

Researchers and some seed growers question whether huge numbers of leafcutting bees are necessary to pollinate the alfalfa crop.  Growers talk about the plants shutting down bloom when pollination takes place too fast, as if the plant had been knocked down in a fight.

Rapid pollination is the opposite of deadheading.  Rather than remove flowers or immature fruits to keep the plant blooming, the alfalfa seed grower hastens fruit and seed maturation by relying on a large bee population to pollinate flowers as rapidly as they come into bloom.

If there are no pollinators in the alfalfa field, the flowers stay in bloom for a long period of time, and new flowers keep coming into bloom.   Conversely, the more pollinators there are foraging on the crop, the fewer flowers will be in bloom.   The more pollinators, the faster the rate of pod development, and the sooner the plant shuts down bloom.  Ironically, a large population of pollinators causes a crunch in flower resources for themselves later in the season.

Deadheading, frequent picking, rapid seed pod development - all these phenomena demonstrate that pollination is an interaction that transcends the separate entities - pollinators and plants - that participate in it. Where flowers and fruits are important for our crops and gardens, we humans participate in that interaction in unexpected ways.

(Read Part I)

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September 5, 2003
Copyright 2003, Karen Strickler. All rights reserved.