Discover secrets of bean flowering

In a few months, you’ll be out walking fields and watching for soybeans to begin flowering.

Ever wonder why some soybean varieties flower sooner than others; why flowers are purple, blue or pink; or what it means when there are different-colored flowers on plants grown from the same seed lot? Han Kandel, North Dakota State University Extension agronomist, broadleaf crops, provides the following guide:

Indeterminate beans

Soybean varieties are classified as indeterminate, semi-determinate or determinate in growth habit. Indeterminate varieties develop leaves and flowers simultaneously throughout the first part of their reproductive period. Varieties grown the Dakotas and Minnesota are mainly indeterminate.

Soybean development is driven by photoperiod, temperature (heat units) and the interaction between the two.

“Soybeans must reach at least the first trifoliolate in growth before they can be induced to flower. Soybean in the northern United States has shorter minimum night-length requirements for the onset of flowering. However, even within a variety, variations in time of flowering may occur from year to year with the same day length. This is closely associated with temperature conditions,” Kandel says.

Each variety depends on a critical period of darkness to change from the vegetative to the reproductive state.

An early-maturity group variety is sensitive to a shorter night and requires fewer hours of darkness to initiate flowering than does a later-maturity group variety.

A late-maturity group variety requires longer hours of darkness to initiate flowering, which allows a longer period of vegetative growth prior to bloom, and it also matures later in the season.

For example, when a variety adapted to a southern latitude (shorter days and longer nights) is grown further north, the longer day lengths and cooler night temperatures cause it to remain growing vegetatively longer, resulting in a tall plant, delayed flowering and delayed maturity.

A variety adapted to a northern latitude, when grown further south under shorter day lengths and warmer nights, will cease vegetative growth earlier than normal, resulting in smaller plants, earlier maturity and a reduced yield potential. The actual calendar date that a variety matures is highly influenced by the latitude where the crop is grown.

Flower colors

Soybean flowers are usually white or purple; pink flowers are also possible. A pure variety will have flowers of the same color.

Flowers are mostly self-pollinated. About 50% to 80% of the total flowers on the plant actually produce pods.

Soybean flowering will start on the third to sixth node on the main stem, depending on vegetative stage when flowering begins. Flowering will progress up and down the plant. Branches will eventually also flower. Within each cluster of flowers along the main stem, the flowering will occur from the base to the tip. Therefore, basal pods are always more mature.

Source: North Dakota State University Extension Service Crop Pest Report

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This article published in the February, 2012 edition of DAKOTA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.