Late-maturing soybean face damage from early killing frost and need special attention for harvest and storage.

University of Missouri Extension soybean specialist Bill Wiebold says to harvest frost-damaged soybean based on seed moisture content rather than how plants look after frost.

Delaying harvest after damaging frost results in grain shatter and subsequent yield loss. Soybean breeders select for shattering tolerance, and normally the seam of the soybean pods holds together. However, the halves rupture if left in the field too long after maturity or damage.

When harvesting, adjust combine settings to allow for plants with wetter than normal stems and leaves, Wiebold said. Seeds also will shrink to smaller than normal size, and seed will be more oblong in shape than normal, but frost-damaged soybean should store almost as easily as normal soybean, Wiebold said.

Below-freezing temperatures will not affect plants that showed signs of maturing, such as yellow leaves and leaf drop, before freezing.

But damage can occur even when temperatures are too warm for ice to form, Wiebold said, because cool temperatures can damage the plant’s enzymes.

“Killing freeze” usually means a temperature as low as 28 degrees for four hours, but the freezing point of water within leaves is less than 32 degrees, making leaves more susceptible to freeze damage.

Ice crystals form inside plant cells when temperatures drop quickly below freezing. Freezing water expands and tears plant cells and membranes, causing cell contents to leak outside the cells.

Damage may not be uniform through the field. Cold air is heavy and drains down slopes to pool in lower parts of he field. Even small dips can catch and hold cold air and put plants at risk of freezing.

Soils lose heat more slowly than air, so damage varies in different areas of the canopy. Leaves, stems and pods near the top of the plant are most vulnerable while those closer to the warmer ground may get less damage.

Fall colors

Fall foliage color is not just for show, MU Extension horticulturist David Trinklein said, because leaf shedding is part of the dormancy process that helps trees survive winter.

During the spring and summer, leaves make food for the trees. Chlorophyll absorbs sunlight to transform carbon dioxide and water into carbohydrates such as sugars and starches through photosynthesis.

The shorter days of late summer trigger the dormancy process. A layer of cells called an “abscission layer” forms at the point where the leaf stem attaches to a branch or twig. Sugars, still manufactured by the leaf but blocked from leaving, turn into colorful pigments known as anthocyanins, which usually are red or purple.

In addition, chlorophyll starts to break down. That is when yellow and gold pigments called carotenoids get to shine. These pigments are present throughout the growing season but are masked by chlorophyll’s dark green color.

Leaf color intensity depends on temperature, light and the availability of water throughout the year. Color-watchers favor a steady supply of mild, sunny days and cool but not freezing nights for the best chance of fall brilliance. Leaves also need some moisture for colors to intensify.

Contrary to common belief, frost is not necessary for trees to begin their color show, Trinklein said. Early frosts may even tarnish leaf color.

The Missouri Department of conservation’s fall colors report is available online at nature.mdc.mo.gov/discover-nature/fall-colors.

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