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Cold Weather and Crops

April 20, 2001
The drop in temperatures to below freezing--25° or 26°F in Central Illinois and probably below freezing in much of the state--on April 17 and 18 raises questions about freeze injury on crops that are in the field.

With the warm temperatures early in April, the wheat crop has grown rapidly, almost catching up to its normal stage of growth, following cool temperatures an¡d slow growth in March. The growth stage ranges from second node visible (or palpable)about Feekes stage 7in the southern part of Illinois, to first node palpablestage 6in Central Illinois, to strong upright growth (stage 5, plants 6 to 8 inches tall) in northern Illinois. The greatest concern over freeze injury is in those fields with the most advanced growth. Any loss of leaf tissue at stage 7, when more than half of the eventual leaf area is exposed, could affect the yield potential by limiting leaf area for photosynthesis. Of more concern would be direct damage to the small head that is developing inside the stem, above the upper node. By stage 7, the head may be 6 inches or so above the soil surface; you can check its location by splitting the stem.

Available information would suggest that temperatures in the mid-20s will probably cause injury of leaf tissue in fields that are in growth stage 7, especially when plants have been growing rapidly. The varieties in the variety trial at Urbana on April 18 showed a lot of difference in the amount of frost on the leaves; those few varieties with a more "floppy" growth habit (wide leaves oriented horizontally) had accumulated much more frost than varieties with upright leaves. Frost on the leaves does not guarantee injury, especially when the weather has been cool and dry and the leaves have been somewhat hardened against such injury. But I expect some leaf burn where leaves were directly exposed to the cold night sky, and therefore experienced a lot of radiational cooling.

Direct injury to the head is much less likely than leaf injury in wheat at these stages. The stem in which the head is positioned is upright in orientation and so does not radiate heat to the sky. Soils are still relatively warm and will radiate to the plants, helping to increase leaf and stem temperature. Leaves above the head will also provide protection from low air temperatures. Most direct freeze injury of wheat heads takes place when the head is in the boot, near or at the top of the plant and therefore much more exposed to the sky and to the cold air. Fortunately, full boot stage is still a week or more away in southern Illinois; so the fact that the freeze took place now rather than a week later is an advantage.

Corn and soybean crops that were planted in late March or early April have emerged in some places, and these crops are much less adapted to resist freeze injury than is wheat. The larger the corn or soybean plants, the greater the chance of freeze or frost injury. Fortunately, cool temperatures for the past week slowed growth of emerged crops, and this will probably limit damage. Corn leaves above the surface will likely be killed by temperatures below 30°F, but this is a small amount of leaf tissue and its loss should not be a major problem. Green leaf tissue should start to appear within a few days of the freeze; if it doesn't, then dig and check plants to see if injury might have occurred deeper in the soil. Where soils are dry, it is possible for cold air to filter down and affect plants below the soil surface. Death of the growing pointusually about 3/4 inch deepis not likely from these temperature drops, but it could happen, especially in low-lying parts of fields, into which cold air drains. Such injury will be easy to see; if new leaf tissue doesn't appear, dig plants and see if the plant looks frozen and mushy down an inch or so.

Emerged soybean plants have their growing point above the soil surface and so can suffer direct freeze injury more easily than small corn plants. The cotyledons protect the growing point to some extent if the cotyledonary leaves have not emerged. The nearness of the growing point to the warmer, radiating soil surface also helps. And the plant has auxiliary buds where the cotyledons are attached that are better protected against cold and that can regenerate branches if the part of the plant above the cotyledons freezes. There is no guarantee that soybean plants won't be killed by these low temperatures, however, even though anecdotal evidence suggests that this may not be very likely. The only thing to do is to watch the crop carefully to see if plants start to regrow once temperatures warm up again.

With the lack of recent rainfall in parts of Illinois, dry soils are starting to be of concern and might affect decisions regarding replanting crops or even planting them for the first time. Turning up moist soil with tillage is one sure way to lose soil moisture, and it pays to consider whether or not that last (or even that first) tillage trip is necessary. It is risky to plant deeper to try to place the seed in moisture this early in the season when soils are still cool. If soils are dry enough that the crop will need rain in order to emerge, or if there is still adequate moisture at the normal planting depth, then planting at 1-1/2 or 1-3/4 inches deep is still appropriate.--Emerson Nafziger

Author: Emerson Nafziger

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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