Freeze Warning and Now what!
The past couple of days we have seen several parts of the county that have experienced cold temperatures
ranging from 27 – 32 degrees. Concerns for crop progress on late seeding of the following crops may
affect the maturity and quality
Planting delays this spring due to the slow warm-up and excessive moisture have created concerns as to
whether or not the later planted and longer season crops will make it to maturity before this area
experiences a hard killing frost. It is a question that has been brought to the forefront by the freeze
warning posted for tonight.
A killing frost is a temperature which kills the plant tissue. If your crops have frost (low temperature)
damage it is the result of tissue death. Critical stages and temperatures for various major crops are
described as follows:
Wheat, durum and other small grains: The further along in the filling stages the small grain plant is
when a frost occurs, the less potential for damage. Head emergence through flowering is when the wheat
plant is most sensitive to frost. At these stages, temperatures below 30 degrees for any length of time can
cause spiklet sterility, shriveled kernels and severe yield reductions. Frozen immature spikes will turn
white.
Frost at the milk to soft-dough stage generally results in small, shriveled kernels. Kernels will have a
whitish or grayish color or the kernel will have a blistered or wrinkled appearance, depending on the
development stage. Test weight and germination is generally lowered and yield loss can be moderate to
severe. The exact temperature at which damage will occur is hard to pin-point as it depends on how cold
it gets along with how long it stays there, but the injury threshold is generally considered to be about two
hours at 28 degrees. Once at the hard-dough stage, small grains can withstand temperatures down to 25
degrees with only slight damage to the bran and some slight kernel shriveling. However, germination of
the crop can still be impacted by frost at the hard-dough stage and grain that may be saved for seed should
be tested for germination and vigor.
Flax: is most susceptible during flowering and early boll stage. Immature seeds can be killed by
temperatures from 28-32 degrees. After flax reaches dough stage it is more resistant to frost. Research
conducted in 1988 to1991 in Canada showed that temperatures of below 25 degrees were reached before
damage to leaves or the fine stems holding the boll was recorded. Tests also showed temperatures had to
get down into the 20 degree range before the main stems were damaged.
Sunflower: is most susceptible at bud and flowering. Temperatures of 28 degrees to 30 degrees can
result in damaged buds and sterile sections or rings in the flowering head. After pollination and petal
wilting and drying (late R-6) sunflower can withstand temperatures as low as 25 degrees F with only
minor damage. If 25 F temperatures occur at the bud stage this will often damage stalk tissue below the
bud and seeds will not develop.
Corn: usually is damaged by temperatures in 30 degree range or less. Colder temperatures will kill entire
stalk. If only leaves above the ear are frosted by a light frost, then kernel development will continue. If
entire stalk, earshank and leaves are frozen, kernel development will cease and soft shriveled corn will
result. If corn is at around 35 percent moisture or if a black layer has formed at the base of the kernel the
plant is physiologically mature and kernels will develop normally despite frost. At the half milk line
development stage yield losses due to frost are generally in the range of 5 to 10 percent with only a slight
impact on test weight. For corn in the dent stage, yield losses will typically range from 25 to 40 percent
with more reduction in test weight and the higher moisture content of the corn may present challenges in
terms of harvest and storage.
Soybean: Soybean plants are easily damaged by temperatures in the 28 to 32⁰F range. Research
information from Wisconsin has shown that all varieties tested had reduced yields when frost occurred at
or before the R6 growth stage. The R6 growth stage is defined as ‘full seed’ – the stage at which time
pods contain green seeds that fill the pod cavity at one of the four uppermost nodes on the main stem with
a fully developed leaf and occur in more than half the plants in the field. The greatest yield losses
occurred when frost damaged plants at the R5 growth stage. The number of beans per plant and reduced
bean size contribute to overall yield loss.
Soybean fields planted to narrow row spacing (6 or 7 to 12 inches) may have slightly more tolerance to
light frosts than soybean planted in wider rows (30 to 36 inches). The heavy plant canopy of the solidseeded, closely drilled beans tends to hold the soil heat better and therefore protects the plants to some
degree. With a dense canopy a light frost (30 to 32 F, during a short period) may kill the upper leaves but
the frost may not penetrate into the lower canopy. The killed leaves initially turn blackish. Once the upper
leaves have been damaged an additional frost event will most likely penetrate deeper into the canopy and
do damage to the leaves and stems.
If the plants have reached the R7 growth stage, yield reductions due to below freezing temperatures may
be limited. The R7 growth (beginning maturity) occurs when one normal pod on the main stem has
reached its mature pod color. A frost between R6 and R7 may or may not affect yield, depending on
temperature and duration of freeze.
Frost-damaged soybeans generally can be harvested as long as the plants reached the R6 growth stage at
the time the killing frost occurred. Frost-damaged soybeans may have higher moisture content and
possibly are more difficult to thresh. Adjust the combine by reducing the concave clearance and adjust the
cylinder speed if needed. Remove as much chaff and green plant material as possible before storing
beans.
Severely frost damaged plants may dry down more slowly. The beans will need to be dried to a safe
moisture level for storage (12% for 6 months). Electronic moisture meters may underestimate the
moisture content in green and immature soybeans and actual moisture content may be 1.0 to1.5
percentage points higher.
Green and immature soybeans are considered damaged seed at the elevator. Elevators may discount loads
with green and or immature soybeans and in some cases may reject entire loads if the damage levels are
high.
Remember temperatures of 32 F degrees at weather stations or farmsteads may result in temperatures of
28 F to 29 F degrees in low lying areas of fields. Also, time of exposure to freezing temperatures will
influence degree of damage done. Usually 2 to 4 hours duration of a critical low temperature will cause
damage. Two other factors which may influence critical frost temperatures are soil moisture and wind
velocity. When soil moisture is higher, frost injury is somewhat reduced due to heat slowly released from
the stored heat in the soil within a plant canopy. Wind movement also helps reduce freezing to some
degree and is better that a still cold night with no air movement. Cloudy nights are also better than clear
nights.