North Dakota State University Extension Service - Burke County
Burke County AgAlert
May 27, 2008
COLD TEMPERATURES
Winter Wheat:
Light frosts can burn leaf tips causing the plant/field to turn a tan/yellow
color. Plants usually recover quickly with warm weather. The jointing (5 ½
leaf) stage is when the internodes begin to elongate and the developing
head/growing point is now above the soil surface. The growing point can be
severely injured when temperatures fall below 24° F for 2 hours. A severe
frost can cause death of the growing point on the main stem, but growth from
later tillers can reduce the overall damage.
Alfalfa:
Established, mature stands of alfalfa can handle temperatures in the mid
20’s. Temperatures of 22° F are considered the threshold where significant
frost damage injury will occur, although alfalfa may injured with
temperatures of 28°F if not acclimated to the cold weather. If alfalfa is
12- inches or higher, severe frost damage will cause the top half of the
stem to wilt. If the wilted stems do not recover in 2 to 3 days the damage
is severe. Frost damaged plants/stems will have a silver or white color.
Injured stems that do not recover will re-initiate new growth from the crown
and the first cutting will be lost. If there is enough forage to cut, then
the alfalfa should be cut right away before extensive leaf drop occurs.
Canola:
The growing point is above ground once the cotyledons emerge. Canola in the
cotyledon stage can withstand temperatures as low as 22° F; especially if
the plants have been hardened to sub-freezing temperatures. If temperatures
fall below 25° F for an extended time there will most likely be some level
of injury. When evaluating frost injury, a waiting period of three to four
days is needed to see if the growing point is alive. If the growing point is
a light green fleshy color, the plant is alive. If the growing point is
black or a dark green color, then most likely the plant is dead. The stem of
dead plants will be wilted, black in color, and will be laying on the soil
surface. A good sign that the plant will recover is that after 3 days a new
leaf will be emerging in the center of the growing point. Often the older
leaves or cotyledons will wilt or even die and the overall appearance of the
field will look tough, if the growing point is alive the crop will recover.
Flax:
The growing point is above ground once the cotyledons emerge. Flax that is
in the cotyledon stage is not tolerant to a hard frost below 27° F. Flax
that is in the 2-leaf stage and has hardened off can stand temperatures in
the low 20's. Similar frost evaluations and injury symptoms for flax are
described in the canola section.
Field Peas/Lentils:
Field pea and lentil have similar emergence as small grains. The growing
point is below the ground with the seed piece until the peas or lentils are
in the 5th node stage or 4 inches high. Peas are very tolerant to a hard
frost and are more tolerant than other broadleaf crops. If temperatures fall
below 20° F, stems/plants will wilt and appear to be severely injured;
however, recovery should be fast due to the fact the growing point is below
the ground.
Small grains/Corn:
Small grains/corn in the 1 to 4-leaf stage can handle temperatures in the
low 20's without major injury. If temperatures fall below 20° F, that is
considered a severe frost. Emerged leaves might wilt or even die; however, a
new leaf should begin to emerge 3 to 4 days after a severe frost has
occurred. Small grains in the jointing stage (4 ½ leaf barley, 5-leaf wheat)
or corn that is in the 5-leaf stage will be more sensitive to a frost of 28°
F for the growing point is now above the soil surface. Once the crop hits
the jointing stage, the growing point is above the ground and the length of
the frost is the question, not how cold it was. Winter wheat is also very
tolerant to cold temperatures, especially if it has been hardened off.
Spider Mite
Dry weather conditions are conducive for the development of spider mites on
evergreens and broad-leaved ornamentals. Evidence of mite damage on
evergreens is a sudden graying or browning of the needles. With heavy
infestations, webbing will often be present between the needles and dropping
of needles may begin to occur.
Spider mite infestations can be easily detected by holding a piece of white
paper under a suspected branch or leaf. Tap the branch or leaf lightly. If
spider mites are present they will fall onto the paper and appear as small
greenish or red crawling specks. Spider mite infestations on evergreens can
be suppressed by forceful spraying with water at 7-10 day intervals.
Insecticides having activity against spider mites are Orthenex, Bifenthrin,
Malathion, TetraSan, and Spectracide. One or two repeat applications at 1 to
1 1/2 week intervals with either insecticide may be necessary for complete
control. When spraying of tender ornamentals is necessary, the wettable
powder formulation of an insecticide should be used in order to prevent
foliar injury which can occur when oil base sprays are used on hot days.
Dan Folske
Extension Agent/Burke County
Soil Conservation Technology Specialist
PO Box 280
Bowbells, North Dakota 58721
dan.folske@ndsu.edu
701-377-2927