Smooth crabgrass (D. ischaemum) and large
crabgrass (D. sanguinalis) are warm season annuals that
reproduce primarily by seeds. Large crabgrass can also reproduce
by long, rooting tillers. Seeds of both plants can germinate from
mid spring through summer when soil temperatures at the 1/4”
level are 55° to 60°F for 5 to 10 consecutive days and can
continue germinating in soil temperatures to 95°F. These plants
reproduce from seeds found in the soil throughout much of the growing
season when moisture and light are present and soil temperatures
are warm. Both crabgrasses are branched and grow upright or prostrate.
Large crabgrass is normally mat forming, often has purple stems,
and can grow to more than 3 feet in height. Smooth crabgrass is
usually 15 inches or less. The leaves of large crabgrass are pale
blue green, hairy on both sides, and with edges that may be rough.
They are flat, sharply pointed, 1/4 to 2/5 inch wide, and 2 to 6
inches long. Smooth crabgrass leaves are normally dull green, sometimes
purple tinged, and with margins that may be rough. They are sharply
pointed, sparsely hairy near the base, 1 to 4 inches long, and 1/8
to 1/4 inch wide. Large crabgrass flowers are borne in a raceme
with 3 to 13 purplish finger-like spikes up to 6 inches long. They
occur in whorls at end of stout stalk during August and September.
The flowers of smooth crabgrass are borne in a raceme with 2-to-6
finger-like, purplish spikes up to 4 inches long, and at end of
flowering stalk. They also occur August and September.
Occurrence - Both crabgrasses can be found in
most warm, moist, fertile lawns in full sun where turf is thin or
mowed too short. They tolerate hot, dry, compacted soils after establishment,
and may spread aggressively to crowd out desirable grasses.
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Non-Chemical Control – Maintaining a dense,
healthy lawn through proper turf selection, establishment, and maintenance
is the recommended non-chemical control for the crabgrasses. Avoid
light, frequent irrigation, spring cultivation, short mowing, and
summer fertilization. They can also be hand-pulled or mechanically
removed.
Chemical
Control - Apply preemergence herbicides before germination
when soil temperatures stabilize at 50 - 55°F for several consecutive
days; postemergence herbicides should be applied when plants are
small.
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