J. H. Dunn
and D. D. Minner Department of Horticulture, College of Agriculture
beautifies and adds greater usefulness to home landscapes, recreational
areas and public and industrial grounds. Proper management is the
most effective factor in weed control, but unfavorable climate,
insects, diseases or abuse by man permits weeds to invade the turf.
Under such conditions careful use of appropriate herbicides permits
the turf to recover its original value.
A dense stand
of healthy grass provides the best weed control. Most weeds are
opportunists which invade weakened lawns and turf; thus, the fight
against weeds starts with good management. All cultural practices
such as mowing, fertilizing, watering, etc. should be done in a
manner and time that will favor the grass rather than the weeds.
Height of mowing influences competition against weeds such as crabgrass-the
higher the cut, the lower the infestation. Frequent light sprinkling
encourages shallow rooted weeds and seed germination. Less frequent
deepsoak watering that maintains a dry surface layer provides the
grass with a competitive advantage.
light, soil moisture and other factors determine the time and extent
of weed germination and development. Some germinate in early spring
while others sprout in summer or fall. If conditions are favorable
a weed may be particularly abundant in a given year, but under different
conditions, the next year it may be little in evidence.
Weeds such as
crabgrass and knotweed, which germinate in the spring from seed
and mature in summer or fall each year, are designated as summer
annuals. Chickweed, henbit and others germinate in fall or late
winter and mature in late spring. These are called winter annuals.
herbicides (chemicals applied to soil to prevent growth from seeds)
must be applied in spring to control summer annuals but in fall
to control winter annuals. After weeds appear, postemergence herbicides
must be used.
live more than two years but may produce seeds for new plants each
year. Like biennials, which require two years to complete a life
cycle, they store food in fleshy roots for next year's growth. Generally,
postemergence herbicides are required for control and are more effective
in the first years growth. Late fall often may be a good
time to apply such herbicides. Old established plants, which have
large taproots, may be controlled more effectively by hand pulling
or digging. Henbit--upright, squarish stems, blue & lavender flowers;
winter annual. Plantain, (narrow or buckhorn)--long, narrow leaf,
seedhead atop wiry stem; perennial. Plantain, (broadleaf or Rugels)--thick,
oval leaves, long seedhead; perennial. Goosegrass--coarser than
crabgrass, leaf base flat, whitish; summer annual. Nimblewill--fine
wiry stems, frail inconspicuous seeds, winter dormant; perennial.
Yellow Nutgrass (Nutsedge)--triangular stems, yellowish green; treat
as perennial. Mallow--upright stems, five point leaf; summer annual
or biennial. Chickweeds--common--light green delicate vine; winter
annual; mouse-ear--dark green hairy leaves; perennial. Shepherds
Purse--young leaf-like dandelion, triangular seed pod; winter or
summer annual. Knotweed--tough viny stems, leaves small & single;
spring germinating annual. Spurge (prostate or mat)--plant sap milky,
stems often matted; summer annual. Bindweed--creeping vine, leaf-like
arrowhead, flower white or pink; perennial.
give key identification characteristics of weeds more frequently
found in lawns and turf. Crabgrass and dandelion are omitted because
most readers are likely to be familiar with them already.
herbicides are formulated with reliable safety factors, application
rates higher than those recommended may cause injury to turf. Many
people "over-apply" herbicides, especially when using fertilizer-herbicide
combinations. The user needs to follow directions on containers
carefully to avoid overdoses. Lower safe rates are effective if
applied when weeds are most susceptible.
treatments are applied before weeds sprout from seeds. Apply 2-4
weeks ahead of germination. Less effective control may be expected
if applied more than a month before. Removing clippings and sprinkling
immediately after application will help move materials down to the
herbicides such as 2,4-D, Trimec, DSMA, etc. are applied after weeds
appear. Liquid sprays are more effective than dry materials, especially
on hard-to-kill weeds. Apply postemergence materials when weeds
are growing vigorously. Tough old weeds are hard to kill, and if
mature seeds are already formed, the lawn is likely to be infested
again next year.
are safest because they give off less vapors which might damage
other plants. Volatile ester formulations should not be used around
ornamental plants. Select a time when winds are calm to prevent
spray drift. Using wax bars or granules impregnated with herbicides
near ornamentals will minimize such hazards.
combinations are extremely popular because they combine two operations.
Combinations with pre- emergence chemicals are generally effective
since both the fertilizer and herbicide action are dependent upon
contact with the soil. Postemergence herbicide action depends more
upon absorption by leaves, and granules in such combinations do
not adhere well to smooth-surfaced leaves. They will stick better
if applied when weed leaves are damp. "Weed and feed" materials
present a conflict in desirable actions. Proper time for weed control
often does not coincide with the most desirable time and rates for
fertilizing. If used for follow-up fertilizations, there is danger
of herbicide overdose.
Hard-to-kill weeds may be eradicated by spot applications of non-selective
herbicides such as dalapon, amino triazole (amitrole), cacodylic
acid or glyphosate. Thoroughly wet the foliage with the solutions,
but avoid run-off and excess accumulation in the soil. Dalapon is
effective mostly against weedy grasses. Use one of the others for
broadleaf weeds. Glyphosate and cacodylic acid usually decompose
in a few days, but amitrol and dalapon will require three to six
weeks before the spot can be reseeded.
can be eliminated by excluding all light with pieces of tarred paper,
black plastic, etc. Keep the cover in place at least three months.
spreaders can be used for applying granular herbicides. Be sure
to adjust the spreader to apply recommended rates. If possible,
apply one-half the desired rate in one direction and the remaining
half at right angles to the first.
applicators, compressed air sprayers or types attached to a garden
hose are effective for liquid applications. High pressures cause
mists subject to drift and should be avoided. Sprinkler cans or
sprinkler nozzles attached to a gallon container can be used on
A sprayer used
for 2,4-D applications should not be used to spray garden or flower
plants. Cleaning procedures are not always reliable. To be safe,
have a separate sprayer for weed killing purposes.
weeds is of little value unless enough desirable grass is present
to fill in bare spots. A reseeding program deserves first consideration
if the turf is so weak that it will not recover once weeds are eliminated.
Study soil and other conditions to determine reasons for low vigor
of the original turf.
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