Weeds In Lawns
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Please view our other pages
Lawn Articles;
Violet
Clover
Thistle
Crab Grass
Dandilion
Dead Nettle
Weeds in grass, are plants out of place. A plant may be desirable in one situation and a grassy
weed in another. For example, creeping bentgrass plants can invade a Kentucky bluegrass lawn
and cause unsightly patches. On a golf green, however, creeping bentgrass is highly desirable as
the predominant plant. Weeds in grass detract from the beauty of lawns due to the contrast in
color and texture between the desired grass plants and the weeds. In addition, weeds compete
with the desired grass plants for available water and nutrients, usually resulting in thinning of
desirable plant cover.
Weed Identification and Characteristics
Lawn weeds may conveniently be divided into two classes based on the way in which they emerge
from the seed. Monocots emerge with a single seed leaf whereas dicots emerge with two seed
leaves. Most monocot weeds found in turfgrass are from the family Gramineae and are termed
weedy grasses . Examples include crabgrass, annual bluegrass, tall fescue, and quackgrass.
Dicots, on the other hand, are termed broadleaf weeds and include such plants as dandelion,
clover, ground ivy, knotweed, and plantain.
Weedy grasses and broadleaf weeds are further divided into groups according to the plants'
length of life. Perennial weeds have a life of more than two years, though new seeds may be
produced every year. Biennial weeds have a life of two years, generally storing up food reserves
in the leaves and roots the first year and producing seed in the second year. The biennial weeds
often are grouped with perennial weeds since control is similar. Annual weeds germinate from
seed, grow, flower, and produce seed in less than one year. Summer annuals germinate in the
spring and mature in the fall, whereas winter annuals germinate in fall or late winter and mature in
late spring.
Effective control of weeds in grass is based on correct identification. Many books and charts are
available to help in identifying common garden weeds. For additional help in weed identification,
inquire at your county extension service.
Methods of Control—Cultural
weed in another. For example, creeping bentgrass plants can invade a Kentucky bluegrass lawn
and cause unsightly patches. On a golf green, however, creeping bentgrass is highly desirable as
the predominant plant. Weeds in grass detract from the beauty of lawns due to the contrast in
color and texture between the desired grass plants and the weeds. In addition, weeds compete
with the desired grass plants for available water and nutrients, usually resulting in thinning of
desirable plant cover.
Weed Identification and Characteristics
Lawn weeds may conveniently be divided into two classes based on the way in which they emerge
from the seed. Monocots emerge with a single seed leaf whereas dicots emerge with two seed
leaves. Most monocot weeds found in turfgrass are from the family Gramineae and are termed
weedy grasses . Examples include crabgrass, annual bluegrass, tall fescue, and quackgrass.
Dicots, on the other hand, are termed broadleaf weeds and include such plants as dandelion,
clover, ground ivy, knotweed, and plantain.
Weedy grasses and broadleaf weeds are further divided into groups according to the plants'
length of life. Perennial weeds have a life of more than two years, though new seeds may be
produced every year. Biennial weeds have a life of two years, generally storing up food reserves
in the leaves and roots the first year and producing seed in the second year. The biennial weeds
often are grouped with perennial weeds since control is similar. Annual weeds germinate from
seed, grow, flower, and produce seed in less than one year. Summer annuals germinate in the
spring and mature in the fall, whereas winter annuals germinate in fall or late winter and mature in
late spring.
Effective control of weeds in grass is based on correct identification. Many books and charts are
available to help in identifying common garden weeds. For additional help in weed identification,
inquire at your county extension service.
Methods of Control—Cultural
The most effective method of controlling lawn weeds is to maintain a dense and vigorously growing turf cover. Weeds in lawns are often an
indication of problems in the grass plant environment, and killing the weeds without correcting the underlying problem will lead to unsatisfactory
results. For example, a problem with knotweed is usually an indication of severe soil compaction. Control of knotweed without correction of the
soil compaction will only lead to sparse soil cover until the area is again invaded by weeds that grow in compacted soil.
Often lawn weeds can be controlled simply by altering the cultural practices to favor the grass plants rather than the weeds. The cultural controls
may include raising (or lowering) the mowing height, changing the frequency of mowing, lengthening (or shortening) the period between
irrigations, increasing (or decreasing) the application of fertilizer, or aerifying the soil.
A combination of proper cultural practices plus prudent use of chemical is sometimes necessary to control lawn weeds effectively in turf.
Whenever herbicides are used, container labels should ALWAYS be read and followed carefully.
Preemergence Herbicides
Preemergence herbicides affect germinating seeds. To be effective, the herbicide should be applied two to three weeks before weed seeds
germinate. Consequently, preemergence herbicides are most effective against annual weeds. For control of summer annuals such as crabgrass,
application of preemergence herbicides between May 5 and May 20 in the Twin Cities area is most effective in a normal year.
Postemergence Herbicides
Postemergence herbicides are used to kill weeds after the weed plants are up and growing. To be effective, most postemergence herbicides
must be absorbed through the leaves; consequently, liquid sprays generally work better than dry, granular materials. However, granular
formulations may be the most practical way for homeowners to apply these materials.
Postemergence herbicides are most effectively applied when lawn weeds are young and growing vigorously.
Selective postemergence herbicides are usually used to control annual, biennial, and perennial broad-leaved weeds because they will kill many
broadleaf plants without damaging grass plants. These herbicides can severely damage or kill trees, shrubs, and flowers; thus, they should be
used with great care near these plants.
Postemergence herbicides may be applied any time the weeds are actively growing, the air temperature is 60–80 degrees F, there are no winds,
and there is no rain in the forecast for 48 hours. Most effective control of perennial broadleaf weeds is obtained when applied in early fall
(August 15–October 15) or in spring (May 1–June 1). For some weeds, repeated application at 20–30 day intervals may be required for control.
Nonselective postemergence herbicides kill all plants, both desirable and undesirable. These herbicides can be used to spot treat perennial
grassy weeds that are not affected by selective herbicides. To spot treat an area, thoroughly wet the weed foliage with herbicide solution.
Preemergence or selective preemergence granular materials may be applied with a fertilizer spreader. The spreader must set or be calibrated to
apply recommended rates. The preferred method of application is to apply one-half the recommended rate in one direction and one-half at a right
angle to that direction.
Compressed air sprayers or sprayers attached to a garden hose are effective for liquid applications. Remember that the higher the pressure and
the finer the mist, the more likely it is that the herbicide will drift and injure shrubs, trees, flowers, and vegetables. Sprinkler cans can be used on
small areas, but once any applicator has been used to apply herbicides, it should not be used to spray other pesticides on ornamental or garden
plants. You should use a separate sprayer for killing weeds. Be sure to clean the sprayer thoroughly after each use.
Courtsy of; Brad Pedersen, Extension Specialist, Department of Horticulture
Bob Mugaas, Extension Educator, Hennepin County
indication of problems in the grass plant environment, and killing the weeds without correcting the underlying problem will lead to unsatisfactory
results. For example, a problem with knotweed is usually an indication of severe soil compaction. Control of knotweed without correction of the
soil compaction will only lead to sparse soil cover until the area is again invaded by weeds that grow in compacted soil.
Often lawn weeds can be controlled simply by altering the cultural practices to favor the grass plants rather than the weeds. The cultural controls
may include raising (or lowering) the mowing height, changing the frequency of mowing, lengthening (or shortening) the period between
irrigations, increasing (or decreasing) the application of fertilizer, or aerifying the soil.
A combination of proper cultural practices plus prudent use of chemical is sometimes necessary to control lawn weeds effectively in turf.
Whenever herbicides are used, container labels should ALWAYS be read and followed carefully.
Preemergence Herbicides
Preemergence herbicides affect germinating seeds. To be effective, the herbicide should be applied two to three weeks before weed seeds
germinate. Consequently, preemergence herbicides are most effective against annual weeds. For control of summer annuals such as crabgrass,
application of preemergence herbicides between May 5 and May 20 in the Twin Cities area is most effective in a normal year.
Postemergence Herbicides
Postemergence herbicides are used to kill weeds after the weed plants are up and growing. To be effective, most postemergence herbicides
must be absorbed through the leaves; consequently, liquid sprays generally work better than dry, granular materials. However, granular
formulations may be the most practical way for homeowners to apply these materials.
Postemergence herbicides are most effectively applied when lawn weeds are young and growing vigorously.
Selective postemergence herbicides are usually used to control annual, biennial, and perennial broad-leaved weeds because they will kill many
broadleaf plants without damaging grass plants. These herbicides can severely damage or kill trees, shrubs, and flowers; thus, they should be
used with great care near these plants.
Postemergence herbicides may be applied any time the weeds are actively growing, the air temperature is 60–80 degrees F, there are no winds,
and there is no rain in the forecast for 48 hours. Most effective control of perennial broadleaf weeds is obtained when applied in early fall
(August 15–October 15) or in spring (May 1–June 1). For some weeds, repeated application at 20–30 day intervals may be required for control.
Nonselective postemergence herbicides kill all plants, both desirable and undesirable. These herbicides can be used to spot treat perennial
grassy weeds that are not affected by selective herbicides. To spot treat an area, thoroughly wet the weed foliage with herbicide solution.
Preemergence or selective preemergence granular materials may be applied with a fertilizer spreader. The spreader must set or be calibrated to
apply recommended rates. The preferred method of application is to apply one-half the recommended rate in one direction and one-half at a right
angle to that direction.
Compressed air sprayers or sprayers attached to a garden hose are effective for liquid applications. Remember that the higher the pressure and
the finer the mist, the more likely it is that the herbicide will drift and injure shrubs, trees, flowers, and vegetables. Sprinkler cans can be used on
small areas, but once any applicator has been used to apply herbicides, it should not be used to spray other pesticides on ornamental or garden
plants. You should use a separate sprayer for killing weeds. Be sure to clean the sprayer thoroughly after each use.
Courtsy of; Brad Pedersen, Extension Specialist, Department of Horticulture
Bob Mugaas, Extension Educator, Hennepin County
