14 common garden invaders and the best ways to control them
April 13, 2009
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Crabgrass (Digitaria species)
This infamous summer annual thrives in warm, moist areas. Seeds germinate in early spring in warmer climates, later in cooler areas. As the plant grows, it branches out at the base; stems can root where they touch the soil.
In flower beds, pull crabgrass before it sets seed. To thwart crabgrass in lawns, keep the turf well fertilized and vigorous, so it will provide tough competition for weeds. Also water your lawn deeply, but infrequently; this tactic will dry out crabgrass roots, killing the weeds or at least diminishing their vigor. Solarization can control crabgrass if high temperatures are achieved.
Use corn gluten as a preemergence treatment. If chemical control is necessary, in ornamental beds only, use a postemergence herbicide control that kills grasses.
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Bindweed (Convolvulus arvensis)
Also called wild morning glory, bindweed grows in open areas. Its 1- to 4-foot-long stems crawl along the ground and twine over and around other plants.
Pulling usually doesn't eradicate it ― the stems break off, but the weed returns from the roots. To control its spread, you'll have to dig the roots out repeatedly (persistence is required). It's important not to let bindweed set seed, since the hard-coated seeds can sprout after lying dormant for 50 years!
Best control is prevention. Remove flowers before they set seed, and pull or hoe seedlings. Kill established plants by regularly cutting to the ground any stems that have reached six inches tall.
For chemical control, in midsummer, when bindweed is at the height of its growth season but has not yet set seed, spot-treat isolated patches with glyphosate.
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Bermuda grass (Cynodon dactylon)
A fine-textured and fast-growing perennial, Bermuda grass is frequently planted as a lawn in warm climates. In other sorts of lawns and in gardens, though, it can be a difficult weed. It spreads by underground stems (rhizomes), aboveground runners (stolons), and seed.
If you have a Bermuda grass lawn, use an 8-inch-deep barrier or edging to prevent it from advancing into other parts of the garden. Dig up stray clumps before they form sod, being sure to remove all the underground stems; any left behind can start new shoots.
Repeated pulling and digging are generally necessary to stop this weed; mulches will slow it down, but it eventually grows through most of them. For chemical control, use a selective postemergence herbicide.
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Spotted Spurge (Chamaesyce maculata)
This annual weed produces large quantities of seed within just a few weeks of germination and scatters them widely. It grows from a shallow taproot and forms a low mat of branching stems that exude a milky juice when cut.
Prevention is the best control. Hoe or pull young seedlings early, before they bloom and set seed. Apply a 1-inch layer of fine mulch to suppress germination in garden beds.
A vigorous, well-fertilized lawn competes well against spotted spurge. If chemical control is necessary in lawns, use a preemergence product in late winter before seeds germinate, following label directions.
Spot treat spurge plants with herbicidal soap when they are young. For spurge growing in cracks in pavement, use a hand weeder.
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Yellow nutsedge (Cyperus esculentus)
Also known as yellow nutgrass, this perennial weed thrives in moist areas in much of the country. Its bright green leaves grow from the base in groups of three; grass leaves, in contrast, grow in sets of two. The flower head is golden brown. Small, roughly round tubers (nutlets) form at the tips of the roots; the weed spreads by these tubers as well as by seed.
Hoe or pull nutsedge when it's young and still small ― when plants have fewer than five leaves or are less than 6 inches tall. Older, taller plants are mature enough to produce tubers; when you dig or pull the plant, the tubers remain in the soil to sprout. Repeatedly removing top growth eventually weakens tubers.
For small patches in lawns, dig deeply (8 inches); remove the whole patch, then refill with soil and seed or sod the patch.
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Yellow oxalis (Oxalis corniculata)
A very aggressive perennial weed, yellow oxalis (also called yellow wood sorrel) is happy in sun or shade, and spreads quickly by seed.
Seedlings start out from a single taproot, which soon develops into a shallow, spreading, knitted root system. Tiny yellow flowers are followed by elongated capsules that can shoot seeds as far as 6 feet.
Dig out small plants early. If you have a lawn, keep it vigorous to provide competition; water deeply but infrequently, since frequent light watering encourages this shallow-rooted weed.
You can also use a preemergence herbicide on turf and around ornamentals listed on the label. Spot-treat oxalis in garden areas with glyphosate.
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Dandelion (Taraxacum officinale)
Dandelion is particularly vigorous in cold-winter climates. It grows from a deep, fleshy taproot and spreads by windborne seeds. Flowering begins in spring and often continues until frost.
A healthy lawn can outcompete dandelions, so thicken the turf by overseeding and by proper fertilizing, watering, and mowing.
Pull dandelions while they're small, before they produce a taproot and set seed. Once the taproot has formed, you must remove all of it, since new plants can sprout from even a small piece. A dandelion weeder with a forked blade is helpful, or use a hand weeder with a bent shaft.
For chemical control, use a selective postemergence herbicide labeled for dandelions in turf.
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Plantain (Plantago species)
Plantains are perennials that form rosettes of dark green leaves marked from end to end with distinctive parallel veining. Leaves of P. lanceolata (buckhorn plantain) are long and narrow; those of P. major (broadleaf plantain) are broadly oval. They love damp, heavy soil.
To reduce infestations in lawns, keep the turf thick through consistent fertilizing; aerating will help, too. Dig out plantains before they set seed. Be sure to remove as much of the roots as possible (a dandelion weeder is helpful here), since these weeds can regrow from any pieces of the rootstalk that remain.
For chemical control, use a preemergence product or spot-treat plantains in the garden with glyphosate, taking care not to get the chemical on desirable plants.
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Common mallow (Malva neglecta)
Also known as cheeseweed (thanks to the fruits, which resemble a round of cheese), common mallow is a widespread annual or biennial weed with broad, lobed leaves and pinkish white, five-petaled flowers.
Hoe or pull these weeds when they're young. Mature plants have a long, tough taproot that is difficult to extract from the soil, and they are of course more likely to have set seed.
For chemical control, use a preemergence herbicide to prevent seedlings from becoming established in lawns and around ornamentals.
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Poison oak, Poison ivy
Poison oak is most common along the West Coast. In the open or in filtered sun, it forms a dense, leafy shrub; in the shade, it's a tall-growing vine. Its leaves are divided into three leaflets with scalloped, toothed, or lobed edges.
Poison ivy looks similar; it's common east of the Rockies and also grows in eastern Oregon and eastern Washington. Usually found in shady areas and at the edges of woodlands, it sprawls along the ground until it finds something to climb, then becomes a vine.
A resin on both poison oak and poison ivy causes severe contact dermatitis in most people. Control the both with an appropriately labeled herbicide, such as glyphosate (be sure to avoid getting these chemicals on other plants).
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Blackberry (Rubus species)
Wild blackberry can be a vexing weed almost anywhere in the United States, but it's particularly troublesome in the Northeast, the Southeast, and many areas of the West. Plants spread rapidly by underground runners and by seed.
Pull young plants in spring, before they develop a perennial root system. To kill established clumps, repeatedly prune back the stems as they sprout; this eventually exhausts the roots. Or mow the tops and dig out the roots; repeat the process as new canes grow from roots left behind.
You can also cut stems to the ground and apply glyphosate to the stubs as soon as possible after cutting. Spot-treat any new shoots with glyphosate as they appear.
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Puncture vine (Tribulus terrestris)
With its sharp, thorny burs that poke into tires, paws, and bare feet, puncture vine is painfully familiar to gardeners in much of the country. An annual weed often found in dry areas, it forms a dense, low mat 5 to 15 feet in diameter.
For best control of small infestations, hoe or dig plants before they can set seed, cutting below the crown to prevent regrowth. Once you've removed puncture vine growing in lawns, improve the soil with compost and sow grass seed in bare spots to prevent the weeds from reestablishing.
For chemical control, preemergence herbicides containing trifluralin may be used on some lawn grasses and ornamentals in late winter or early spring. For postemergence control in lawns, use a selective herbicide.
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Purslane (Portulaca oleracea)
Not to be confused with the large-flowered ornamental purslane sold in nurseries (the 'Wildfire' strain, variously ascribed to Portulaca oleracea, P. umbraticola, and P. grandiflora), weedy purslane is a low-growing summer annual found throughout the country.
It thrives in moist conditions but can withstand considerable drought. Its fleshy, dark green leaves are edible, with a tart, lemony flavor.
Purslane is easy to pull or hoe. But pieces of stem can reroot readily, so be sure to remove them from the garden. Also remove plants that have begun to flower, since they can ripen seed even after they've been pulled. Don't compost any part of the plant.
Also known as couch grass or devil's grass, quack grass is an aggressive perennial that produces an extensive mass of long, slender, yellowish white branching rhizomes (underground stems) that can spread laterally 3 to 5 feet.
Thoroughly dig the area and remove all visible pieces of rhizome; this will slow the weed's growth for a few years. You can also suppress quack grass by smothering it; leave the cover in place for at least a year.
For chemical control, use preemergence herbicides on turf grasses and around ornamentals listed on the label. Or spot treat with an herbicide containing glyphosate, taking care to avoid contact with desirable plants.
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