A guide to controlling common lawn weeds with herbicides, fertilizers, and hand-weeding.
Everyone would like to have a lawn free of weeds, but it requires quite a bit of work and planning. Synthetic herbicides are most often employed, but these chemical weed killers can be dangerous to people, pets and turf. You need to attack the underlying problems that weaken lawns and favor weeds to let you limit the use of herbicides.
The best way to eliminate weeds in your yard is to grow a thick, vigorous lawn. Dense grass crowds out weeds and blocks the sunlight their seeds need to germinate.
No single herbicide, weeding technique or lawn care tactic works against all weeds. How you attack the weeds in your lawn depends on which you have. Lawn weeds fall under three broad categories: unwanted grasses; sedges; and broadleaf plants. Annual weeds complete their life cycle in one season and reproduce from seeds. Perennials live several years and spread underground as well as by seed, making them harder to control.
Healthy Lawns Control Weeds
Most lawn weeds take root wherever they can find the space and receive sunlight to germinate. These plants cannot establish themselves in healthy, thick grass. Sensible lawn care plan will help stop weed problems before they have a chance to start.
Under-fertilized grass can lead to a sparse lawn that loses the competition with weeds. Too much fertilizer helps certain weeds such as annual bluegrass, Bermuda grass and crabgrass. Following the application rates on your lawn fertilizer to properly feed your lawn, and using a fertilizer with a high percentage of controlled-release nitrogen, such as sulfur-coated urea, ureaform or IBDU will provide a long lasting feeding.
The frequency and timing of fertilizer application vary depending on your lawn type and the length of your growing season. Most northern lawns need only one or two applications of fertilizer annually—once in fall and sometimes a second time in spring. Southern lawns might require three feedings—early to mid-spring just after the grass greens up, early summer and again in early fall.
Frequent, light watering causes shallow roots and helps annual bluegrass, crabgrass, chickweed, sedges and other weed seeds germinate. If you water too little, the lawn suffers while spotted spurge, Bermuda grass, and other weeds adapted to drier soil thrive. Instead, provide your lawn with infrequent, deep soakings. Lawns need about 1 inch of water per week. You can set an empty tuna can on the lawn to determine when you have applied 1 inch of water.
Mowing too close weakens turf by reducing the ability of a grass leaf to produce enough nutrients. It also lets light hit the soil surface, which helps weed seeds germinate and grow. Check with your local agricultural extension service for the recommended range of mowing heights for your grass type. Mowers should typically be set to between 2 and 4 inches.
Sometimes weeds help you determine soil problems which will allow you to correct them so your landscape favors lawn grasses and discourages weeds. For instance, ground ivy grows best where the soil surface remains damp and in areas too shady for good grass growth. Aeration is a good way to promote drainage and remove tree branches in shady areas to allow more light to reach the lawn.
Hand-weeding is the best defense on small lawns where there are a manageable number of weeds. This is the most effective against annual broadleaf weeds removing young weeds before they flower and seed is the simplest way to prevent them from spreading.
Catching perennial weeds early is crucial. Dandelions, for example, develop deep taproots that are hard to pull once they mature. Yank the entire plant, including the root—any root pieces left underground will grow new plants. If new sprouts grow, pull them repeatedly to eventually starve and kill the weed.
Weeding is easiest when the soil is moist. Tools like the dandelion digger help get at the root by probing deep into the soil. Once the weed is out, promptly reseed the bare spot; otherwise, new weeds will fill it in.
Perennial weeds such as dandelions should be pulled when they are young and is easier when soil is moist. Use a sharp spade or dandelion digger angled downward toward the center of the plant, and loosen the soil around it. Use the tool to pry the weed upward while pulling it and be careful not to break off the roots. Fill in the hole with additional soil if necessary and reseed if needed.
Use herbicides as a last resort such as if your lawn has been taken over by weeds. Follow directions carefully since improper application can damage or kill turf and other desirable plants.
Read the instruction carefully as some herbicides work only within a certain temperature range; others work only when applied at a specific time of year. Be careful not to apply during windy weather.
Preemergence herbicides kill germinating seeds before seedlings break through the soil. Crabgrass is the primary target. The most common pre-emergence herbicides are synthetic. Natural, nontoxic pre-emergence herbicides made from corn gluten are safer, though you might have to apply them for several seasons for them to be fully effective. Three quality products are Concern Weed Prevention Plus, WOW! and WeedzSTOP. A drawback to these and most other pre-emergence herbicides is that they kill germinating lawn seed. Check product labels carefully.
Post-emergence herbicides kill existing weeds that are actively growing. These come in two basic forms: contact and systemic. Contact herbicides kill only the part of the plant they touch. Most act quickly and work best against annual weeds. Systemic herbicides circulate inside the plant, killing the whole thing. They're more effective than contact herbicides on perennial weeds, though repeat treatments might be needed. Weed-Away and Weed Warrior, are systemic and selective to kill broadleaf weeds only. They won't kill weed grasses. Glyphosate, the active ingredient in Roundup and other products, is a systemic, nonselective herbicide that kills broadleaf weeds and weedy grasses. But because it also kills turf and other desirable plants, it's safest to use it on your lawn when you want to kill an entire section and then replant it. For this reason spot treatment is better.
Weed-and-feed products combine fertilizer and herbicides. The main problem with these products is that the recommended time for weed control doesn't coincide with the best time and rates for fertilizing. Your lawn may also receive an overdose of herbicide when used for follow-up fertilizing. WOW Plus!, corn gluten with added organic fertilizer, is the safest weed-and-feed.
Crabgrass is an annual with branching, spreading stems. Its coarse, blue-green to purplish leaf blades can be smooth or hairy, depending on the species. Flower heads with several fingerlike spikes rise from narrow stems.
Crabgrass thrives in lawns mowed shorter than 2 inches, underfed lawns, and those watered frequently and lightly. Thick, deeply irrigated turf is the best control. Dig crabgrass before it seeds. Preemergence crabgrass herbicides are available and should be applied in spring before soil temperature reaches a steady 60 degrees F.
Dandelion is a broadleaf perennial recognized by bright-yellow flowers and a large, flat rosette of leaves rising from a long, fleshy taproot. Dandelions favor thin turf.
Pull or dig out young plants before they go to seed. Then cut any regrowth from leftover root pieces. You can also spot-treat weeds with a selective broadleaf weed killer.
White clover is a broadleaf perennial that used to be included in grass seed mixes. Also called white Dutch clover, it's distinguished by three-lobed leaves with a crescent-shaped white band. The plant spreads by creeping stems and thrives in sparse, undernourished turf with excessive moisture.
Control it by watering well, applying nitrogen fertilizer and avoiding excessive applications of phosphorus. Spot-treat with a selective broadleaf weed killer and a second treatment often is needed.
Ground ivy is a broadleaf perennial with square stems and bright-green rounded leaves with scalloped edges. It reproduces by seed and creeping stems that root as they touch the ground.
Also called creeping Charlie, it prefers damp soil and shade. Improve drainage and water less. Pull stems and roots of young plants. Spot-treat with a broadleaf post emergence herbicide.
Yellow woodsorrel is a broadleaf perennial, although it might act as an annual in some regions. Also known as oxalis, it has cloverlike leaves and yellow flowers, each with five petals. Plants spread by roots and seed.
This weed is difficult to control, and does best in thin turf watered frequently and lightly. Water thoroughly and fertilize properly. Dig out small plants or spot-treat isolated ones with a post emergence weed killer. Prevent new weeds with a pre-emergence herbicide with oxalis on the label.
Quackgrass is a perennial grass with flat light-green to blue-green leaves. It spreads by seeds and aggressive underground stems.
Thoroughly dig out roots and pointed rhizomes—remaining pieces regenerate new plants. Spot-treat with a nonselective weed killer.
Yellow nutsedge is grass-like perennial sedge with triangular stems and 1/4-inch-wide leaves. Also called yellow nutgrass, it reproduces by seed and tubers that grow at the root tips. Tubers often persist in the soil, making established plants difficult to control.
Mow high in early to midsummer and water infrequently though thoroughly. Spot-treat with post emergence herbicides labeled for nutsedge. As with most weeds, control is easiest when plants are small.
Spotted spurge is a broadleaf annual that grows close to the ground in a fast-spreading mat. Its small leaves are green with a brown-red spot on top. Cut stems exude a milky liquid.
Spotted spurge reseeds heavily. A high-mowed, well-fertilized and vigorous lawn provides tough competition. Pull isolated plants before they seed. Spot-treat with a post emergence weed killer and use appropriate pre-emergence herbicides to prevent new weeds.
See my related article for 10 Steps to Improve Lawns: knoji.com/10-steps-to-improve-lawn/
National Pesticide Information Center
University Of Minnesota Extension