Weeds in the Lawn: How to Identify Them and What to Do About Each

The single biggest mistake in lawn weed control is treating all weeds as if they were the same problem. They are not. The right approach for crabgrass, the right approach for dandelions, and the right approach for nutsedge are three different approaches, and applying the wrong one to the wrong weed produces no result — sometimes at substantial cost. Identification and timing matter more than the choice of product.

The classifications that determine treatment

Every lawn weed falls into one of four categories based on two distinctions: broadleaf vs. grassy, and annual vs. perennial. The right treatment for each category is different, and the right timing follows from the life cycle of the specific weed.

Broadleaf annual weeds

Common examples: chickweed, purslane, knotweed, lambsquarters. These germinate from seed each year, complete their life cycle in a single season, and rely on producing a new crop of seed to return the following year.

The right treatment is either a pre-emergent herbicide applied before the weeds germinate (early spring or, for some species, late fall) or a broadleaf post-emergent herbicide while the plants are small and actively growing. A pre-emergent prevents germination by creating a chemical barrier in the soil; a post-emergent kills plants that have already germinated.

Broadleaf perennial weeds

Common examples: dandelion, clover, ground ivy, plantain, violets. These return from the same root system year after year, often with deep taproots or spreading rhizomes that resist simple removal.

Pre-emergents do not work on perennial weeds — the plant is already established and the seed-prevention mechanism is irrelevant. The right treatment is a post-emergent broadleaf herbicide applied when the plant is actively growing. Fall application is usually more effective than spring; the plant is translocating resources to the root system in fall and carries the herbicide deep with it. Spring application kills the visible plant but often does not eliminate the root.

Grassy annual weeds

The dominant example is crabgrass. Goosegrass and foxtail are common in some regions. These germinate from seed each spring as soil temperatures warm, grow vigorously through summer, and die at first frost — after dropping seeds that produce the next year's crop.

Crabgrass control is overwhelmingly about pre-emergent application timing. The pre-emergent must be in the soil before the seeds germinate, which means application in early spring — usually when forsythia is in bloom or when soil temperatures reach about 55 degrees. After germination, controlling crabgrass post-emergent requires specific herbicides that are different from the standard broadleaf products, and effectiveness decreases as the plants mature.

Grassy perennial weeds

Common examples: nutsedge (technically a sedge, not a grass), quackgrass, bermudagrass invading cool-season lawns. These are the most difficult lawn weed category.

Standard pre-emergents and standard broadleaf herbicides do not control these. Specific products are required: halosulfuron or sulfentrazone for nutsedge, glyphosate (which will also kill the surrounding lawn) for spot-treatment of perennial grasses, or more selective products for specific situations. Many of these weeds are easier to manage by hand-pulling or by addressing the underlying soil or moisture conditions that favor them, rather than chemical control.

Weed identification: a few key examples

Crabgrass vs. tall fescue

Crabgrass is light green, grows in a star-shaped pattern flat against the ground, and appears mid-summer in thin areas. Tall fescue, a desirable lawn grass in many regions, has coarser, darker leaves and grows in upright clumps. Homeowners commonly confuse the two and misapply crabgrass treatment to tall fescue clumps, which is not what crabgrass control is for.

Dandelion vs. cat's ear (false dandelion)

Both produce yellow flowers, both have toothed leaves. Dandelion leaves are smooth and have a single thick taproot. Cat's ear has hairy leaves and multiple taproots. Some broadleaf herbicides work on one but not the other.

Nutsedge vs. grass

Nutsedge looks like a slightly lighter, slightly faster-growing grass. The distinguishing feature is the triangular stem — "sedges have edges," as the saying goes. Rolling the stem between fingers reveals three flat sides. True grasses have round stems.

Clover

White clover has three-leaflet leaves and small white flowers. It was historically a deliberate component of lawn seed mixes (it fixes nitrogen from the air, which fertilizes the surrounding grass). It only became a "weed" when broadleaf herbicides became widely available and the lawn ideal shifted to monoculture grass. Many homeowners now actively welcome clover; some lawn care services will not.

The strategy that actually works long-term

Chemical weed control is one component of lawn weed management, and over-emphasized in most homeowner approaches. The other components matter more.

Mow at the right height

Most cool-season lawns benefit from mowing at three to four inches. Most warm-season lawns are mowed lower. Short mowing weakens the grass and creates open soil for weeds to invade. The single most effective weed-control practice in most lawns is mowing higher.

Water deeply and infrequently

Shallow watering produces shallow-rooted grass that competes poorly with weeds. Deep watering encourages deep grass roots that crowd out invading weed seedlings. For more, see Watering the Lawn.

Overseed to thicken the lawn

A thick, healthy lawn outcompetes weeds before chemical control is needed. Overseeding cool-season lawns in early fall is one of the highest-leverage practices in lawn improvement.

Fertilize appropriately

Adequate (not excessive) nitrogen produces a vigorous lawn that resists weed invasion. For more, see Lawn Fertilizer: What to Apply, When.

Address soil issues

Many persistent weed problems point to underlying soil conditions. Compacted soil favors plantain and prostrate knotweed. Wet, poorly drained areas favor sedges and creeping charlie. Acidic soil favors moss and certain weed species. A soil test and addressing the underlying condition often resolves a weed problem that herbicide alone would only mask.

When to use chemical control, and how

Pre-emergent crabgrass control in early spring is consistently the highest-value chemical application in cool-season lawns. Broadleaf weed control in fall is consistently the highest-value broadleaf application. These two applications, done at the right time with appropriate products, accomplish most of what is achievable through chemistry.

Spot-treatment of specific weeds is more efficient than blanket application of "weed-and-feed" combination products, which apply herbicide to large areas of lawn that do not need it. The "weed-and-feed" approach is convenient but ecologically heavier than necessary.

For more on the practices that prevent weeds before chemistry becomes necessary, see Watering the Lawn and Lawn Fertilizer.