What is horseweed?

Horseweed, also called marestail, is a native winter annual that jumps on bare, disturbed ground. Basic horseweed identification is easy once it bolts: a single tall stem crowded with narrow, hairy leaves, shaped a bit like a horse’s tail or a bottle brush, with a spray of tiny white flowers on top.

For homeowners it’s mostly a weed of unfinished landscapes, new construction lots, gravel areas and neglected beds. For PA farmers it’s a much bigger deal, because glyphosate-resistant horseweed is now widespread across the state, and some populations resist other herbicide groups too.

It isn’t poisonous and it isn’t a lawn invader, but a single missed plant can seed a whole area for next year. A few minutes of pulling at the right time saves a lot of work later.

Horseweed identification: what to look for

Horseweed looks completely different at different times of year, so it helps to know both stages.

• Rosette (fall to early spring): a flat, circular cluster of small, hairy, slightly toothed leaves hugging the ground. It sits like this through winter.
• Bolting stem (late spring to summer): a single upright stem shoots up, covered from bottom to top with narrow, hairy leaves that shrink toward the tip. Mature plants are usually 3 to 6 feet tall.
• Flowers (midsummer into fall): the top of the stem branches into many tiny flower heads with whitish rays and yellowish centers.
• Seeds: small, with fluffy white bristles, and a single plant can release a huge number of them on the wind.

The stem usually stays unbranched until near the top, which gives a stand of horseweed its stiff, bristly look.

Where it grows in Pennsylvania

Horseweed grows statewide wherever there’s open, disturbed soil and full sun. In central PA you’ll see it in no-till crop fields, along highways, in gravel lots and on freshly graded construction sites.

Around the house it moves into any ground that sits bare through fall: a new lawn that didn’t get seeded before winter, a pile of topsoil, the gravel area beside a shed or a garden left unmulched after harvest. Because its seed blows in from far away, a yard can go from no horseweed to a full stand in one season. It rarely survives in a regularly mowed lawn.

Horseweed look-alikes

Native daisy fleabanes are close relatives and grow in the same places. They have fewer, broader leaves and much bigger, showier white daisy flowers, so they’re easy to separate once they bloom.

Common ragweed shares the same disturbed ground but has fern-like, deeply divided leaves and green pollen spikes. Mugwort has lobed leaves with silvery undersides and a strong sage smell, and it spreads in patches by underground stems.

How to get rid of horseweed

Horseweed is an annual with a simple taproot, so you don’t need to dig deep. The trick is getting it before it gets tall and goes to seed.

Hand pulling and hoeing. In yards and gardens, this is usually all you need. Rosettes come out easily in fall or early spring. Taller plants pull fine from moist soil too; just get them before they flower in midsummer. Mowing slows bolted plants but doesn’t stop them, and they often branch and flower lower down.

Cultural control. Don’t leave soil bare over fall and winter. Seed new lawns or cover graded areas with straw and grass seed as soon as you can, mulch garden beds, or plant a cover crop such as cereal rye in vegetable gardens. A dense lawn or crop canopy keeps most seeds from ever taking hold.

Chemical control. If you do use herbicide, timing matters more than the product. Extension guidance says to treat plants while they’re small rosettes, under about 5 inches tall, because larger plants are very hard to kill. Glyphosate alone is not reliable in Pennsylvania because resistant horseweed is so common. Broadleaf herbicides containing 2,4-D or dicamba can work on young rosettes where those products are labeled for the site.

• Read and follow the product label. The label is the law, and it tells you whether the product is allowed on lawns, beds, gravel or bare ground.
• Applying herbicide for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, so large lots or fields go to a licensed applicator.

Keeping it from coming back

The best prevention is simply keeping soil covered. Finish landscape projects with seed, mulch or plantings before fall, and keep gravel areas raked and maintained.

If you have a gravel lot or path where horseweed keeps sprouting, landscape fabric under a proper depth of stone cuts down on new seedlings; our guide to landscape fabric under gravel covers when it helps. Check bare areas each fall and spring and pull any rosettes you find.

Questions we get about horseweed (marestail)

Similar weeds

Common RagweedAmbrosia artemisiifoliaPrickly LettuceLactuca serriolaGiant RagweedAmbrosia trifida