What is common ragweed?

Common ragweed is a native summer annual that comes up fast on any patch of bare, disturbed soil. It’s a weed of gardens, new construction, field edges and thin lawns, and it’s best known for the pollen that fuels fall hay fever. Common ragweed identification is worth learning because pulling a few plants early in the summer is far easier than dealing with a full-grown stand in August.

Ragweed is native to Pennsylvania and does feed some birds and insects, so it isn’t evil in the wild. In a yard or garden, though, there’s little reason to keep it around.

Common ragweed identification: what it looks like

Seedlings show up early in spring, often before most summer weeds. They have thick, rounded seed leaves and a stem that may be spotted purple, followed by finely divided true leaves.

• Leaves: Deeply cut and fern-like, a little hairy and rough. The lower leaves sit in opposite pairs; leaves higher on the stem switch to alternate.
• Stems: Branching and hairy, forming a bushy plant usually 1 to 3 feet tall, though it can top 5 feet on rich soil.
• Flowers: No petals. In August, slender spikes of tiny greenish-yellow, nodding flower heads appear at the branch tips. These are the male flowers that shed pollen. Female flowers are small and hidden where the upper leaves meet the stem.
• Seed: A small, hard, top-shaped seed with a crown of little points.

A mowed plant can still flower at just a couple of inches tall, so don’t assume a low plant isn’t ragweed.

Where it grows in Pennsylvania

Anywhere soil has been turned over, ragweed is ready. New lawns, freshly graded yards, utility trenches, roadside ditches and garden beds left bare are classic spots. It tolerates compacted, dry or poor soil and partial shade, so it’s not picky.

The seed is long-lived in the ground. Dig into a spot that hasn’t been disturbed in years and a flush of ragweed often comes up the next spring from seed that had been buried there.

That’s why it’s so common the year after a septic repair, a new patio or a regrading job. The soil isn’t dirty or contaminated; old seed simply got brought back up to the light. A single healthy plant can drop thousands of seeds, so letting even a few go to seed restocks the ground for years.

Common ragweed look-alikes

Goldenrod gets blamed for ragweed’s hay fever because it blooms bright yellow at the same time. Goldenrod is insect-pollinated, has showy flowers and simple, narrow leaves. Ragweed has no showy flowers at all.

Giant ragweed is the bigger cousin, with large three- to five-lobed leaves. Mugwort has similar cut leaves, but they’re silvery and woolly underneath and smell like sage; it’s a perennial that spreads by roots.

How to get rid of common ragweed

The good news: ragweed is an annual, so it doesn’t come back from the root. Kill this year’s plants before they set seed and you’re winning.

• Pull or hoe: Young plants pull out easily, especially after a rain. Hoeing seedlings in May and June is quick work. If you’re sensitive, wear gloves, since the leaves can irritate skin, and pull before flowering to avoid a face full of pollen.
• Mow: Mowing reduces size but plants regrow from the base and can still flower low. Pull or dig rather than rely on mowing in a garden or bed.
• Mulch and cover: In beds, a 2 to 3 inch layer of shredded mulch keeps most seed from sprouting. On bare ground, seed grass or plant something right away.
• In lawns: Ragweed rarely survives in thick, regularly mowed turf. If it’s there, the lawn is thin. Broadleaf lawn herbicides containing 2,4-D, dicamba or MCPP work well on young, actively growing plants in late spring and early summer.
• In beds: A preemergence herbicide labeled for ornamental beds can stop seedlings if applied before they emerge in early spring; check that it’s labeled for your plants.

Always read and follow the product label; the label is the law. Make sure the product is labeled for the site, whether that’s lawn, landscape beds or near water. Applying herbicide for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, so larger jobs should go to a licensed lawn-care company.

Keeping ragweed from coming back

Ragweed is a bare-soil weed. The best prevention is simply not leaving ground exposed: mulch beds, keep turf thick and mowed at a healthy height, and seed or plant disturbed areas promptly after any digging or grading.

Because seed lingers in the soil, expect some new plants for a few years after a construction project. Pull them early each summer and the seed bank will drop off over time.

Questions we get about common ragweed

Similar weeds

Giant RagweedAmbrosia trifidaHorseweed (Marestail)Erigeron canadensisCommon LambsquartersChenopodium album