Giant ragweed

About

Giant ragweed is a huge native annual, usually 3 to 12 feet tall, that shades out anything near it. Its wind-blown pollen causes severe hay fever, and some farm populations resist glyphosate and other herbicides.

Also called great ragweed

How to identify it

Look for

Large, rough, sandpapery leaves grow in opposite pairs and are usually split into 3 to 5 deep lobes like a hand. Stems are tall, stout and rough. Slender spikes of green male flowers top the plant in summer, and the seed is a large, crown-shaped husk about 1/3 inch long.

Look-alikes

Young plants whose first leaves aren't lobed can look like cocklebur or sunflower; common ragweed seedlings look similar but are less than half the size.

Where it grows

Rich, moist bottomland, stream banks, field edges, fencerows and gardens, especially in the Corn Belt.

When

One of the first summer annuals to sprout, in early to late spring; it flowers in midsummer and ripens seed in late summer.

How it spreads

Seed only: from about 150 seeds per crowded plant to a few thousand on open-grown ones; most seeds are gone from the soil within two years.

Most common in

Where it is regulated

Listed as a noxious weed in Delaware and Illinois.

What to do, gentlest first

Start at the top. Weed killers are a last step; always read and follow the label.

1. Cut or pull before the pollen and seed Prevent

   Remove plants before they flower in midsummer. Because few seeds last more than two years in the soil, two seasons of stopping seed can clear a patch.

   The pollen causes severe hay fever in sensitive people.

2. Pull seedlings early By hand

   Pull or hoe in spring while plants are small; big plants have tough stems and are much harder to remove.

   Wear gloves and long sleeves; the leaves and stems are rough.

3. Mulch beds Barrier

   Seeds sprout best from half an inch to 2 inches deep and rarely from deeper than 4 inches, so a thick mulch over bare soil helps keep seedlings down.

4. Burn down seedlings with a contact spray Organic spray

   Horticultural vinegar (acetic acid), fatty-acid soap, pelargonic acid or clove-oil sprays burn what they touch within hours. They work best on young seedlings in warm, sunny weather, and you will need to repeat, because they don't move into roots and bigger plants regrow.

   They kill any plant they touch, so shield crops. Strong vinegar can burn skin and eyes; wear goggles and gloves and follow the label.

5. Spot spray young plants Conventional spray

   In lawns, a broadleaf herbicide such as 2,4-D; in beds, spot spray glyphosate on the weed only. Some farm populations resist glyphosate and ALS herbicides, so pull any survivors.

   Read and follow the label. Many lawn weed killers also kill or damage vegetables and flowers, so keep spray and drift off garden plants.

Uses, wildlife and pets

For wildlife

Rodents eat large numbers of the fallen seeds over winter (SARE).

Pets

Not listed by the ASPCA.