Johnsongrass

About

A tall, aggressive perennial grass from the Mediterranean, brought in as forage in the early 1800s. Thick rhizomes let it take over gardens and field edges, and stressed leaves can poison grazing animals.

Also called Johnson grass

How to identify it

Look for

Stands 2 1/2 to 7 feet tall with smooth leaves up to 2 feet long and a bold white midrib. The ligule is a membrane with a hairy fringe. Large, open, purplish seedheads appear in summer, and the rhizomes are thick, fleshy, cream-colored and often spotted with purple.

Look-alikes

Young plants look like corn or sudangrass. Fall panicum is another look-alike; the white midrib and thick rhizomes point to johnsongrass.

Where it grows

Field edges, ditches, roadsides, fence lines, vegetable gardens and neglected lots in full sun.

When

Shoots rise from rhizomes in mid-spring and from seed soon after, flowers from early summer into fall, and ripens seed in September and October.

How it spreads

Spreads by hundreds of seeds per plant and by thick rhizomes whose pieces sprout new plants; equipment moves both.

Most common in

Where it is regulated

Listed as a noxious weed in about 19 states, including Illinois and North Carolina.

What to do, gentlest first

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

1. Mow before it flowers Prevent

   Repeated mowing keeps it from setting seed, but it won't kill the rhizomes. Clean tools and equipment after working in patches.

2. Dig young plants and rhizomes By hand

   Dig young plants with their rhizomes. Repeated digging or hoeing of new shoots drains the food stored in the roots.

   Chopped rhizomes regrow, so remove all the pieces you can.

3. Glyphosate or a grass-selective spray Conventional spray

   From June to August, spot-spray glyphosate on plants at least 18 inches tall, up to early flowering. Grass-selective clethodim or sethoxydim can be used around many broadleaf plants.

   Read and follow the label. Many lawn herbicides kill garden plants, so keep spray and drift off beds and vegetables.

Uses, wildlife and pets

Pets

Not listed by the ASPCA, which does list its relative Sudan grass as toxic to horses. After frost or drought, johnsongrass can build up prussic acid (cyanide) and nitrates that poison livestock, and Oklahoma State Extension advises never putting horses on it.