Ontario Weeds: Johnson grass

Table of Contents

1. Name
2. Other Names
3. Family
4. General Description
   • Photos and Pictures
5. Stems and Roots
6. Flowers and Fruit
7. Habitat
8. Similar Species
9. Related Links

Name: Johnson grass, Sorghum halepense (L.) Pers.,

Other Names: sorgho d'Alep, sorgho de Johnson

Family: Grass Family (Gramineae)

General Description: Perennial reproducing by seed and by large, coarse underground stems (rhizomes).

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Photos and Pictures

Johnson grass (A - infestation in field; B - large whitish rhizomes produced after plants head out).

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Stems & Roots: Stems 50-270cm (2-9ft.) tall, 5-20mm (1/5-4/5in.) in diameter, smooth and stiff or wiry; leaves 20-60cm (8-24in.) long, 1-2cm (2/5-4/5in.) wide, bright green, and smooth; leaf sheaths split with smooth overlapping margins; ligule membranous, 2-5mm (1/12-1/5in.) long; no auricles; collar (junction between leaf blade and leaf sheath) often with purplish blotches on its sides; rhizomes thick and fleshy, at first white to pinkish, but turning chestnut brown over winter, as much as 1cm (2/5in.) in diameter and up to 1m (40in.) long, with many nodes and inter-nodes, frequently rooting from the nodes, and the internodes partially covered with brown scale-like sheaths.

Flowers & Fruit: Inflorescence up to 50cm (2ft.) long, with whorls of upright branches; at first compact, later spreading and open; spikelets arranged in pairs at each node along the thin branches, 1 member of the pair is sterile (non-seed bearing), rather dull in texture and has a short stalk; the second member of each pair is fertile (contains the grain or "seed"), rather plump, about 5mm (1/5in.) long by 2.5mm (1/10in.) wide, does not have a stalk, and may have a twisted awn 1-1.5cm (2/5-3/5in.) long. Flowers from July to September.

Habitat: Johnson grass is presently known to occur in 13 counties in southern and southwestern Ontario.

Similar Species: Young plants of Johnson grass might be mistaken for very thin plants of corn, Sudan grass, or an annual grain sorghum. They can be distinguished from corn by the size and shape of the mother seed attached to the primary root of the seedling, or if growing as a shoot from an overwintered rhizome, by the presence of the thick, hard, brownish rhizome. Older plants can be distinguished by their large, open, non-silky inflorescences with plump seeds, and in autumn by the presence of coarse, whitish, rope-like, branching rhizomes (horizontal underground stems) about 1cm (2/5in.) in diameter in late autumn. Most rhizomes are killed by Ontario winters, but 2 or 3 stands are apparently hardy enough that their rhizomes are not killed, and these plants are acting as true perennials. Also compare with Common reed.

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Related Links

... on general Weed topics
... on weed identification, order OMAFRA Publication 505: Ontario Weeds
... on weed control, order OMAFRA Publication 75: Guide To Weed Control

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