Johnsongrass: complete guide

Overview

Johnsongrass (Sorghum halepense (L.) Pers.), also known as Aleppo grass or wild sorghum, is an aggressively growing grass from the Poaceae family. Native to Mediterranean and Central Asia regions, it has naturalized in many areas worldwide where it causes severe agricultural problems.

Johnsongrass is notorious for its incredible ability to spread via seeds and underground rhizomes. In many areas, it is considered a serious invasive weed causing significant agricultural productivity losses. This article covers identification and control strategies for gardeners and farmers.

Appearance and bloom

Johnsongrass grows as an upright perennial grass from 0.5-1.5 meters tall (sometimes reaching 2 meters). The stems are sturdy, green to dark green, smooth and hollow inside. The leaves are long, to 50 cm, and 5-15 mm wide with a distinct midvein.

The leaves are green to yellow-green, sometimes with red venation on young tops. They grow alternately along the stem. The base of each plant is surrounded by numerous daughter plants emerging from underground rhizomes.

Flower heads appear at stem tops from July to October, depending on region. These are feathery, loose and open, 20-50 cm long. Each individual flower is small and green to purplish. The plant produces thousands of seeds per stem.

An important feature: Johnsongrass forms thick underground rhizomes (horizontal stems) that can extend 2-3 meters per year. These rhizomes can survive years in the soil and produce new plants.

Ideal location

Johnsongrass grows in practically all locations: full sun, partial shade, wet areas, dry areas. The plant is highly adaptable. It grows well along roadsides, ditches, field edges, abandoned areas and anywhere soil is disturbed.

The plant prefers moderately warm temperatures and grows optimally in temperate to subtropical climates.

Soil

Johnsongrass tolerates virtually all soil types: sand, loam, clay, salt. It even grows on very poor and polluted soils. The plant grows well from pH 4-9.

The plant can establish on disturbed, compacted soils where other plants struggle. This is a key feature of its invasiveness: it can establish where conventional agriculture cannot.

Watering

Johnsongrass has moderate water requirements but tolerates both drought and water saturation. Shortly after establishment, it grows rapidly if water is available. Once established, it overwinters easily via underground rhizomes.

The plant grows well along banks and in marshes but also performs well in dry situations.

Pruning and control

Pruning does not worsen Johnsongrass problems; in fact, it can stimulate more growth if rhizomes are damaged. Thus mechanical control is difficult.

Effective control requires:

1. Herbicides: Specific non-selective herbicides are needed (professional advice)
2. Digging: Complete root removal of all rhizomes (very laborious)
3. Exclusion: Prevent spreading via seeds and rhizomes
4. Mulch coverage: Black plastic for 12+ months can kill rhizomes

Maintenance calendar (control)

Spring: Early detection of new growth. Remove small plants before flowering. Begin herbicide application on new shoots.

Summer: Monitor regularly. Remove flowering stems before seed formation. Herbicides are most effective during growth.

Autumn: Remove all dead stems and flowers before seed spread. Check underground rhizomes.

Winter: In milder climates, plant area continues growing. Plan long-term control strategy.

Winter hardiness

Johnsongrass is winter hardy to zone 6, sometimes zone 5. Underground rhizomes survive hard winters and regrow in spring. This is part of its invasiveness.

Warnings

Johnsongrass can accumulate toxic substances (hydrocyanic cyanide) in leaves under certain conditions. This makes it dangerous for livestock. The plant must be carefully handled around animal areas.

In many regions, Johnsongrass is formally declared an invasive species and must not be deliberately cultivated. Check local regulations.

Closing remarks

Johnsongrass is not recommended for gardens or landscaping. It is a serious invasive weed causing incredible damage to agriculture. Prevention is the best policy.

See local agricultural services for professional control advice. For more information, visit www.gardenworld.app/en.

Protecting your garden starts with preventing invasive species! Visit www.gardenworld.app/en for advice.