Johnson grass (Sorghum halepense) is a perennial grass recognized globally as one of the world’s most aggressive and problematic weeds. Originally introduced as a forage crop, it quickly escaped cultivation and established itself as a persistent threat in agricultural and natural environments. Its ability to rapidly colonize and dominate fields has earned it a reputation among the ten most troublesome weeds worldwide. This article explores the nature of Johnson grass, examining its physical characteristics, its dual reproductive strategy, the consequences it poses to farming and livestock health, and the integrated strategies required for its control.
Physical Identification and Origin
Johnson grass is a robust, warm-season perennial grass that can reach impressive heights of six to ten feet. Its leaves are linear and lance-shaped, characterized by a prominent white midrib that runs down the center of the blade. The plant’s mature seed head, known as a panicle, is open and loosely branched, often displaying a distinctive purplish or reddish-brown tint.
The plant is native to the Mediterranean region and Western Asia. Its widespread global distribution began with human intervention when it was intentionally introduced to the Southern United States in the early 1800s, primarily as a perennial forage crop. It quickly escaped cultivation through contaminated seed and the movement of its underground structures, establishing itself in nearly every continental state and becoming a noxious weed.
Biological Mechanism of Persistence
The difficulty in eradicating Johnson grass stems from its highly effective, dual reproductive strategy involving both seeds and underground stems called rhizomes. The extensive network of fibrous, branching rhizomes allows the plant to survive harsh conditions and regenerate rapidly. These rhizomes are cream or tan, can grow to a half-inch or more in diameter, and can spread for hundreds of feet each year, creating a dense, tangled sod.
Rhizome segments store energy reserves, and each piece is capable of sprouting a new plant. Mechanical tillage can inadvertently spread the infestation by fragmenting the underground network. New rhizomes begin growing early in the season, and a single plant can produce 200 to 300 feet of new rhizomes annually, allowing new shoots to quickly outgrow competing vegetation and aggressively colonize a large area.
In addition to its rhizome system, Johnson grass is a prolific seed producer, contributing to a persistent soil seed bank. A single plant can produce thousands of seeds, which are dispersed by water, machinery, and contaminated grain. These seeds can remain viable in the soil for up to six years, ensuring that new seedlings emerge long after the parent plants have been controlled. This combination of regenerating rhizomes and long-lived seeds makes complete eradication extremely difficult with a single control method.
Agricultural and Health Consequences
The presence of Johnson grass poses significant threats to agriculture, primarily through aggressive competition with commercial crops. The weed’s high growth rate and extensive rhizome system allow it to aggressively compete with row crops, such as corn, cotton, and soybeans, for limited light, water, and soil nutrients. This competition leads directly to substantial crop yield reductions and significant economic losses for farmers.
Beyond crop interference, Johnson grass presents a health risk to livestock, particularly cattle, due to potential toxicity. When the plant is subjected to stress from conditions like drought, frost, or physical damage, it can accumulate toxic levels of hydrocyanic acid, also known as prussic acid. This compound is released when the plant’s cells are broken down, preventing the animal’s blood cells from releasing oxygen to body tissues, causing rapid death from asphyxiation.
Prussic acid concentration is highest in the leaves of young, new growth or in wilted forage. The plant can also accumulate nitrates to toxic levels, especially when heavily fertilized with nitrogen, creating a dual poisoning threat. While well-cured hay is considered safer regarding prussic acid, high nitrate levels can persist in dried forage, requiring careful testing before feeding to ensure animal safety.
Management and Control Techniques
Effective control of Johnson grass requires an integrated, long-term management approach that targets both the rhizomes and the seed bank. Chemical control often involves the targeted application of systemic herbicides like glyphosate, which must be timed correctly to translocate the chemical deep into the rhizome system. Post-emergence, grass-selective herbicides such as clethodim and fluazifop are also used, particularly in broadleaf crops, to control new growth without harming surrounding plants.
Mechanical control techniques weaken the plant and prevent seed production. Repeated, close mowing can deplete the rhizome’s stored energy reserves and stop the plant from setting seed. Deep tillage in the fall can bring rhizomes to the soil surface, exposing them to winter freezing temperatures. Tillage must be frequent, as a single pass can fragment and spread the rhizomes, worsening the infestation.
Cultural control methods focus on suppressing the weed through competitive planting and field practices. Planting highly competitive cover crops or using dense crop rotation cycles can help shade the soil and reduce Johnson grass establishment. Sanitation is also crucial, involving the cleaning of farm machinery and using certified, weed-free seeds to prevent the introduction and spread of seeds and rhizome fragments into new areas.