Johnsongrass

Quick facts

For identification. Not a growing guide.

How to recognize it in each season.

1. 1

   Spring emergence (from seed and from rhizomes)

   Mar to May, every year

   • Recognize it: a big coarse grass with a white midrib
     Johnsongrass comes up as a stout, coarse grass with broad blades half an inch to an inch wide, each carrying a conspicuous whitish midrib down the centre. Shoots from rhizomes appear first and grow fast; seedlings are slimmer and come later. Dig one up: if there is a thick, fleshy, creeping underground stem with purple-brown scaly nodes attached, it is johnsongrass and not a sorghum crop plant or shattercane.
   • Keep animals off short spring regrowth
     This is the highest-risk foliage of the year. Young shoots and regrowth under about 18-24 inches carry the highest concentration of the compound that releases hydrogen cyanide, and cattle, sheep, and horses can die within an hour of grazing it. Do not turn animals onto a paddock with short johnsongrass regrowth, and do not offer freshly pulled plants to livestock as green feed.

   Key signs

   • Broad blades with a white midrib, coming up faster than surrounding grass
   • Thick scaly rhizomes when you dig a shoot
   • Short regrowth in any paddock or hay field - the peak prussic acid risk
2. 2

   Rapid growth to 3-8 feet

   May to Jul, every year

   • Mow or cut repeatedly - and start before the seedheads form
     A single mowing does nothing but stimulate rhizome shoots. Cut the stand low and repeat every three to four weeks through the whole season, for two to three seasons, to run the rhizome reserves down. In a garden or fence line, dig entire rhizome systems out when the soil is moist and follow the runners as far as they go, then check the same ground monthly - anything you snapped off will resprout.
   • Do not till or disc an infested area
     Rhizome pieces with a single node will start new plants, so a tiller or disc turns one patch into a field of evenly spaced plants. This is how johnsongrass moved through row-crop country. If ground must be worked, do it once and commit to relentless follow-up; otherwise leave it undisturbed and rely on cutting and competition.
   • Check for prussic acid risk before any grazing
     Cyanide risk stays high in fast-growing young foliage and rises again in any drought-stressed stand. The standard rule is not to graze johnsongrass under 18-24 inches, not to graze drought-stressed stands, and not to turn hungry animals onto it. If in doubt, cut a sample and have your extension office or vet arrange a prussic acid or nitrate test rather than guessing.

   Key signs

   • Stems reaching 3-8 feet and shading out the crop or pasture
   • Rhizomes running along tilled edges and fence lines
   • Drought stress, which raises cyanide risk in foliage that looks fine
3. 3

   Open purplish seedhead (stop it here)

   Jun to Sep, every year

   • The seedhead is the clearest ID against sorghum
     Johnsongrass carries a large, loose, OPEN purplish panicle up to a foot long, with branches spreading well away from the stalk. Grain sorghum, sudangrass, and shattercane all carry a tighter, more compact head. Combined with the rhizomes underground, the open panicle is what separates the weed from the crop - and shattercane, which also has a looser head, is an annual with no rhizomes at all.
   • Cut seedheads before they ripen
     A single plant can set tens of thousands of seeds, and buried seed stays viable for several years - some studies find a decade or more. Cutting or mowing before the heads colour and harden is the highest-value hour you will spend, because it stops the patch reloading the soil seed bank while you work on the rhizomes.
   • If you spray, spray at boot stage or in early fall
     Systemic herbicide only reaches the rhizomes when sugars are moving downward - at the boot-to-early-heading stage, and again on actively growing regrowth in early fall before frost. Choose a product whose label lists your site: grass-selective products exist for broadleaf crops, non-selective products for spot treatment, and pasture labels carry grazing and haying restrictions. Follow the label exactly - it is legally binding.

   Key signs

   • Large open purplish panicles - the diagnostic seedhead
   • Seed colouring and hardening - cut before this
   • Boot stage and fall regrowth - the two windows when systemic treatment reaches the rhizomes
4. 4

   Frost and winter (the most dangerous weeks of the year)

   Oct to Feb, every year

   • After a frost, keep every animal off it
     Frost ruptures the plant's cells and releases a burst of hydrogen cyanide into wilting foliage. This is when most johnsongrass poisonings happen, because the field looks finished for the year. Keep livestock off frosted johnsongrass until the plants are completely brown and dry - commonly given as at least 7 to 14 days after a killing frost - and remember that a light frost that only nips the tops can leave dangerous green regrowth lower down.
   • The tops die, the rhizomes survive the winter
     Top growth is killed by hard frost, but the rhizome network - a dense mat of thick underground stems in the top foot of soil, loaded with the sugars pulled down in fall - overwinters intact through most of its range and resprouts vigorously. Winter is for planning next season's cutting schedule and marking the patch boundary, not for tilling.

   Key signs

   • Frosted, wilting green foliage - the single most dangerous state for grazing animals
   • Regrowth after a light frost, which is more dangerous than the frosted tops
   • Rhizome mats in the top foot of soil, alive under dead stalks

Hazards