Johnsongrass

A coarse 3-8 foot grass with a white leaf midrib, thick creeping rhizomes, and an open purplish seedhead - a listed noxious weed in many states that also kills cattle and horses with prussic acid after frost or drought. It is the rhizomes that make it confusable with the sorghum crop, and the frost that makes it lethal.

How it takes over: seed establishes in disturbed ground, then a dense rhizome mat in the top foot of soil spreads laterally and regrows from single-node fragments, so tillage multiplies it and one mowing a year stimulates it.

Nobody plants johnsongrass now, so what matters is how it establishes and why it is so hard to remove. It arrives as seed — in contaminated crop seed and hay, in manure, on tillage and harvest equipment, in floodwater, and in bird droppings — and germinates readily in warm, disturbed, sunny ground. Seedlings are the one genuinely vulnerable stage; a young plant can be killed outright before it builds any underground reserve.

Within a few weeks of establishing it starts producing rhizomes: thick, fleshy, creeping underground stems with purple-brown scaly nodes, concentrated in the top foot of soil and running outward for yards. A single first-year plant can produce a substantial rhizome mat in one season, and every node on it can send up a shoot. A rhizome piece with one node will start a new plant, which is why discing and rototilling an infested field distributes it evenly rather than removing it.

Above ground it grows fast and tall, reaching 3 to 8 feet and shading out crops and pasture grasses. A single plant can set tens of thousands of seeds, which stay viable in soil for several years and in some studies a decade or more. So it is attacking on both fronts at once: an expanding perennial rhizome mat, and a replenished seed bank.

It likes full sun, fertile moist ground, and disturbance, and is at its worst in the warm South and the row-crop Midwest. It is limited mainly by cold — rhizomes are damaged by hard sustained freezing — and by dense shade and vigorous competing perennial cover, which is the biological weakness control strategies are built around.

Control means depleting the rhizomes: cut every three to four weeks all season for two to three years, never till, keep the ground under dense competing cover, and time any systemic herbicide to the boot stage or fall regrowth.

Johnsongrass control is a campaign against the rhizome mat, and every method comes down to the same principle: force the rhizomes to keep spending and never let the leaves pay them back.

Cut on a schedule. Mow or cut low, and repeat every three to four weeks through the whole growing season, for two to three seasons. One mowing a year does nothing except remove the seedheads and stimulate new shoots. If you can only do one thing, cut before the seedheads ripen — that at least stops the seed bank reloading while you work.

Never till or disc it. Every pass cuts the rhizomes into single-node cuttings and plants them across the field. The one exception is a deliberate program used in some agricultural settings, where repeated deep tillage in a hot, dry summer is used specifically to expose rhizomes to desiccation — that only works with sustained dry heat and total commitment, and done half-heartedly it simply multiplies the stand.

Dig small patches properly. In a garden or fence line, work moist soil and follow every rhizome to its end. Bag what you remove; do not compost it and do not move the soil. Then re-check the same ground monthly for a full season, because anything you snapped will resprout.

Compete it out. A dense, tall, vigorous perennial sod suppresses johnsongrass hard. In pasture, manage fertility and grazing so the desirable grass stays thick; in gardens, keep beds mulched and closely planted. This is what stops a cleared area refilling.

If you use herbicide. Timing is the whole game: apply a systemic product at the boot to early heading stage, or on actively growing fall regrowth before frost, when sugars move down to the rhizomes. Spraying short spring shoots wastes the product. Choose a product whose label lists the site you are treating — grass-selective products exist for use in broadleaf crops, non-selective products for spot treatment, and pasture and hay labels carry their own grazing and haying intervals. Follow the label rate, timing, and restrictions exactly; the label is legally binding. Expect repeat treatment for two or more seasons, and note that herbicide-killed johnsongrass can still be dangerous to graze while it is wilting, for the same reason frosted plants are.

Where the poisonings happen: prussic acid largely dissipates as cut forage cures, but nitrate does not - so hay made from a stressed or heavily fertilized stand stays dangerous all winter. Test suspect hay rather than guessing.

Most johnsongrass deaths happen around cut, wilting, or stored forage, so it is worth being precise about which poison behaves how.

Prussic acid (hydrogen cyanide). The plant stores a compound that releases cyanide when leaf cells are ruptured — by chewing, by trampling, by wilting, and above all by frost. Concentrations are highest in young shoots and regrowth under about 18-24 inches, in drought-stressed plants, and in frosted foliage. Death can come within an hour, so this is not something you notice and then react to. The one piece of good news is that cyanide is volatile: it largely dissipates as cut forage dries and cures, so properly made, fully cured hay is much lower risk than the standing plant. Green chop and freshly cut wilting material are the dangerous forms, because the cells have been ruptured and the gas has not yet escaped.

Nitrate. This one works the opposite way and catches people out. Johnsongrass accumulates nitrate, especially after heavy nitrogen fertilization and after a drought broken by rain, and nitrate is stable — it does not break down when the plant is cut, dried, and baled. Hay made from a nitrate-loaded stand stays dangerous for as long as you keep it. Nitrate concentrates in the lower stem, so cutting higher leaves more of it in the field.

Practical rules. Do not graze johnsongrass under 18-24 inches, when it is drought-stressed, or for at least 7 to 14 days after a killing frost — and only once the plants are fully brown and dry, since a light frost can leave dangerous green regrowth below nipped tops. Never turn hungry animals onto it. If you have cut hay off a stand that was stressed or heavily fertilized, have it tested for nitrate before feeding; your extension office or veterinarian can arrange it. Testing costs a fraction of one animal.

Horses have an additional, separate problem: long-term grazing of sorghum species has been associated with a syndrome of hind-limb weakness, incoordination, and urinary incontinence. Keep horses off johnsongrass pasture entirely.

Is it johnsongrass or sorghum, shattercane, or a native grass; why mowing once and tilling both fail; what to do in a garden, a pasture, and a crop field; and how long control actually takes.

Is this johnsongrass or my sorghum crop? Dig it up. Johnsongrass has thick, fleshy, creeping rhizomes with purple-brown scaly nodes; grain sorghum, sudangrass, and sorghum-sudan hybrids are annuals with fibrous roots and no rhizomes. Above ground, johnsongrass carries a large loose open purplish panicle up to a foot long, while the crop heads are tighter and more compact. Both have the white leaf midrib, so the midrib alone will not settle it.

Is it shattercane? Shattercane is a weedy annual sorghum with a looser head than the crop, but it is an annual with no rhizomes, so it cannot come back from underground — preventing seed set genuinely ends it. Digging one plant up answers the question in ten seconds.

Could it be a native grass? Big bluestem, indiangrass and other tall natives are the ones people sometimes wonder about. None of them has johnsongrass's combination of a broad blade with a strong white midrib, thick scaly creeping rhizomes, and a big open purplish panicle; indiangrass has a narrow golden plume, and big bluestem's seedhead splits into the distinctive three-fingered turkey foot.

I mowed it and it came back thicker. Expected. Cutting removes leaves and the rhizomes push up more shoots. Only repeated cutting every three to four weeks, over years, actually depletes them.

I tilled it and now it is everywhere. Also expected, and it is the classic mistake — a rhizome piece with one node becomes a plant. Stop tilling, switch to scheduled cutting plus competition, and treat the whole tilled area as infested.

It is in my garden or along a fence. Dig, following every rhizome to its end in moist soil, bag the material, and re-check monthly for a season. Do not compost it. Keep the ground mulched or densely planted afterwards.

It is in my pasture. Manage the immediate animal risk first — keep stock off short regrowth, drought-stressed stands, and anything frosted — then mow at three-to-four-week intervals and consider a fall systemic treatment on regrowth, using a product whose label lists pasture and observing its grazing and haying intervals.

My animals got into it. What now? Prussic acid poisoning is fast: staggering, laboured breathing, and bright cherry-red mucous membranes and blood, sometimes within minutes. Call a veterinarian immediately and move the rest of the herd off. This is one of the few plant poisonings where minutes genuinely decide the outcome.

How long will control take? Two to three years of consistent work on an established stand, plus watching for seedlings from the seed bank for several more.

Promoted as a miracle forage grass in the 1800s and named for the Alabama planter who spread it, johnsongrass escaped to become one of the world's worst agricultural weeds - and a textbook case of a deliberately introduced crop turning noxious.

Johnsongrass is not an accidental arrival. Native to the Mediterranean and western Asia, it was brought to the American South in the early 1800s and deliberately promoted as a forage and hay grass — it was fast, tall, drought-tough, and productive, which is exactly the profile that also makes a superb weed. It takes its name from Colonel William Johnson, an Alabama plantation owner who planted it widely on his river-bottom land in the 1840s and encouraged neighbours to do the same. The name stuck as the plant spread.

Within a few decades the enthusiasm curdled. Farmers found that the rhizomes made it impossible to remove from cropland, that it invaded everything around the fields it was sown in, and that it killed livestock after frost and drought. By the early twentieth century it was recognized as a serious pest, and it now appears on state noxious-weed lists in many states, restricting its sale and movement, and it is ranked among the worst agricultural weeds in the world.

It has one more modern twist. Johnsongrass can hybridize with cultivated sorghum, and the offspring can carry crop traits into a rhizomatous perennial weed — a real concern wherever herbicide-tolerant sorghum is grown near johnsongrass stands. The whole arc is one of the clearest cautionary tales in American agriculture: a plant chosen for vigour, planted on purpose, and impossible to take back.

Worst across the warm South and the row-crop Midwest (roughly zones 6-11), limited northward by rhizome-killing winters; the control calendar shifts with season length, and the frost-poisoning risk arrives earlier the further north you are.

Johnsongrass is a warm-season grass and its range reflects that, running roughly from zone 6 through zone 11 and thinning out northward as winters get hard enough to kill rhizomes outright.

The South and Gulf states: its heartland. Long hot seasons mean multiple growth flushes, so the three-to-four-week cutting cycle has to start early and run late, and there is more of everything to do. Frost comes late, which means the dangerous frosted-foliage window arrives at the end of a very long growing season.

The row-crop Midwest and mid-South: where it does the most economic damage, moving through corn, soybean, and sorghum ground along tilled edges and waterways. This is also where crop hybridization with cultivated sorghum matters most.

Zone 6 and the northern edge: cold winters damage rhizomes in the top foot of soil, so stands are patchier and often confined to sheltered ground, south-facing slopes, and warm urban edges. Frost arrives early, which means the most dangerous grazing weeks come sooner — keep the 7-to-14-days-after-a-killing-frost rule in mind well before you would in the South.

The arid West: established mainly along irrigated ground, canal banks, and river bottoms, where it can be aggressive despite the surrounding dryness. Drought stress in unirrigated stands raises prussic acid risk sharply.

A warming note: milder winters have been letting johnsongrass persist further north than it once did, so a stand appearing at the edge of its historical range is worth taking seriously rather than assuming winter will handle it.

Everywhere the method is the same: never till, cut every three to four weeks, keep the ground densely covered, treat at boot stage or on fall regrowth, and keep animals off short, stressed, or frosted foliage.