Ornamental · Poaceae
Sorghum-sudangrass
Sorghum × drummondii
A coarse, towering summer grass with broad green blades and thick, cane-like stalks, carrying a real cyanide warning for grazing animals.
Full sun
Every 7–14 days
45–90 days
USDA 3–11
Overview
Sorghum-sudangrass grows into a wall of coarse green, a hybrid of sorghum and sudangrass built more for bulk than beauty. It is a fast, heat-loving annual, pushing from seed to several feet of thick, leafy growth in a matter of weeks. Broad blades stack up the stems in a dense stand that growers keep as a cover crop and forage, where the mass of foliage shades the ground and the spent crop returns organic matter to it.
The part that does the heavy work stays hidden below the blades. Overhead, the tall canopy of broad leaves throws deep shade that buries weed seedlings, and a single mid-season cut at 3 to 4 feet sends the roots thickening and driving down to fracture packed soil. Those roots weep sorgoleone, a natural compound that holds many weed seeds shut, so a rough patch grows back green and even.
Behind the lush green sits a real poison. The blades and stems of this grass, like its sorghum kin, carry cyanogenic compounds that release prussic acid, the cyanide form, and they pack in nitrate whenever the plant is stressed. That makes the growing foliage dangerous to horses and grazing livestock, worst of all while it is young, drought-shrunk, wilted, or frosted. The green you see is grown for the soil, and any grazing on it is a real hazard.
Care
The short version: it wants full sun, warm soil, and little more. Drop seed after frost into well-drained ground, water it rarely since the blades ride out drought, and cut the stand at 3 to 4 feet before the stems harden to wood.
Light
It stands in full sun and open heat and grows fastest there. The broad green blades widen and the stems shoot up quickest where summers run hot and nothing shades the plant, so give it a bright, exposed patch of ground.
Water
Once its roots take hold, the plant is drought-tolerant, and in a dry spell the blades roll toward dormancy rather than dying back, so water reaches it only every 7 to 14 days and not at all when rain does. Standing water and soggy ground it cannot take. Note too that drought-shrunk plants hold the most cyanide in their tissue and must not be grazed.
Soil
Its roots settle into almost any well-drained loam and spread across a wide pH from 5.5 to 9.0. The one ground it will not take is waterlogged soil.
Temperature
The blades keep growing through real heat, across roughly 32°F to 100°F (0°C to 38°C), and the stand looks its greenest and tallest near the top of that range. Frost, though, kills it outright, and the blackened, collapsed stand is how the cover crop ends in fall.
Humidity
The plant pays no mind to humidity, and plain outdoor air is all its blades ever need. What swells the leaves and lifts the stems is heat and sun, not moisture hanging in the air.
Feeding
Nitrogen at planting, about 75 to 100 pounds per acre, fills out the tall, leafy biomass. Hold back, though, since heavy nitrogen against low phosphorus and potassium loads more cyanide and nitrate into the green tissue.
Pruning
One clean mow-kill at about 3 to 4 feet is the key cut, and it drives a fresh surge of roots and tillers that break compaction and thicken the stand. Cut before the plant climbs past 4 feet toward its full 5 to 12 feet, or the stems coarsen into woody, cane-hard stalks that resist the blade and rot down slowly.
Repotting
none
Size & spacing
In a close stand it towers to 5 to 12 feet (1.5 to 3.7 m) when left uncut, each plant a fan of blades about 1 to 3 feet (30 to 91 cm) across. That crowded wall of foliage is the whole point, since its shade is what buries the weeds.
When to plant
This is a summer grass through and through, with no stomach for cold. Sow it in late spring or summer once frost has gone and the soil warms past 60°F to 65°F (16°C to 18°C), let the blades pile up through the heat, and end it with a cut or the first fall frost that turns the whole stand black.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: not recommended
- Plant: Late spring to summer after all frost danger has passed and soil is warm (above about 60-65 degrees F)
- Harvest: Mow or harvest at about 3-4 ft; grown for biomass/cover, not bloom
Hardiness
USDA hardiness zones: 3–11
Temperature: down to -40°C
Troubleshooting
Most of the trouble with this grass is one of two things: a poisoning risk to animals, or a plant that has grown past what you can handle. Watch the growth stage, because the same green forage that is useful at one height turns dangerous or unmanageable at another. Match what you see below and act before it becomes a problem.
Grazing animals crumple, gasping, in minutes
This is acute cyanide (prussic acid) poisoning, the deadliest thing hidden in this grass. The green tissue releases hydrogen cyanide from a compound called dhurrin, and it runs highest in young blades under 18 to 24 inches, in the tender regrowth after a cut, and in stems left drought-shrunk, wilted, or frosted. Signs strike within minutes and animals seldom last two hours, so keep livestock off that growth, wait 5 to 7 days after a frost, and reach a vet the instant you suspect it.
Fix:
- This is a veterinary emergency - contact a veterinarian immediately
- Remove all animals from the forage at once
- For pets, call ASPCA Animal Poison Control at (888) 426-4435
Stems coarsen into thick, woody, cane-hard stalks
Left past 4 feet, the stand shoots toward its full 5 to 12 feet and the stems thicken and harden into coarse, woody stalks as much as half an inch across. That cane-like mass blunts the mower, breaks down slowly, and litters the ground into spring. Cut at about 3 to 4 feet before the stalks stiffen, then terminate the plant or let winter blacken it so the residue has time to rot.
Fix:
- Mow at about 3-4 ft before stalks turn woody
- Terminate (mow or let winterkill) before residue becomes excessive
- Chop or mow finely to speed breakdown
Stressed leaves store nitrate, and frost blackens the crop
Drought and heavy nitrogen pack nitrate into the leaf tissue, and that stressed forage can poison livestock on its own, on top of the cyanide. Test any stressed or heavily fed growth before feeding, and dilute or set aside blades that test high. As a frost-tender warm-season annual, the plant blackens and collapses at the first hard freeze, which is the usual end of the cover crop.
Fix:
- Test stressed forage before feeding
- Dilute or avoid feeding high-nitrate forage
- Accept winterkill as the cover-crop termination method
Toxicity
Take this one seriously. The plant releases prussic acid (cyanide) and stores nitrate in its green tissue, and both can kill horses and grazing livestock fast, worst of all in young, drought-stressed, wilted, or frosted growth. House pets and people face lower risk because they do not graze it, but the foliage is not safe to chew, so keep animals and children away from it.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Caution
The ASPCA lists Sudan grass as non-toxic to cats, so a bite of a green blade is not the danger here. The cyanide still gathers in young, wilted, or frosted growth, so keep a cat from chewing on stressed plants.
Dogs Caution
The ASPCA does not flag this grass as toxic to dogs, and most dogs pass over the coarse blades. The real hazard lies in stressed or frost-bitten foliage, so steer a grass-chewing dog away from it just in case.
Horses Toxic
Horses suffer the most. Past acute cyanide poisoning with labored breathing, foaming, and tremors, they develop a distinct cystitis and ataxia syndrome: wobbly hind legs, a paralyzed bladder that dribbles urine, and permanent nerve damage. Pregnant mares can abort or carry malformed foals. Do not graze horses on this forage, and the danger deepens in young, drought-stressed, wilted, or frosted growth.
Rabbits Caution
A grazing rabbit meets the same cyanide packed into young, wilted, drought-stressed, or frosted blades. Keep rabbits off this growth, since a small animal can take a fatal dose in moments.
Livestock Toxic
Cattle, sheep, and goats are the classic victims and die fast from cyanide and nitrate poisoning, with rapid breathing, staggering, and convulsions inside 15 minutes to 2 hours. The risk climbs on young blades under 18 to 24 inches, on fast regrowth, and on drought-stressed, wilted, or frost-damaged plants, which must never be grazed.
Birds Caution
Poultry and other birds that peck at the foliage meet the same cyanogenic compounds that endanger grazing animals. The seed is used in feed, but keep birds off young, stressed, or frosted green growth.
People Caution
This is a forage and cover crop, not a food plant, so no part of it is eaten raw. The fresh green foliage holds the cyanide and nitrate that poison livestock, so do not nibble it and keep children from chewing on the stalks.
Keep grazing animals off young (under 18-24 in), drought-stressed, wilted, or frost-damaged growth and off fast regrowth - these have the most cyanide and can kill within an hour. Wait at least 5-7 days after frost; properly cured hay/silage is generally safer. Not for use as forage for horses without testing. Contact a vet or ASPCA Animal Poison Control at (888) 426-4435 immediately if poisoning is suspected.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Breeders have shaped this grass for different jobs, from more digestible forage to taller stands to cover-crop bulk. These are the main types you will run into.
Brown midrib (BMR) types
A recessive low-lignin trait paints a reddish-brown streak down the leaf midrib and makes these types more digestible and higher in energy as forage, though the softer stems stand up to wind and weather less well.
Photoperiod-sensitive types
These hold back their seedhead until day length drops below about 12 hours and 20 minutes, so the leafy stems climb very tall and stretch the window for cutting high-quality vegetative forage.
Forage hybrids vs cover-crop selections
Both carry coarser stems and broader leaves than true sudangrass with stronger regrowth and yield, but the forage types are bred for grazing and silage while the cover-crop types are bred for biomass, smothering weeds, and breaking up compaction.
How to propagate
There is only one way to grow it, and it could hardly be simpler: drop seed into warm ground and watch the blades come up.
SeedEasy
Late spring to summer once soil is warm (above about 60-65 degrees F) and frost danger has passed · Germinates in days; reaches mowable 3-4 ft and dense canopy within several weeks
Wait until the soil warms past about 60°F to 65°F (16°C to 18°C) and frost is gone, then sow, ideally at least two months before fall frost. Drill the seed at 20 to 40 pounds an acre or scatter 30 to 50 pounds, set about half an inch to 2 inches deep. Green shoots break the surface within days and knit into a mowable 3 to 4 foot wall of blades in a few weeks.
Myths
Sorghum-sudangrass looks and acts the same as grain sorghum or perennial Johnsongrass.
It does not. This is a frost-tender annual hybrid grown as a summer cover and forage grass, apart from grain sorghum and apart from the perennial weed Johnsongrass. The three are Sorghum relatives that share the prussic-acid hazard, yet the annual cover crop is not perennial Johnsongrass, and the two grow and spread quite differently in the field.
Cured hay is as dangerous as the fresh green plant.
It is not. Most of the prussic acid leaves the tissue as the forage dries into hay or ensiles into silage, so well-cured hay and silage are far safer than fresh young, stressed, or frosted growth. Horses are the exception, and even they should not be fed it long-term without testing.