Ornamental · Poaceae
Sorghum-sudangrass
Sorghum × drummondii
A fast, towering summer grass that smothers weeds and busts compaction, with a real cyanide warning for grazing animals.
Full sun
Every 7–14 days
45–90 days
USDA 3–11
Overview
Sorghum-sudangrass is a hybrid summer grass grown for muscle, not looks. Cross sorghum with sudangrass and you get a fast, heat-loving annual that can shoot from seed to several feet of dense growth in a matter of weeks. Farmers and gardeners plant it as a cover crop and forage, where it shades out weeds and pumps organic matter back into the soil.
Its real talent is underground. The thick canopy smothers weed seedlings, and a single mid-season mowing at 3 to 4 feet sends the roots driving deep to break up compacted soil. The roots also leak sorgoleone, a natural compound that keeps many weed seeds from sprouting, which is part of why it cleans up a weedy plot so well.
There is a serious catch. This grass and its sorghum relatives carry cyanogenic compounds that release prussic acid, the poison form of cyanide, and they pile up nitrate when the plant is stressed. That makes it dangerous to horses and grazing livestock, especially when the growth is young, drought-hit, wilted, or frosted. Grow it for the soil and treat any grazing as a real hazard.
Care
The short version: full sun, warm soil, and not much fuss. Sow it after frost into well-drained ground, water sparingly since it shrugs off drought, and mow at 3 to 4 feet before it turns woody.
Light
Give it full sun and all the heat you can. This grass thrives where summers are hot and grows fast in the open, so a sunny, exposed plot is exactly what it wants.
Water
It is genuinely drought-tolerant once established and nearly shuts down rather than dying in a dry spell, so water only every 7 to 14 days and skip it when rain does the job. Avoid overwatering and steer clear of soggy ground. Just remember that drought-stressed plants hold the most cyanide and must not be grazed.
Soil
It takes almost any well-drained loam and tolerates a wide pH from 5.5 to 9.0, which is a broad range for any crop. The one thing it will not abide is waterlogged soil.
Temperature
A true warm-season grass, it grows in the heat across roughly 32°F to 100°F (0°C to 38°C) and loves the high end of that. Frost kills it outright, which doubles as the way a cover crop is terminated in fall.
Humidity
Plain outdoor humidity suits it fine, so there is no figure to manage. Heat and sun drive its growth far more than moisture in the air.
Feeding
Feed nitrogen at planting, around 75 to 100 pounds per acre for full biomass. Go easy, though, because too much nitrogen paired with low phosphorus and potassium pushes up the cyanide and nitrate the plant stores.
Pruning
The key move is a single mow-kill at about 3 to 4 feet, which triggers a burst of root growth and tillering that busts compaction and builds biomass. Cut or terminate before the plant runs past 4 feet toward its full 5 to 12 feet, or the stalks turn woody and the residue gets tough and slow to break down.
Repotting
none
Size & spacing
Sown as a dense stand, it climbs to 5 to 12 feet (1.5 to 3.7 m) if left unchecked, with each plant spreading about 1 to 3 feet (30 to 91 cm). The thick canopy is the point, since that shade is what smothers the weeds.
When to plant
This is a warm-season annual with no tolerance for cold. Sow it in late spring or summer once frost is past and the soil has warmed above 60°F to 65°F (16°C to 18°C), let it run through the heat, and then mow it or let the first fall frost kill it off.
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 either a poisoning risk to animals or a plant that has gotten away from you. Watch the growth stage closely, because the same forage that is useful at one height becomes dangerous or unmanageable at another. Match what you see below and act before it becomes a problem.
Grazing animals collapse suddenly with labored breathing
This is acute cyanide (prussic acid) poisoning, and it is the most dangerous thing this grass does. The plant releases hydrogen cyanide from a compound called dhurrin, and the levels run highest in young growth under 18 to 24 inches, in fast regrowth after cutting, and in drought-stressed, wilted, or frosted plants. Signs hit within minutes and animals rarely last two hours, so keep livestock off that growth, wait 5 to 7 days after a frost, and call a vet the moment 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
Stalks turn tall, woody, and hard to cut
Left past 4 feet, this grass races toward its full 5 to 12 feet and the stems lignify into coarse, woody stalks up to half an inch thick. That mass fights the mower, breaks down slowly, and gets in the way of spring planting. Mow at about 3 to 4 feet before the stalks toughen, then terminate or let it winterkill so the residue clears in time.
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 foliage builds up nitrates and frost ends the crop
Drought and heavy nitrogen push nitrate into the leaves, and that stressed forage can poison livestock on its own, apart from the cyanide risk. Test any stressed or heavily fertilized growth before you feed it, and dilute or skip forage that tests high. As a frost-tender warm-season annual, the plant blackens and dies at the first hard freeze, which is the usual way a cover crop is terminated.
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, and both can kill horses and grazing livestock fast, with the worst risk in young, drought-stressed, wilted, or frosted growth. House pets and people are at 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
ASPCA lists Sudan grass as non-toxic to cats, and a nibble of blade is not the worry here. Still, the cyanide builds 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 will leave it alone. The real hazard sits in stressed or frost-bitten foliage, so steer a grass-chewing dog away from it just in case.
Horses Toxic
Beyond acute cyanide poisoning with labored breathing, foaming, and tremors, horses 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 climbs in young, drought-stressed, wilted, or frosted growth.
Rabbits Caution
A grazing rabbit faces the same cyanide that builds in young, wilted, drought-stressed, or frosted blades. Keep rabbits off this growth, since a small animal can take a fatal dose fast.
Livestock Toxic
Cattle, sheep, and goats are the classic victims and die quickly from cyanide and nitrate poisoning, with rapid breathing, staggering, and convulsions inside 15 minutes to 2 hours. The risk peaks on young growth 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 can 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 there is no part you eat raw. The fresh foliage carries 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 richer forage to taller stands to cover-crop brawn. These are the main types you will run into.
Brown midrib (BMR) types
A recessive low-lignin trait, marked by a reddish-brown leaf midrib, makes these types more digestible and higher in energy as forage, though they stand up to wind and weather less well.
Photoperiod-sensitive types
These hold off on forming a seedhead until day length drops below about 12 hours and 20 minutes, so they grow very tall and stretch the window for cutting high-quality vegetative forage.
Forage hybrids vs cover-crop selections
Both have coarser stems and broader leaves than true sudangrass with stronger regrowth and yield, but forage types are bred for grazing and silage while 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 not be simpler: direct-seed it into warm ground and stand back.
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
Start from seed once the soil warms past about 60°F to 65°F (16°C to 18°C) and frost is behind you, and aim to sow at least two months before fall frost. Drill it at 20 to 40 pounds an acre or broadcast 30 to 50 pounds, setting the seed about half an inch to 2 inches deep. It comes up within days and forms a mowable 3 to 4 foot canopy in a few weeks.
Myths
Sorghum-sudangrass is the same as grain sorghum or perennial Johnsongrass.
It is a frost-tender annual hybrid grown as a summer cover and forage grass, not grain sorghum and not the perennial weed Johnsongrass. All three are Sorghum relatives that share the prussic-acid hazard, but the annual cover crop should not be confused with perennial Johnsongrass, which behaves very differently in the field.
Properly cured hay is just as dangerous as the fresh growing plant.
Most of the prussic acid dissipates as the forage cures into hay or ensiles into silage, so well-made hay and silage are far safer than fresh young, stressed, or frosted growth. Horses are the exception and still should not be fed it long-term without testing.