Ornamental · Poaceae
Sorghum-sudangrass
Sorghum × drummondii
A commanding summer grass that fills a plot with living green and quiet soil-building, with a real cyanide caution for any animal that grazes it.
Full sun
Every 7–14 days
45–90 days
USDA 3–11
Overview
Sorghum-sudangrass earns its keep through purpose, not polish. Born from a cross of sorghum and sudangrass, it is a fast, heat-loving annual that turns a bare summer plot into several feet of dense green within a matter of weeks. Growers fold it into the working season as a cover crop and forage, where it draws a leafy curtain over the weeds and quietly enriches the ground it stands on.
What sets the tone for a plot happens out of sight, at the roots. The tall canopy shades weed seedlings into retreat, and a single midsummer mowing at 3 to 4 feet turns the plant's energy downward, driving roots deep to open up compacted soil. Those roots also release sorgoleone, a natural compound that keeps many weed seeds dormant, which is why a neglected corner comes back looking composed and clean.
For all its easy presence, it keeps a serious secret. Like its sorghum relatives, it holds cyanogenic compounds that release prussic acid, the cyanide form of the poison, and it gathers nitrate whenever stress sets in. That places it firmly off-limits to horses and grazing livestock, most of all while the growth is young, drought-hit, wilted, or touched by frost. Let it play its role in the soil, and treat any grazing as the genuine hazard it is.
Care
The short version: it asks for full sun, warm soil, and a light touch. Sow it after frost into free-draining ground, water it seldom since it takes dry spells in stride, and mow at 3 to 4 feet before the stand turns woody.
Light
Give it a place in full sun and open heat, and it settles in at once. This is a grass that comes into its own where summers run hot and unshaded, so an exposed plot flatters it and sets a quick pace of growth.
Water
It is drought-tolerant to its core once established, easing toward dormancy rather than decline through a dry spell, so a drink every 7 to 14 days is plenty and rain will often stand in for you. Keep it clear of overwatering and heavy, soggy ground. And hold this in mind: drought-stressed growth carries the most cyanide and must never be grazed.
Soil
It keeps easy company with almost any well-drained loam and settles across a broad pH from 5.5 to 9.0, a generous span by any measure. The one condition it refuses is waterlogged ground.
Temperature
A true warm-season grass, it thrives through the heat across roughly 32°F to 100°F (0°C to 38°C) and looks its best near the top of that range. Frost closes it out entirely, which doubles as the graceful way a cover crop takes its leave in fall.
Humidity
There is no dial to turn here, as ordinary outdoor humidity suits it completely. Its growth answers to heat and sun far more than to any moisture in the air.
Feeding
Offer nitrogen at planting, on the order of 75 to 100 pounds per acre, to bring on full biomass. Keep a measured hand, though, since a heavy dose against low phosphorus and potassium lifts the cyanide and nitrate the plant holds.
Pruning
The season turns on a single mow-kill at about 3 to 4 feet, which sends up a wave of root growth and tillering that breaks compaction and builds biomass. Cut or terminate before it presses past 4 feet toward its full 5 to 12 feet, or the stalks harden into woody stems and the residue lingers, tough and slow to break down.
Repotting
none
Size & spacing
Sown as a close stand, it rises to 5 to 12 feet (1.5 to 3.7 m) when left to its own devices, each plant claiming about 1 to 3 feet (30 to 91 cm) of ground. That dense screen is the whole intention, since its shade is what puts the weeds to rest.
When to plant
This is a creature of high summer with no appetite for cold. Set it out in late spring or summer once frost has left and the soil has warmed past 60°F to 65°F (16°C to 18°C), let it carry the heat of the season, and then close the chapter with a mowing or the first frost of fall.
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
The story of this grass turns on two possible problems: a poisoning risk to animals, or a stand that has simply outpaced its keeper. Growth stage is the thread to follow, since the very forage that serves you at one height turns dangerous or unwieldy at another. Read the situation below and step in while the timing is still yours.
Grazing animals fall suddenly, laboring for breath
This is acute cyanide poisoning, known also as prussic acid, and it is the gravest turn this grass can take. The plant releases hydrogen cyanide from a compound called dhurrin, and it runs highest in young growth under 18 to 24 inches, in the swift regrowth after a cut, and in plants left drought-stressed, wilted, or frosted. Trouble arrives within minutes and animals seldom last two hours, so keep livestock off that growth, allow 5 to 7 days after a frost, and reach a veterinarian 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
Stalks rise tall and woody and defy the mower
Left beyond 4 feet, the stand climbs toward its full 5 to 12 feet and the stems harden into coarse, woody stalks up to half an inch thick. That heavy mass resists the mower, breaks down slowly, and crowds the spring planting to come. Mow at about 3 to 4 feet before the stalks toughen, then terminate the stand or let winter take it so the residue has time to clear.
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 gathers nitrates, and frost draws the season to a close
Drought and a heavy hand with nitrogen push nitrate into the leaves, and that stressed forage can poison livestock on its own account, quite apart from the cyanide. Test any growth that has been stressed or heavily fed before offering it, and dilute or set aside whatever tests high. As a frost-tender warm-season annual, it blackens and dies at the first hard freeze, the customary and quiet way this cover crop is brought to a close.
Fix:
- Test stressed forage before feeding
- Dilute or avoid feeding high-nitrate forage
- Accept winterkill as the cover-crop termination method
Toxicity
This is the line to hold. The plant gives off prussic acid (cyanide) and stores nitrate, and either can take a horse or grazing livestock quickly, with the sharpest danger in young, drought-stressed, wilted, or frosted growth. House pets and people stand at a safe remove because they do not graze it, yet the foliage is no thing to chew, so keep animals and children clear of it.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Caution
The ASPCA sets Sudan grass down as non-toxic to cats, so a passing nibble of a blade is not the concern. Even so, cyanide collects in young, wilted, or frosted growth, so keep a curious cat from chewing on stressed plants.
Dogs Caution
The ASPCA does not mark this grass as toxic to dogs, and most will pass it by without a glance. The real danger lies in stressed or frost-bitten foliage, so guide a grass-chewing dog away from it as a simple precaution.
Horses Toxic
Horses fare the worst of all. Beyond a sudden cyanide poisoning marked by labored breathing, foaming, and tremors, they can slip into a distinct cystitis and ataxia syndrome: unsteady hind legs, a paralyzed bladder that dribbles urine, and lasting nerve damage. Pregnant mares may lose or misform a foal. Keep horses off this forage entirely, and know the danger deepens in young, drought-stressed, wilted, or frosted growth.
Rabbits Caution
A grazing rabbit meets the very same cyanide that collects in young, wilted, drought-stressed, or frosted blades. Keep rabbits well away from this growth, for a small creature can take a fatal dose in moments.
Livestock Toxic
Cattle, sheep, and goats are the classic losses, falling quickly to cyanide and nitrate poisoning with rapid breathing, staggering, and convulsions within 15 minutes to 2 hours. The peril crests on young growth under 18 to 24 inches, on swift 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 finds a place in feed, yet keep birds off young, stressed, or frosted green growth.
People Caution
This is a forage and cover crop, never a plant for the table, so there is no part to be eaten raw. The fresh foliage carries the cyanide and nitrate that poison livestock, so leave off tasting it and keep children from chewing 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 drawn several characters out of this one grass, from finer forage to towering stands to workmanlike cover-crop cover. These are the forms you are most likely to meet.
Brown midrib (BMR) types
A recessive low-lignin trait, announced by a reddish-brown line along the leaf midrib, lends these types finer digestibility and richer energy as forage, at the cost of standing a little less firmly through wind and weather.
Photoperiod-sensitive types
These hold their seedhead in reserve until day length slips below about 12 hours and 20 minutes, so they reach great height and lengthen the season for cutting leafy, high-quality forage.
Forage hybrids vs cover-crop selections
Both wear coarser stems and broader leaves than true sudangrass and bring stronger regrowth and yield, but forage types are shaped for grazing and silage while cover-crop types are shaped for biomass, smothering weeds, and easing compaction.
How to propagate
There is a single, unfussy way into this grass, and it suits the season perfectly. Sow the seed straight into warm ground and let the summer take it from there.
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 for the soil to warm past about 60°F to 65°F (16°C to 18°C) with frost safely gone, then sow, ideally a good two months before the fall frost arrives. Drill it at 20 to 40 pounds an acre or broadcast 30 to 50 pounds, settling the seed about half an inch to 2 inches down. It rises within days and gathers into a mowable 3 to 4 foot canopy over a few short weeks.
Myths
Sorghum-sudangrass is merely another name for grain sorghum or perennial Johnsongrass.
It is neither. This is a frost-tender annual hybrid raised as a summer cover and forage grass, apart from grain sorghum and apart from the perennial weed Johnsongrass. The three share a place in the Sorghum family and the same prussic-acid hazard, yet the annual cover crop should never be mistaken for perennial Johnsongrass, whose conduct in the field is a different story entirely.
Well-cured hay is every bit as dangerous as the fresh, growing plant.
It is not. The greater part of the prussic acid slips away as the forage cures into hay or ensiles into silage, which leaves good hay and silage far safer than fresh young, stressed, or frosted growth. Horses stand as the exception, and even they should not be fed it over the long term without testing.