Ornamental · Poaceae
Sorghum-sudangrass
Sorghum × drummondii
A fast, sky-high summer grass that'll smother weeds and loosen packed soil, though it comes with a genuine cyanide warning for grazing animals.
Full sun
Every 7–14 days
45–90 days
USDA 3–11
Overview
Sorghum-sudangrass isn't grown for looks, and that's fine, because it's all about muscle. Cross sorghum with sudangrass and you've got a fast, heat-loving annual that can rocket from seed to several feet of thick growth in just a few weeks. Farmers and gardeners lean on it as a cover crop and forage, since it shades out weeds and adds a load of organic matter to the soil.
Want to know where the real magic happens? Down at the roots. That thick canopy smothers weed seedlings up top, and one mid-season mow at 3 to 4 feet sends the roots charging deep to bust up compacted soil. Those roots also leak sorgoleone, a natural compound that stops loads of weed seeds from sprouting, so a weedy patch tends to come back a whole lot cleaner.
Here's the catch, and it's a big one. This grass and its sorghum cousins carry cyanogenic compounds that release prussic acid, which is the cyanide form of the poison, and they build up nitrate whenever the plant gets stressed. That makes it genuinely risky for horses and grazing livestock, especially when the growth is young, drought-hit, wilted, or frosted. So grow it for the soil, and treat any grazing as the real hazard it is.
Care
The short version: it wants full sun, warm soil, and honestly not much else. Sow it after frost into ground that drains well, water it only now and then since it shrugs off dry spells, and mow at 3 to 4 feet before it goes woody.
Light
Give it full sun and all the heat you've got. It's happiest where summers run hot and open, so a bright, exposed spot is exactly what it's after, and it'll grow fast for you.
Water
Once it's settled in, it's really drought-tolerant, and it'll nearly shut down in a dry spell instead of dying, so you only need to water every 7 to 14 days, and you can skip it when the rain pitches in. Don't overwater it, and steer clear of soggy ground. Just keep in mind that drought-stressed plants hold the most cyanide, so those can't be grazed.
Soil
It's not picky at all, and it'll take just about any well-drained loam, plus a wide pH from 5.5 to 9.0, which is a pretty generous range. The one thing it won't put up with? Waterlogged soil.
Temperature
As a true warm-season grass, it grows right through the heat, roughly 32°F to 100°F (0°C to 38°C), and it loves the upper end of that. Frost kills it flat out, and that's actually how a cover crop like this gets wrapped up in fall, so there's no cold-weather worry.
Humidity
Nothing to manage here, since ordinary outdoor humidity is plenty. What really pushes its growth is heat and sun, way more than any moisture in the air.
Feeding
Give it nitrogen at planting, somewhere around 75 to 100 pounds per acre, and it'll build all the biomass you want. Go easy though, because too much nitrogen with low phosphorus and potassium bumps up the cyanide and nitrate it stores.
Pruning
The move that matters is one mow-kill at about 3 to 4 feet, which kicks off a burst of root growth and tillering that busts compaction and piles on biomass. Cut or terminate before it shoots past 4 feet toward its full 5 to 12 feet, or the stalks turn woody and the leftovers get tough and slow to break down.
Repotting
none
Size & spacing
Sown thick, it'll climb to 5 to 12 feet (1.5 to 3.7 m) if you let it, with each plant taking up around 1 to 3 feet (30 to 91 cm). That dense canopy is the whole point, since the shade is what smothers the weeds.
When to plant
This one's a warm-season annual, so cold is a hard no. Wait for late spring or summer once frost's out of the picture and the soil's warmed above 60°F to 65°F (16°C to 18°C), let it ride out the heat, and then just mow it or let the first fall frost take it down.
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
Nearly everything that goes wrong here is one of two things: a poisoning risk for animals, or a plant that's simply gotten too big for you. Keep half an eye on how tall it's getting, because the same forage that's handy at one height turns dangerous or unmanageable at another. Spot your issue below and jump on it early.
Grazing animals drop fast and can't catch their breath
This is acute cyanide poisoning, also called prussic acid, and it's easily the scariest thing this grass does. The plant gives off hydrogen cyanide from a compound called dhurrin, and it's highest in young growth under 18 to 24 inches, in the fast regrowth after a cut, and in drought-stressed, wilted, or frosted plants. Signs show up within minutes and animals rarely make it past two hours, so keep livestock off that growth, hold off 5 to 7 days after a frost, and call a vet the moment you suspect something's wrong.
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 get tall, woody, and a pain to cut
Leave it past 4 feet and it'll race toward its full 5 to 12 feet, with the stems hardening into coarse, woody stalks up to half an inch thick. All that mass fights the mower, breaks down slowly, and gets in the way come spring planting. So mow at about 3 to 4 feet before the stalks toughen, then terminate it or let it winterkill 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 leaves stack up nitrates, and frost calls it a season
Drought and a heavy dose of nitrogen push nitrate up into the leaves, and that stressed forage can poison livestock all by itself, on top of the cyanide. Test any growth that's been stressed or heavily fertilized before you feed it, and water it down or skip it if it tests high. Since it's a frost-tender warm-season annual, it blackens and dies at the first hard freeze, which is usually how a cover crop like this gets terminated anyway.
Fix:
- Test stressed forage before feeding
- Dilute or avoid feeding high-nitrate forage
- Accept winterkill as the cover-crop termination method
Toxicity
Don't brush this part off. The plant gives off prussic acid (cyanide) and stores up nitrate, and both can kill horses and grazing livestock in a hurry, with the worst of it in young, drought-stressed, wilted, or frosted growth. Pets and people are at lower risk since they aren't out grazing it, but the foliage still isn't safe to chew, so keep animals and kids away from it.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Caution
Good news for cat folks: the ASPCA lists Sudan grass as non-toxic to cats, so a nibble of a blade isn't the real worry here. Still, cyanide builds up in young, wilted, or frosted growth, so it's worth keeping a curious cat off stressed plants.
Dogs Caution
The ASPCA doesn't flag this grass as toxic to dogs either, and honestly most dogs will just leave it alone. The real risk is in stressed or frost-bitten foliage, so nudge a grass-chewing dog away from it, just to be safe.
Horses Toxic
Horses get the worst of it. On top of sudden cyanide poisoning with labored breathing, foaming, and tremors, they can develop a distinct cystitis and ataxia syndrome: wobbly hind legs, a paralyzed bladder that dribbles urine, and permanent nerve damage. Pregnant mares can even abort or carry malformed foals. Don't graze horses on this forage, and remember the danger climbs in young, drought-stressed, wilted, or frosted growth.
Rabbits Caution
A grazing rabbit runs into the same cyanide that builds up in young, wilted, drought-stressed, or frosted blades. Keep rabbits off this growth, because a small animal can take a fatal dose in no time.
Livestock Toxic
Cattle, sheep, and goats are the usual victims, and they go down fast from cyanide and nitrate poisoning, with rapid breathing, staggering, and convulsions inside 15 minutes to 2 hours. The risk really spikes on young growth under 18 to 24 inches, on fast regrowth, and on drought-stressed, wilted, or frost-damaged plants, so those should never be grazed.
Birds Caution
Poultry and other birds that peck at the foliage can run into the same cyanogenic compounds that endanger grazing animals. The seed does get 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's no part you'd eat raw. The fresh foliage carries the same cyanide and nitrate that poison livestock, so don't nibble it, and keep kids 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 nudged this grass in a few directions, whether that's richer forage, extra height, or straight-up cover-crop power. Here's what you'll usually come across.
Brown midrib (BMR) types
You can spot these by the reddish-brown streak down the leaf midrib, a sign of a recessive low-lignin trait that makes them easier to digest and higher in energy as forage, even if they don't stand up to wind and weather quite as well.
Photoperiod-sensitive types
These hold back on forming a seedhead until the days drop below about 12 hours and 20 minutes, so they get really tall and give you a longer stretch to cut leafy, high-quality forage.
Forage hybrids vs cover-crop selections
Both have coarser stems and broader leaves than true sudangrass, with stronger regrowth and yield, but the forage kinds are bred for grazing and silage while the cover-crop kinds are bred for biomass, smothering weeds, and breaking up compaction.
How to propagate
How do you grow it? Just one way, and it couldn't be simpler. Drop the seed straight into warm ground and let it do its thing.
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
Hold off until the soil warms past about 60°F to 65°F (16°C to 18°C) and frost's behind you, then sow, ideally at least two months before the fall frost rolls in. You can 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's up within days and knits into a mowable 3 to 4 foot canopy in just a few weeks.
Myths
Sorghum-sudangrass is basically the same thing as grain sorghum or perennial Johnsongrass.
Nope. It's a frost-tender annual hybrid grown for summer cover and forage, not grain sorghum and definitely not the perennial weed Johnsongrass. Sure, all three are Sorghum relatives that share the prussic-acid hazard, but the annual cover crop isn't the same as perennial Johnsongrass, and the two act nothing alike out in the field.
Cured hay is just as risky as the fresh, growing plant.
Not really. Most of the prussic acid clears out as the forage dries into hay or ferments into silage, so well-made hay and silage are a lot safer than fresh young, stressed, or frosted growth. Horses are the exception, though, and even then they shouldn't be fed it long-term without testing.