Ornamental · Poaceae
Sorghum-sudangrass
Sorghum × drummondii
A towering, fast summer grass that smothers weeds and loosens packed soil, with a real cyanide caution for anything that grazes it.
Full sun
Every 7–14 days
45–90 days
USDA 3–11
Overview
If you have never grown a cover crop before, sorghum-sudangrass is a forgiving place to start. It comes from crossing sorghum with sudangrass, and the result is a fast, heat-loving annual that rushes from a handful of seed to several feet of thick green in just a few weeks. Growers reach for it as a cover crop and forage because it shades out weeds and builds the soil's organic matter, and it asks little of you in return.
The best of it happens where you cannot see, down among the roots. That dense canopy blankets weed seedlings and starves them of light, and when you give the stand a single mid-season mow at 3 to 4 feet, the roots answer by plunging deep and prying apart compacted soil. Those same roots seep a natural compound called sorgoleone that quietly keeps many weed seeds from waking up, which is a big reason a messy, weedy patch comes back looking so much cleaner.
There is one thing to take to heart, and it helps to know it up front. This grass, like its sorghum cousins, holds cyanogenic compounds that give off prussic acid, the cyanide form of the poison, and it banks up nitrate whenever the plant is under stress. That makes it a genuine danger to horses and grazing livestock, most of all when the growth is young, drought-hit, wilted, or touched by frost. The kind approach is to grow it for the soil and treat any grazing as the real hazard it is.
Care
The short version: give it full sun and warm soil, and relax, because it does not need much from you. Sow it after the last frost into ground that drains well, water only now and then since it handles dry spells with ease, and mow at 3 to 4 feet before the stalks harden.
Light
Give it full sun and every bit of heat you can offer. This is a grass that comes alive when summers are hot and open, so a bright, unshaded spot is exactly the gift it wants, and it rewards you with fast growth.
Water
Once it has settled in, it is honestly drought-tolerant, easing into a near standstill in a dry spell instead of giving up, so you only need to water every 7 to 14 days and can skip that when the rain helps. Keep it out of soggy ground and try not to overwater. One thing to hold onto: a drought-stressed plant carries the most cyanide and must never be grazed.
Soil
It is wonderfully easygoing about soil, happy in almost any well-drained loam and content across a wide pH from 5.5 to 9.0, which is a generous range for any crop. The only thing it truly cannot forgive is waterlogged ground.
Temperature
As a true warm-season grass, it grows happily through the heat, roughly 32°F to 100°F (0°C to 38°C), and it is at its best near the top of that. A frost ends it outright, and that is simply how a cover crop like this gets closed out in fall, so there is no need to fret over the cold.
Humidity
There is nothing to fuss over here, because ordinary outdoor humidity suits it just fine. What really drives its growth is heat and sunshine, far more than any moisture in the air.
Feeding
Offer nitrogen at planting, somewhere around 75 to 100 pounds per acre, and it will build all the biomass you hoped for. Go gently, though, since piling on nitrogen while phosphorus and potassium stay low nudges up the cyanide and nitrate the plant holds.
Pruning
The one move that matters is a single mow-kill at about 3 to 4 feet, which sparks a rush of root growth and tillering that breaks up compaction and packs on biomass. Cut or terminate before it sprints past 4 feet toward its full 5 to 12 feet, or the stalks go woody and the leftover residue turns tough and slow to rot down.
Repotting
none
Size & spacing
Planted thick, it can rise to 5 to 12 feet (1.5 to 3.7 m) when left alone, with each plant taking up about 1 to 3 feet (30 to 91 cm) of room. That crowded canopy is the whole idea, since the shade it throws is what smothers the weeds for you.
When to plant
Think of this as a heat-lover with no patience for cold. Wait until late spring or summer, once frost has passed and the soil has warmed above 60°F to 65°F (16°C to 18°C), then let it soak up the summer. When the first fall frost arrives it will finish the plant for you, so there is nothing tricky to time.
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
Almost every problem here comes down to one of two things: a poisoning risk to animals, or a plant that has simply outgrown you. The trick is to keep an eye on how tall it is getting, since the very same forage that helps at one height turns dangerous or hard to handle at another. Find your situation below and step in early, and it stays manageable.
Grazing animals suddenly go down, gasping for breath
This is acute cyanide poisoning, sometimes called prussic acid, and it is the one truly frightening thing this grass can do. The plant gives off hydrogen cyanide out of dhurrin, a natural compound in its tissue, and those levels climb highest in young shoots under 18 to 24 inches, in the quick regrowth after a cut, and in plants left drought-stressed, wilted, or frosted. Trouble shows within minutes and animals seldom last two hours, so keep livestock well away from that growth, hold off for 5 to 7 days after a frost, and phone 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
Stalks grow tall and woody and get hard to mow
Let it slip past 4 feet and it will charge toward its full 5 to 12 feet, and the stems harden into coarse, woody stalks as much as half an inch across. All that mass drags on the mower, rots down slowly, and can crowd your spring planting. The easy fix is to mow around 3 to 4 feet before the stalks toughen, then terminate it or let the winter finish 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 leaves store nitrates, and frost closes out the crop
Drought and a heavy hand with nitrogen drive nitrate up into the leaves, and that stressed forage can harm livestock all on its own, quite apart from the cyanide. Test any growth that has been stressed or fed hard before you offer it, and water it down or set it aside if the numbers come back high. Being a frost-tender warm-season annual, the plant blackens and dies at the first hard freeze, which is the gentle, natural way this cover crop bows out.
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 part to take to heart. The plant gives off prussic acid (cyanide) and stores nitrate, and together they can kill a horse or grazing livestock quickly, with the greatest danger in young, drought-stressed, wilted, or frosted growth. House pets and people sit at much lower risk since they are not out grazing it, but the leaves are still not safe to chew, so gently keep animals and children away from it.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Caution
The ASPCA actually lists Sudan grass as non-toxic to cats, so a quick nibble of a blade is not the real concern. Even so, cyanide gathers in young, wilted, or frosted growth, so it is worth keeping a curious cat from chewing on stressed plants.
Dogs Caution
Dogs are not on the ASPCA's toxic list for this grass, and most will happily ignore it anyway. The genuine risk lives in stressed or frost-bitten foliage, so gently guide a grass-nibbling dog away from it just to be safe.
Horses Toxic
Horses have it worst. On top of sudden cyanide poisoning with labored breathing, foaming, and tremors, they can fall into a distinct cystitis and ataxia syndrome: wobbly hind legs, a paralyzed bladder that dribbles urine, and lasting nerve damage. Pregnant mares may lose a foal or deliver a malformed one. Keep horses off this forage altogether, since the danger only grows in young, drought-stressed, wilted, or frosted plants.
Rabbits Caution
A grazing rabbit meets the very same cyanide that pools in young, wilted, drought-stressed, or frosted blades. Keep rabbits clear of this growth, because a small body can reach a fatal dose remarkably fast.
Livestock Toxic
Cattle, sheep, and goats are the classic casualties, going down quickly from cyanide and nitrate poisoning with fast breathing, staggering, and convulsions within 15 minutes to 2 hours. The hazard peaks on young growth under 18 to 24 inches, on fast regrowth, and on drought-stressed, wilted, or frost-damaged plants, which should never be grazed.
Birds Caution
Poultry and other birds that peck at the leaves can run into the same cyanogenic compounds that threaten grazing animals. The seed does turn up in feed, but keep birds away from young, stressed, or frosted green growth.
People Caution
This is a forage and cover crop rather than something for the table, so there is no part you would eat raw. The fresh leaves hold the cyanide and nitrate that poison livestock, so leave off nibbling 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
Growers have coaxed this grass into a few different forms over the years, some for richer feed, some for extra height, and some for sheer cover-crop strength. Here are the main ones you are likely to meet.
Brown midrib (BMR) types
A recessive low-lignin trait, easy to spot by the reddish-brown streak down the leaf midrib, leaves these types softer to digest and richer in energy as feed, though they lean and weather a little less sturdily in wind.
Photoperiod-sensitive types
These types wait to throw up a seedhead until the days shorten past about 12 hours and 20 minutes, so they climb very tall and give you a longer window to cut leafy, high-quality forage.
Forage hybrids vs cover-crop selections
Both carry coarser stems and broader leaves than true sudangrass along with stronger regrowth and yield, but the forage kinds are bred for grazing and silage while the cover-crop kinds are bred for biomass, choking out weeds, and loosening packed soil.
How to propagate
Good news for a first try: there is really only one way to grow it, and it could hardly be easier. Sow the seed straight into warm ground and let it take over 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 behind you, then sow, ideally at least two months ahead of the fall frost. Drill it at 20 to 40 pounds an acre or scatter 30 to 50 pounds broadcast, tucking the seed about half an inch to 2 inches down. It sprouts within days and knits into a mowable 3 to 4 foot canopy in just a few weeks.
Myths
Sorghum-sudangrass is just another name for grain sorghum or perennial Johnsongrass.
Not so. This is a frost-tender annual hybrid raised for summer cover and forage, not the grain crop and certainly not the perennial weed Johnsongrass. All three do belong to the Sorghum family and share that prussic-acid hazard, yet the annual cover crop is a different plant from perennial Johnsongrass, and the two behave nothing alike out in the field.
Cured hay is every bit as risky as the fresh, growing plant.
Happily, that is not the case. Most of the prussic acid fades away as the forage dries into hay or ferments into silage, which makes good hay and silage far safer than fresh young, stressed, or frosted growth. Horses are the one exception, and even so they should not be fed it over the long haul without testing first.