How to Grow Sorghum-Sudangrass Cover Crop

Sorghum-sudangrass is the go-to summer cover crop when you want a lot of organic matter fast and a root system that punches into compacted soil. Sow it after the soil warms to about 65°F, usually late May through July in most of the US, at roughly 1 to 2 pounds of seed per 1,000 square feet, about an inch deep. Water it in, give it some nitrogen if your soil is poor, and it can grow 5 to 12 feet tall in 60 to 90 days. Mowing it once or twice when it reaches 3 to 4 feet tall forces deeper roots and more tillers. Terminate it before it sets seed, or let the first hard frost kill it, then chop and leave or incorporate the residue. Here’s the full process, including the timing mistakes that cause problems for the next crop.

In this guide

1. What Sorghum-Sudangrass Is
2. When to Plant
3. Preparing the Bed
4. How to Sow Sorghum-Sudangrass
5. Mowing for Deeper Roots and More Biomass
6. When and How to Terminate
7. Safety Note if You Keep Livestock
8. Pairing It With Other Cover Crops
9. How It Compares With Other Summer Cover Crops
10. Small-Garden Tips
11. Troubleshooting
12. Frequently Asked Questions

What Sorghum-Sudangrass Is

Sorghum-sudangrass is a hybrid cross between grain or forage sorghum and sudangrass (Sorghum bicolor ssp. drummondii). Plain sudangrass, sometimes written “sudan grass,” is finer-stemmed and regrows quickly after cutting; the hybrids are taller, coarser and produce more biomass. Both are warm-season annual grasses that love heat, tolerate drought once established, and die with the first frost.

Farmers use it for forage and as a cover crop. In gardens and market plots it’s valued for:

• Biomass: few cover crops produce as much organic matter in a single summer.
• Compaction relief: aggressive fibrous roots penetrate hard layers, especially after mowing.
• Weed suppression: a dense, tall canopy shades weeds, and the roots and residue release natural compounds that inhibit seed germination.
• Nematode suppression: some varieties are reported to reduce certain plant-parasitic nematodes when chopped and incorporated green. Results vary by variety and nematode species.
• Soil rest: it fills the summer gap between a spring crop and a fall crop, or rests a bed for a whole season.

Compared with mulching a bare bed, a living cover crop feeds soil life and builds structure from below; our cover crop vs mulch comparison explains when each makes sense.

When to Plant

Sorghum-sudangrass needs warm soil. Seed sown into soil below about 60–65°F germinates slowly and unevenly, and cool nights stall growth. In most of the US that means:

• Deep South and Gulf Coast (zones 8–10): late April through early August.
• Mid-South, Mid-Atlantic, lower Midwest (zones 6–7): late May through mid-July.
• Northern states (zones 3–5): early June through early July.

To get real benefit, allow at least 6–8 weeks of warm growing time before you need the bed again or before first frost. For maximum biomass and root depth, 10–12 weeks is better. It fits neatly after garlic, spring peas, early potatoes or spring greens, and before a fall-planted cover crop or garlic.

Preparing the Bed

1. Remove or mow down the previous crop and weeds.
2. Loosen the surface lightly with a rake, broadfork or shallow tillage. A firm, fine seedbed gives the best seed-to-soil contact.
3. If your soil is low in fertility, add nitrogen. Sorghum-sudangrass is a grass and responds strongly to nitrogen; a few inches of compost, or a modest application of an organic nitrogen fertilizer, boosts growth on poor soils. On rich garden soil, you may need nothing.
4. Water the bed if it’s dry, so seed lands in moist soil.

How to Sow Sorghum-Sudangrass

Seeding Rates

Method	Per 1,000 sq ft	Per acre (approx.)	Depth
Drilled or in rows	About 1 lb	35–50 lb	1–1.5 in
Broadcast and raked in	1.5–2 lb	50–80 lb	Cover about 1 in

Rates vary by seed supplier and variety, so check the bag. Higher rates give a denser stand with thinner stems, which is easier to mow and breaks down faster. In a small garden, broadcast seed by hand in two passes at right angles for even coverage, then rake it in to about an inch deep and tamp lightly with the back of the rake or by walking a board over it.

Germination and Early Care

Keep the soil moist until seedlings emerge, which takes about 5–10 days in warm soil. After that, the crop is quite drought tolerant, but you’ll get far more growth with an inch of water a week in dry spells. Birds sometimes eat surface seed; burying it fully solves most of that problem.

Mowing for Deeper Roots and More Biomass

Mowing is what turns sorghum-sudangrass from a tall grass into a soil-building tool. When plants reach about 3–4 feet, cut them back to 6–12 inches. Don’t scalp lower than 6 inches; it needs some stubble to regrow. The cut stimulates tillering, so each plant sends up several new stems, and it pushes roots deeper into the soil.

Mowing also keeps stalks softer and thinner, which makes final termination and decomposition much easier. In a long season you can mow two or even three times. A sickle bar mower, flail mower, or heavy string trimmer with a blade handles it well; a standard rotary lawn mower struggles with tall stalks, so mow before they get too woody. Leave the clippings on the bed as a mulch.

When and How to Terminate

End the cover crop before it flowers and sets seed, because mature stalks become very fibrous and slow to break down, and you don’t want volunteer seedlings. Common termination methods:

• Frost kill: the first hard frost (around 28–32°F) kills it. Leave the standing dead residue over winter as a protective cover, then chop it in spring. This is the easiest option in northern gardens.
• Mow and incorporate: flail-mow or chop, let it wilt a few days, then work it shallowly into the top few inches of soil.
• Mow and leave as mulch: chop and leave on the surface for no-till beds; transplant through the residue rather than direct seeding.
• Smother: mow low, then cover with an opaque tarp for three to six weeks in warm weather to kill regrowth.

How Long to Wait Before Planting

Sorghum-sudangrass residue has a high carbon-to-nitrogen ratio, especially when mature. As soil microbes break it down, they temporarily tie up nitrogen, which can starve the next crop. Its natural compounds also inhibit germination of small seeds. Plan on waiting about three to six weeks after incorporating it before sowing small-seeded crops like carrots, lettuce or onions. Transplants and large-seeded crops like beans, squash and garlic cope better. Adding some compost or a nitrogen source at planting helps offset the tie-up.

Safety Note if You Keep Livestock

If you graze animals or feed them the cuttings, be aware that sorghum and sudangrass can accumulate prussic acid (cyanide compounds) in young regrowth, after frost and during drought, and nitrates under stress. These can be dangerous to cattle, sheep, goats and horses. Talk to your county extension agent before using it as forage. In a garden where it’s simply a cover crop, this isn’t a concern.

Pairing It With Other Cover Crops

Sorghum-sudangrass is often mixed with a summer legume such as cowpeas or sunn hemp. The legume fixes nitrogen that the grass will use, and the grass gives the legume something to climb. A typical home mix reduces the sorghum-sudangrass rate by about half and adds the legume at its own reduced rate. In fall, follow it with a winter cover such as cereal rye, crimson clover or hairy vetch to keep living roots in the soil year-round; see how to grow hairy vetch for the next step in that rotation.

How It Compares With Other Summer Cover Crops

Cover crop	Main strength	Height	Best for
Sorghum-sudangrass	Biomass, compaction, weed suppression	5–12 ft	Resting a bed all summer, heavy or compacted soil
Sudangrass	Fast regrowth, finer stems	4–8 ft	Repeated mowing, easier termination
Buckwheat	Very fast (30–45 days), pollinators	2–3 ft	Short gaps between crops
Cowpeas	Nitrogen fixation, heat tolerance	1.5–3 ft	Adding nitrogen, mixing with grasses
Sunn hemp	Nitrogen and biomass	4–7 ft	Warm regions, long summers

Small-Garden Tips

Sorghum-sudangrass is a farm-scale crop, but it works in a backyard with a few adjustments. In raised beds and small plots, sow at the higher broadcast rate so stems stay thin, and plan to cut it with hedge shears, a string trimmer with a blade or a machete rather than a lawn mower. Chop stalks into short pieces before leaving them as mulch or adding them to compost, since long stems are slow to break down. If neighbors are close, remember it can tower over a fence by midsummer; mowing at 3–4 feet keeps it tidy. In very small beds where you need the space again within six weeks, buckwheat may be the better summer choice.

Troubleshooting

• Patchy stand: soil was too cool, seed too shallow (birds) or too deep, or the bed dried out during germination.
• Yellow, stunted plants: nitrogen shortage. Side-dress with compost or an organic nitrogen fertilizer.
• Stalks too woody to mow: you waited too long. Use a brush cutter or let frost kill it and deal with residue in spring.
• Next crop germinating poorly: residue tie-up and allelopathy. Wait longer, add nitrogen and use transplants.
• Volunteers next year: it went to seed. Terminate earlier next time.

Cover crops are one part of building living soil. For compost, amendments and soil testing, see our composting and soil hub, and for adding finished compost to garden beds, read adding compost to raised beds.