New sunn hemp is a fast-growing tropical legume cover crop grown for warm-season biomass, nitrogen fixation, weed suppression, and root activity between cash crops. Plants can reach 4-6 feet or taller in a short window and are most valuable when terminated before heavy seed set. In home gardens it is best used as a summer soil-building crop in open sunny beds with warm soil and good drainage. Key facts: 60–90 days to maturity, 8+ hours of sun, 12–18 " spacing. Not recommended for containers.
Updated June 1, 2026·Backed by 2 cited sources
Overview
At a Glance
The essentials first: timing, light, spacing, seed-starting, container fit, and overall size.
Days to maturity
60–90 days
Sun
8+ hours
Full Sun, 8+ Hours
Spacing
12–18 "
between plants
Seed start
0 weeks
before transplant
Container
Not recommended
Needs 10+ gal if attempted
Height
4–6 ft
at maturity
Planting window
Zone Planting Guide
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Care
Growing Guide
Everything in one place: seed starting, transplant timing, watering, soil, and structural support.
A seedling killer caused by several different fungi working together. It hits vegetables, flowers, herbs, microgreens, and cover-crop seedlings the same way — seeds rot before they emerge, or young seedlings collapse right at the soil line. Wet seed-starting mix and poor airflow in seedling trays are the classic conditions.
Triggers: Wet soil or starting mix, poor drainage, seedlings packed too tightly, contaminated trays or media, and stagnant air all favor damping-off.
Risk fades when: Drying the soil surface and improving airflow slows new spread. Collapsed seedlings don't recover, but the rest of the tray can be saved.
Damping off of coffee seedlings caused by Fusarium sp. — Photo:
Scot Nelson
·
CC0 1.0
Common leaf spot (clover & alfalfa)Pseudopeziza medicaginis, Pseudopeziza trifolii
Moderate
Disease
May–SepPeak window months: May, Jun, Jul, Aug, Sep.
The most common leaf disease of alfalfa and clover cover crops. It makes small, dark brown to black circular spots with slightly fringed edges, often with a tiny raised fruiting disk in the center. Spots start on the lower leaves and move up, and infected leaves yellow and drop, thinning the stand. It is favored by cool, moist weather and is worst in spring and fall.
Triggers: The fungus overwinters in leaf debris and spreads by spores during cool, moist weather. Dry, warm summer weather slows it; cool, moist fall conditions favor a resurgence. A dense canopy that traps humidity raises risk.
Risk fades when: Dry, warm weather slows it. Two days above 82°F or three dry days exits the active window; spotted leaves stay damaged.
Poor establishmentMississippi State Extension states the case in one line: plant establishment is the most critical aspect in any planting.
Moderate
Physiological
Mar, Apr, May, Aug, Sep, OctPeak window months: Mar, Apr, May, Aug, Sep, Oct.
Mississippi State Extension states the case in one line: plant establishment is the most critical aspect in any planting. A cover crop that comes up thin never catches up, and every benefit you sowed it for - nitrogen, biomass, erosion control, weed suppression - scales with the stand you got. Almost all of it is decided in the first week. Seed must reach mineral soil and stay in contact with it. Iowa State is emphatic that seedbed firmness is very important, recommending a cultipacker or roller on tilled seedbeds before planting, then broadcasting onto that firm bed and rolling again for shallow coverage. Depth matters as much as contact: small seed buried too deep never emerges, and Iowa State caps forage seeding depth at half an inch. Broadcast seed sitting loose on a dry surface simply desiccates, which is why MSU says to raise broadcast seeding rates by 25 to 50 percent over drilled rates to compensate for poor seed-to-soil contact and uneven distribution. Then there is temperature. Warm-season species and mixes sown into cool soil sit and rot rather than germinate. And weeds: a crop that emerges into competition thins before it ever closes canopy.
Triggers: Mississippi State Extension: plant establishment is the most critical aspect in any planting; poor seed quality results in uneven stands, stunted growth and limited benefits, so use seed with a recent test date, high germination and pure-seed percentages, and few weed seeds. Increase broadcast seeding rates by 25 to 50 percent over drilled rates to compensate for poor seed-to-soil contact and poor uniformity of distribution; a cultipacker is recommended when broadcast seeding. Broadcasting is not recommended when planting multiple species, because different species require different seeding depths - this is the characteristic failure of a mixed-species stand. Legumes require the correct rhizobium inoculant to fix nitrogen, and inoculant plus fungicide treatment can account for a large fraction of bag weight, reducing actual pure live seed. Iowa State Extension: seedbed firmness is very important; use a cultipacker or roller on tilled seedbeds before planting, broadcast onto a firm tilled seedbed and cultipack or roll for shallow seed coverage and seed-to-soil contact; control seeding depth to no deeper than half an inch; control weeds before planting, since weeds compete with young seedlings for moisture and light. NC State Extension: drill seeding is often the most consistent method, providing improved uniformity and seed-to-soil contact; planting by the recommended dates is crucial for proper establishment, stand and biomass. NRCS defines a weed-free seedbed as not exceeding one weed seedling per square foot at planting. Warm-season species and mixes will not germinate or thrive in cold soil.
On New Sunn Hemp: Cool soil or early weed pressure can thin the stand before the crop gets moving.
Prevention: Sow into warm soil and a clean seedbed.
Risk fades when: A thin stand does not thicken. Once emergence is over, the options are to overseed into the gaps or to accept reduced benefit and terminate on schedule. Fix it at sowing instead. Firm the seedbed before you sow - roll or cultipack a tilled bed. Sow shallow, no deeper than half an inch for small seed, and press or roll the seed in afterward so it contacts mineral soil rather than sitting in loose dust or on top of residue. If broadcasting rather than drilling, raise the seeding rate by 25 to 50 percent. Wait for the right soil temperature: warm-season species and warm-season mixes sown into cool soil will not germinate. Sow into moisture, or irrigate after sowing. Control weeds before planting, not after. Inoculate legume seed with the correct rhizobium. And do not broadcast a multi-species mix if you can drill it, because the species need different depths and one of them will always be wrong.
A rust of alfalfa and clover cover crops that makes small reddish-brown pustules on both leaf surfaces and sometimes on stems. The pustules rupture and release powdery spores, and heavy infection yellows and drops leaves, weakening the stand. It is favored by humid weather and extended leaf wetness.
Triggers: Favored by humidity and extended leaf wetness. Reddish-brown pustules on both leaf surfaces release powdery spores spread by wind. Often associated with nearby spurge as an alternate host.
Risk fades when: Hot or dry weather slows rust. Two days above 88°F or three dry days exits the active window; existing pustules remain.
Feeding & picking
Nutrition & Harvest
How hungry the plant is, what ripe harvest looks like, and how long the crop keeps after picking.
Feeding
Nutrition
Feeding intensityLow feeder
RecipesWorm Castings Topdress
Timing
Harvest
Mow or incorporate when the stand is dense and vigorous, ideally before heavy seed set and before stems become too fibrous.
Expected yield0.2–0.5 lbs/plant
Storage Use immediately as mulch, compost feedstock, or green-manure residue.
Plant relationships
Companion Planting
Helpful neighbors can support growth or deter pests. Keep antagonistic plants separated to reduce stress and competition.
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What you'll need
Growing Supplies
Hand-picked for your New Sunn Hemp, with the extension research behind every recommendation.
Seed starting tray + heat mat
For gardeners who start seeds indoors, this combo improves even germination. Warm-season crops benefit from bottom heat. Look for a rigid tray, cell inserts with drainage, and a heat mat paired with a thermostat.
Source: Utah State University Extension; Iowa State University Extension; Mississippi State University Extension
Nearly every garden benefits from mulch for weed suppression, moisture conservation, and soil temperature moderation. For most home gardeners, quality organic mulch is the better buy over landscape fabric.
Source: Penn State Extension; Wisconsin Horticulture; Illinois Extension
Every gardener benefits from putting water at the root zone instead of on the leaves, because drip and soaker systems reduce foliar disease pressure by limiting leaf wetness and soil splash. A quality kit should include a backflow preventer, filter, pressure reducer, and UV-resistant tubing.
Source: Iowa State University Extension; Colorado State University Extension; UMass Extension
Row cover adds frost protection, speeds early growth, and physically excludes insect pests without spraying. Look for spun-bonded fabric with a stated weight and frost rating, UV resistance, and enough width for hoops or low tunnels.
Source: University of Maryland Extension; University of New Hampshire Extension; Colorado State University Extension
K-State Research and Extension and University of Maryland Extension recommend shade cloth as a heat-management tool for vegetable gardens, with 30 percent shade rating most effective for tomatoes, peppers, and fruiting crops, and 40 to 50 percent for protecting heat-sensitive greens during hot summer months. University of Delaware research found 30 percent black shade cloth tripled marketable yield for bell peppers compared to unshaded plants, and Purdue trials showed shade cloth reduced maximum daily temperatures by 8 to 10 degrees Fahrenheit. Choose knitted polyethylene with reinforced grommets every 18 to 24 inches, mount on hoops or a frame with open sides for airflow, and remove or vent during prolonged wet weather to avoid increased humidity in the canopy.
Source: K-State Research and Extension; University of Maryland Extension; University of Delaware Cooperative Extension; Purdue University Extension
Reflective plastic mulch (white-on-black or silver)
North Carolina State Extension reports that white-on-black plastic mulch can reduce soil temperature by 5 to 20 degrees Fahrenheit and silver mulch by about 6 degrees, the opposite effect of black mulch which warms soil. This makes reflective mulch the appropriate plasticulture choice for hot zones (especially Zone 9a desert and other high-heat low-humidity areas) where overheating limits warm-season crop performance more than cold soil. Silver mulch adds documented aphid and thrips repellency from the reflective surface. Use only with drip irrigation installed underneath, never use plastic mulch without irrigation, and reserve for late spring or early fall plantings where the surrounding heat is the primary stress.
Source: North Carolina State Extension; University of Arizona Cooperative Extension
A soil test gives a baseline for pH and nutrient status so gardeners can add only what the soil actually needs. Prioritize a mail-in or lab-affiliated kit whenever possible because extension guidance notes that laboratory testing is more accurate than instant readers.
Source: University of Maryland Extension; Purdue Extension; Montana State University Extension
University of Minnesota Extension recommends measuring soil temperature 2 to 4 inches below the surface to decide when warm-season crops can actually be planted, because air temperature and average frost dates do not reliably predict whether soil is warm enough for germination. A dedicated soil thermometer with a 4 to 6 inch stainless steel probe gives gardeners a deterministic reading instead of relying on the calendar alone, which matters most in zones with wide last-frost variability. Look for a waterproof stainless steel stem, a clearly marked vegetable-garden temperature range, and a readable analog or digital display at planting depth.
University of Arizona Cooperative Extension notes that most vegetables root in the top 12 to 24 inches of soil and that hot, dry periods require more frequent irrigation, but watering by habit often wets only the top inch while leaving the root zone dry. A dedicated soil moisture meter with a long probe gives gardeners a deterministic reading at root depth instead of guessing from surface appearance, which is most critical in low-rainfall desert zones (Zone 9a Phoenix) and in raised beds or containers that dry from the top down. Look for a single-purpose moisture meter (not a 3-in-1 or 4-in-1 combo, which trade accuracy for feature count) with a probe that reaches 8 to 12 inches and a clear analog or digital display.
Source: University of Arizona Cooperative Extension
UF/IFAS Extension and Texas A&M AgriLife Extension recommend securing or removing trellises, shade cloth, hoop covers, container plants, and lightweight raised-bed accessories before tropical storms and hurricanes, since loose garden items become projectiles in high winds. Most-relevant for Gulf Coast Zone 8b (Houston, Mobile, New Orleans), Florida Zone 9b (Miami, Tampa), and any coastal area within the Atlantic and Gulf hurricane corridors. Galvanized steel ground anchors resist rust in humid coastal soils, and screw-in spiral anchors hold significantly better than driven stakes in saturated soil during storm conditions. Use quick-release fasteners on shade cloth and trellises so they can be removed quickly when a storm watch is issued.
Extension guidance favors bypass designs because they make cleaner, closer cuts on living tissue than anvil types. Look for hardened steel blades that can be sharpened, a comfortable grip, and a cutting capacity matched to real home-garden stems.
Source: University of New Hampshire Extension; Iowa State University Extension; Purdue University Extension
Raised beds improve drainage, let gardeners control soil from day one, reduce compaction, and make gardening more accessible. A quality kit should use rot-resistant, food-safe materials and provide enough depth for productive rooting.
Source: Penn State Extension; University of Delaware Cooperative Extension; Illinois Extension
The most useful mix is three categories: a beginner guide, a reference manual for diagnosis and crop-by-crop lookup, and a soil science book. Look for region-aware editions, strong visuals, and evidence-based authorship.
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Research
Sources
Reference material and extension guidance used to build this growing guide.
university Oklahoma State University Extension, Using Sunn Hemp as a Cover Cropuniversity Oklahoma Cooperative Extension Service PDF, Using Sunn Hemp as a Cover Crop
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