Sunn Hemp

Sunn hemp (Crotalaria juncea) is a fast-growing, warm‑season legume used as a summer cover crop across much of the U.S. It’s chosen for rapid biomass and nitrogen production, strong weed suppression, and notable benefits to soil structure and nematode management, particularly in vegetable and row‑crop rotations (SARE—Managing Cover Crops Profitably, Appendix B; UF/IFAS EDIS).

Benefits of Sunn Hemp as a Cover Crop

• Erosion Control: In vegetative filter strip trials in Hawai‘i, sunn hemp reduced soil loss by 67–77% compared with bare fallow, and reduced total suspended solids in runoff by an average 77% across events (UH-CTAHR watershed study pages; summary of the peer‑reviewed JAWRA paper via K‑State Croplands database](https://www.lib.k-state.edu/gracroplands/selected-results.php?FirstYear=2002&LastYear=2015&endYear=2011&page=212&search=&startYear=2004)).
• Weed Suppression: Rapid canopy closure and dense residues suppress both grass and broadleaf weeds. Research has documented 24–80% reductions in weed growth with sunn hemp systems, and 81–96% suppression of volunteer wheat following wheat harvest in Kansas (SARE LS02‑140 final report; Oklahoma State Univ. Extension). Roller‑crimped sunn hemp mulch has provided strong weed suppression for up to six weeks after termination in tropical systems (SARE OS11‑062).
• Soil Structure Improvement: Deep taproot with vigorous laterals helps open soil; long‑term no‑till research found sunn hemp increased cumulative infiltration ~300%, increased aggregate stability 80%, and reduced bulk density ~5% in the top 3 inches (K‑State Agronomy eUpdate summarizing the study; USDA‑ARS publication summary; UMaine Extension).
• Water Management: By tripling infiltration (above) and slowing/shortening runoff events, sunn hemp stands help retain rainfall and reduce run‑off volume; meta‑analyses of cover crops show ~50% average reduction in runoff volume and 10–40 minutes longer time to runoff, trends to which sunn hemp contributes when grown in-season (K‑State eUpdate; UW–Extension Ag Water Quality review).
• Disease/Pest Break: Sunn hemp is a poor/non‑host for several plant‑parasitic nematodes and produces allelopathic compounds that suppress root‑knot (Meloidogyne spp.), reniform, and can help reduce soybean cyst nematode pressure in rotations (UF/IFAS nematode management EDIS; UMN SCN guide; Minnesota Office for Soil Health (MOSH)).
• Nitrogen Management: As a legume, sunn hemp commonly contributes 100–150 lb N/ac in 9–12 weeks under good conditions, with documented ranges from ~50–150 lb N/ac in the Midwest up to 120 lb N/ac in the Southeast and more in subtropical systems (SARE—Managing Cover Crops Profitably, Appendix B; Ohio State Univ. Extension).
• Biomass Production: Typical dry matter yields range from 1,000–4,000 lb/ac in northern states to >5,000 lb/ac within 9–12 weeks in warmer regions; ratooned or long‑season subtropical stands can greatly exceed this under ideal conditions (Ohio State Univ. Extension; SARE—Managing Cover Crops Profitably, Appendix B; UF/IFAS EDIS). Recent northern trials also report ~4,000 lb/ac DM in 60 days under favorable summer weather (UMaine 2024 variety trial).

Planting and Management Advice

• Seeding Rate (drill and broadcast):
• General NRCS guidance (Tropic Sun): drill 30–50 lb/ac; broadcast 40–60 lb/ac; higher end if terminating in <60 days or with heavy weed pressure; inoculate with cowpea‑type rhizobia (USDA‑NRCS/Univ. of Hawai‘i ‘Tropic Sun’ release).
• State example (Ohio): drill 14–20 lb/ac PLS at ¼–½ in; broadcast with shallow incorporation 16–22 lb/ac PLS (broadcast without incorporation not recommended) (Ohio State Univ. Extension).
• Oklahoma: drill 15 lb PLS/ac; increase by ~25% if broadcasting (Oklahoma State Univ. Extension).
• Seeding Depth:
• ½–1 in is typical; acceptable range ¼–1½ in depending on soil moisture/texture; shallower on fine/moist soils, deeper on coarse/dry soils (USDA‑NRCS ‘Tropic Sun’; Ohio State Univ. Extension; Oklahoma State Univ. Extension).
• Soil Type and pH:
• Adapts to many soils but prefers well‑drained sites; avoids waterlogged or saline soils. pH tolerance broadly 5.0–7.5 (NRCS), and can perform on calcareous soils up to ~8.4 in south Florida (USDA‑NRCS ‘Tropic Sun’; UF/IFAS EDIS; UMaine Extension).
• Planting Time (seasonal windows by region):
• Southeast/Florida: seed April–June (not later than mid‑July) for maximum biomass; soil temps >68°F (UF/IFAS EDIS).
• Southern Great Plains (e.g., OK): ideal after wheat harvest; plant at least 45 days before first killing freeze; sunn hemp is killed by <28°F (Oklahoma State Univ. Extension).
• Midwest (Ohio): reliable establishment window June 10–Aug 16 (Ohio State Univ. Extension).
• Upper Midwest/Cold regions: plant a minimum of 9 weeks before average first fall freeze; sunn hemp winter‑kills (MOSH, Univ. of Minnesota).
• General temperature guidance: plant once soils are consistently ≥65°F (Alabama Cooperative Extension).
• Termination (methods and timing):
• Frost‑sensitive (winter‑kills). Mechanical: mow/shred, roller‑crimp (effective at 3–3.5 ft tall), or till; Chemical termination also effective where permitted. For incorporation, plow/mow‑and‑disk at bud to early bloom for rapid N release and manageable residue (UF/IFAS EDIS; SARE—roller/crimper guidance; Iowa State Extension—method options).
• Allow 1–3 weeks between termination and cash‑crop planting to begin residue breakdown, recharge surface moisture, and reduce “green bridge” risk; follow herbicide label plant‑back intervals if using chemical termination (UMN Extension; MSU Extension; NC State Extension).
• Rotational Considerations:
• Excellent before nematode‑susceptible vegetables and soybeans (for SCN pressure areas) due to non/poor‑host status and allelochemical effects; integrate with broader nematode IPM for lasting impact (UF/IFAS EDIS NG043; UMN SCN guide).
• If grazing or cutting for forage, avoid grazing after flowering when seeds form because sunn hemp seed can contain pyrrolizidine alkaloids (varies by cultivar); grazing is generally restricted to vegetative stages, and rates may be increased on slopes or for forage stands (Ohio State Univ. Extension; UF/IFAS EDIS).
• To avoid excessively lignified residue and slow N release, do not let stands get far past early bloom if you plan to incorporate; roller‑crimping later (flowering) improves mulch persistence for weed suppression (UF/IFAS EDIS; SARE OS11‑062).
• Water Requirements and Drought Tolerance:
• For maximum growth, about 1 inch of water per week is ideal, but sunn hemp is notably drought‑tolerant once established; rated “Excellent” for drought tolerance in Midwest extension guidance (USDA‑NRCS ‘Tropic Sun’; Ohio State Univ. Extension; UMaine Extension).
• Inoculation and Fertility:
• Use a cowpea‑type Rhizobium inoculant for reliable nodulation and N fixation. No N fertilizer is needed; ensure adequate P and K based on soil tests to maximize biomass/N production (USDA‑NRCS ‘Tropic Sun’; Oklahoma State Univ. Extension).

Practical notes:

• Stand establishment is key to weed suppression—control existing weeds at planting and use appropriate seeding rates/depth.
• Broadcast seeding without incorporation is not recommended in Ohio trials; if broadcasting, lightly incorporate and increase the seeding rate 20–30% (Ohio State Univ. Extension; Oklahoma State Univ. Extension).
• In no‑till, sunn hemp has repeatedly improved near‑surface soil physical properties and infiltration compared with no cover, supporting better water capture and reduced runoff in heavy summer storms (K‑State eUpdate; USDA‑ARS summary).

These data synthesize USDA/NRCS releases, university extension bulletins (UF/IFAS, Ohio State, Oklahoma State, UMaine, UMN), SARE publications and recent trials to help you match sunn hemp management to your rotation, region and goals.