Sunn Hemp as a Cover Crop
Legume cover crop
Sunn hemp (Crotalaria juncea) is a fast-growing, warm-season legume grown as a summer cover crop across much of the United States. Gardeners and small farmers value it for building soil and adding nitrogen during the hottest part of the year, when many other cover crops slow down. It is often used to fill a summer gap between a spring crop and a fall planting.
Benefits
As a legume, sunn hemp works with soil bacteria to take nitrogen from the air, which can help feed the crop that follows. It grows tall and dense in a short time, producing a large amount of biomass that adds organic matter to the soil as it breaks down. The rapid canopy shades the ground and helps crowd out summer weeds, while the deep, branching roots improve soil structure and help hold soil in place against erosion. Because results vary with soil and climate, check with your local Cooperative Extension for what to expect in your area.
When to plant
Sunn hemp is a heat-loving plant, so it is planted after the soil has warmed and the danger of frost has passed, generally from late spring through midsummer. Seed is usually broadcast or drilled into a firm, weed-free seedbed and lightly covered, with general seeding ranges available from your local Cooperative Extension. Because it needs warm conditions to grow well, it is not suited to cool-season planting.
Growing it
Once soils are warm, sunn hemp germinates quickly and can reach several feet tall within a couple of months. It performs on a range of soils, including sandy and lower-fertility ground, and it tolerates heat and short dry spells better than many cover crops, needing little attention once established. For nitrogen fixation on ground that has not grown it before, seed is sometimes paired with an appropriate rhizobia inoculant; your local Cooperative Extension can advise on this.
Ending the cover crop
Sunn hemp is easiest to end while the plants are still young and the stems are tender, before they turn woody and hard to handle. Mowing, cutting, or crimping the stand and then turning the residue into the soil are common non-chemical ways to terminate it. In regions with cold winters, a hard frost will kill the plants, so a late-summer stand may winterkill on its own.
Best for
Sunn hemp is a strong fit for gardeners and small farmers who want a fast summer cover to add nitrogen and organic matter during hot weather, especially on sandy or tired soils that need a quick boost between main crops.
General guidance for US growers. Seeding rates and regional mixes vary; your local Cooperative Extension office and NRCS can help you plan. Page updated Aug 4, 2026.