Mustard

Mustards (Sinapis alba, Brassica juncea and related species) are fast-growing, cool‑season brassicas widely used as cover crops for rapid ground cover, nitrogen scavenging, biofumigation, and short-season biomass production. Growers choose mustards for their quick fall growth, ability to suppress some soil‑borne diseases and weeds, and to capture residual soil nitrate before winter or early spring rains. SARE “Brassicas and Mustards” (sare.org)

• Erosion Control: Mustard cover crops have reduced soil loss by up to 82% compared to no cover crop, and across systems cover crops reduce sediment losses by an average of 20.8 tons per acre under conventional till, 6.5 t/ac under reduced till, and 1.2 t/ac under no-till. SARE Ecosystem Services “Covering the Soil to Prevent Erosion” (sare.org)
• Weed Suppression: Fall‑planted mustards suppressed in‑stand weed biomass by ≥50% in 90% of site‑years (Great Lakes trials), with individual studies reporting 51–99% reductions during growth; suppression can be variable by region and season. Agronomy Journal (Björkman et al. 2015) and references therein; see also a recent Southwest study where brown mustard reduced early‑season weed density by up to 56% in one site‑year (barley performed better overall). Weed Science 2025 (acsess.onlinelibrary.wiley.com, bioone.org)
• Soil Structure Improvement: Mustard develops a taproot with fibrous laterals that “compaction‑bust” to roughly 1–3 feet, improving root channels for the next crop. USDA‑NRCS Iowa Mustard Fact Sheet (2025) (nrcs.usda.gov)
• Water Management: Cover crops increase infiltration; synthesis of field studies shows 8–462% higher infiltration with cover crops, while a potato system using mustard green manures recorded 2–10× greater infiltration vs. no green manure. Recent work also shows cover crops can double saturated hydraulic conductivity and raise water‑holding four months after termination. SARE “Increasing Infiltration”; Crop Management (McGuire 2003); Soil Use & Management 2023 (sare.org, acsess.onlinelibrary.wiley.com, bsssjournals.onlinelibrary.wiley.com)
• Disease/Pest Break (Biofumigation): When chopped and incorporated at early bloom, mustards release isothiocyanates that can suppress pathogens like Rhizoctonia, Pythium and some Fusarium; termination method and timing are critical to efficacy. Oregon State Univ. Extension EM 9530 (2024); USDA‑NRCS Iowa Mustard Fact Sheet (2025) (extension.oregonstate.edu, nrcs.usda.gov)
• Nitrogen Management (scavenging): Mustard does not fix N but is an effective “catch crop.” August‑seeded mustard in Minnesota produced 4,000–6,000 lb/ac biomass and took up about 85–105 lb N/ac before winter‑kill; in high‑residual N vegetable ground in California, above‑ground N content has reached ~328 lb N/ac. UMN MOSH; SARE “Brassicas and Mustards” (mosh.umn.edu, sare.org)
• Biomass Production: Typical mustard biomass ranges from roughly 2,000–6,000 lb dry matter/ac in the Upper Midwest, with Iowa reports as low as 200–2,000 lb/ac under short windows; high‑fertility vegetable systems in California have reported up to 8,500 lb/ac. UMN MOSH; USDA‑NRCS Iowa Fact Sheet; SARE “Brassicas and Mustards” (mosh.umn.edu, nrcs.usda.gov, sare.org)

• Seeding Rate (pure stands):
• Drill: 5–12 lb/ac; Broadcast: 10–15 lb/ac. These are widely used rates for mustards as cover crops. SARE/eOrganic, eOrganic (sare.org, eorganic.org)
• NRCS PLS guidance (Iowa): Drill 3 lb/ac PLS; Broadcast 4 lb/ac PLS (lower rates reflect PLS basis and regional guidance). USDA‑NRCS Iowa Mustard Fact Sheet (2025) (nrcs.usda.gov)
• Seeding Depth: 0.25–0.75 inch. SARE/eOrganic, eOrganic (sare.org, eorganic.org)
• Soil Type and pH: Best on well‑drained soils; preferred pH 5.5–8.5; avoid poorly drained sites, especially at establishment. SARE/eOrganic, eOrganic (sare.org, eorganic.org)
• Planting Time (seasonal windows by region; adjust to “about 4 weeks before your first 28°F freeze” rule of thumb):
• Upper Midwest/Northern Corn Belt: July 25–Sept 15 for fall plantings (winter‑kill expected). University of Minnesota Extension (extension.umn.edu)
• Great Lakes (southern portion): Best results planting Aug 13–23; no later than Sept 1–10 for adequate biomass. Agronomy Journal Great Lakes study (acsess.onlinelibrary.wiley.com)
• Northeast: Mid‑July through August for fall mustard (winter‑kills); spring option in April (can frost‑seed in March) where biofumigation before vegetables is desired. Cornell Cover Crop Guide—Fall and Spring Mustard and Spring Mustard (covercrop.org)
• Pacific Northwest (irrigated/arid): Mid–late August into potato rotations; fast fall growth before winter‑kill is common. SARE “Brassicas and Mustards” (sare.org)
• California (coastal/valley annual crops and orchards): October–November seeding typical. UC Davis SAREP Mustards (sarep.ucdavis.edu)
• Southeast (Deep South): Aim for September–early October when soils are 45–85°F. Alabama Cooperative Extension and regional guidance to plant brassicas ~2 weeks earlier than small‑grain/legume winter covers. Clemson Land‑Grant Press (aces.edu, lgpress.clemson.edu)
• Upper Midwest spring window example (Iowa): Early April–mid‑May for spring mustards; fall window early Aug–early Sept. USDA‑NRCS Iowa Mustard Fact Sheet (2025) (nrcs.usda.gov)
• Termination:
• Winter‑kill: Most mustards winter‑kill around ~25°F; where reliable freeze arrives, this provides simple termination. UMass/NE Vegetable Guide (nevegetable.org)
• Biofumigation (for disease/pest suppression): Terminate at early bloom, flail‑chop to finely macerate, and immediately incorporate into moist soil; sealing the surface (roller or tarp) and timely irrigation help retain isothiocyanates. Delays of just a few days after chopping can sharply reduce efficacy. OSU Extension EM 9530 (2024); UMass NE Veg Guide (extension.oregonstate.edu, nevegetable.org)
• Mechanical kill without incorporation: Roller‑crimping is generally unreliable on brassicas; flail mowing is preferred if not winter‑killing. SARE “Brassicas and Mustards” (sare.org)
• Herbicide options: Mustards are sensitive to glyphosate and 2,4‑D; verify labels and rotation restrictions. UC Davis SAREP Mustards (sarep.ucdavis.edu)
• Waiting period after incorporation: For biofumigation, wait about 1–2 weeks before planting the next crop. Cornell Cover Crop Guide—Spring Mustard tips (covercrop.org)
• Rotational Considerations:
• Avoid tight rotations with brassica cash crops (canola, cole crops); mustards can host pests/diseases (e.g., clubroot) and flea beetles/diamondback moth. UMass NE Vegetable Guide (nevegetable.org)
• Herbicide carryover: Brassica cover crops are susceptible to residual broadleaf herbicides from the preceding crop—check labels and intervals. SARE “Brassicas and Mustards” (Precaution) (sare.org)
• Do not allow seed set; mustard volunteers can become problematic weeds. USDA‑NRCS Iowa Mustard Fact Sheet (2025) (nrcs.usda.gov)
• Water Requirements and Tolerance:
• Establish on well‑drained soil with adequate moisture; mustard is not particularly drought‑tolerant and also performs poorly on waterlogged soils; NRCS rates drought tolerance as “Fair.” MSU Extension E2956; USDA‑NRCS Iowa Mustard Fact Sheet (2025) (canr.msu.edu, nrcs.usda.gov)
• For nitrate leaching risk mitigation, mustards and other brassicas are among the most effective families at reducing nitrate leaching (global median ~75% reduction vs. fallow). Journal of Environmental Quality 2022 meta‑analysis (pubmed.ncbi.nlm.nih.gov)

Notes for practical use:

• Fertility for biofumigation: Sulfur is needed to build glucosinolates; many extension guides recommend an N:S of about 6–8:1 when fertilizing mustards. eOrganic/SARE (eorganic.org)
• Regional biomass/N expectations: In Minnesota, August‑seeded mustard has produced 4,000–6,000 lb/ac and captured 85–105 lb N/ac before winterkill; in coastal California vegetables, mustard biomass has reached ~8,500 lb/ac with very high N uptake where residual soil N is high. UMN MOSH; SARE “Brassicas and Mustards” (mosh.umn.edu, sare.org)

This guidance is grounded in university extension, NRCS, SARE, and recent peer‑reviewed research to support practical, region‑specific decisions for using mustard as a cover crop.