Rapeseed

Rapeseed (Brassica napus), a cool-season brassica closely related to canola, is used as a fast-establishing cover crop with a deep taproot. Growers choose rapeseed to reduce erosion, scavenge residual soil nutrients, suppress weeds and some soil-borne pests, and to improve soil structure ahead of cash crops. (SARE overview of brassicas; see species page and chapter) SARE—Brassicas and Mustards

• Erosion Control: In multi-state syntheses, cover crops reduced sediment losses by 20.8 t/ac under conventional till, 6.5 t/ac under reduced till and 1.2 t/ac under no-till; mustard (a brassica like rapeseed) reduced soil loss by up to 82% vs. no cover. In Missouri no-till silage corn, adding a winter cover reduced soil loss from 9.8 to 0.4 t/ac/yr. These data reflect the erosion-control potential when rapeseed provides dense winter cover. SARE—Cover Crops at Work: Prevent Erosion, SARE—Managing Cover Crops in Conservation Tillage Systems
• Weed Suppression: Incorporating overwintered rapeseed before potato reduced midseason weed density 73–85% compared to fallow in research cited by the American Society for Horticultural Science; brassica residues commonly reduce early-season emergence 23–34% and delay emergence by ~2 days. HortTechnology (summary citing Boydston & Hang 1995), SARE—Brassicas and Mustards, eOrganic (allelopathy incl. rapeseed)
• Soil Structure Improvement: Rapeseed’s strong taproot “biodrills,” creating vertical channels that can reach 6+ feet, improving subsequent root growth and infiltration. Under compacted soil, rapeseed formed roughly twice as many deep roots (15–50 cm) as cereal rye, aiding penetration of dense layers; recent research highlights B. napus root mechanical traits that enhance penetration in compacted soils. SARE—Brassicas and Mustards, Plant & Soil—Chen & Weil 2010, Soil & Tillage Research 2023
• Water Management: Literature syntheses show cover crops increase infiltration by 8–462%, thereby reducing runoff and erosion; ARS long-term work with winter covers reduced sediment in runoff by 87% under strip till vs. conventional till. SARE—Cover Crops at Work: Increasing Infiltration, USDA-ARS news release
• Disease/Pest Break: Incorporated rapeseed has reduced Rhizoctonia canker/black scurf and Verticillium wilt in potato systems; nematodes (e.g., Meloidogyne chitwoodi) were reduced up to 80% in trials using brassica green manures. In soybeans, fields following rapeseed showed lower Rhizoctonia root rot in bioassays and occasional reductions in soybean cyst nematode eggs. Effects vary by species, variety, and management. SARE—Brassicas and Mustards (disease and nematode sections), Plant Disease (UIUC multi-year study), OSU Extension biofumigation factsheet (disease examples)
• Nitrogen Management: As a non-legume, rapeseed scavenges nitrate. SARE synthesizes that non-legume covers commonly take up 30–50 lb N/ac and can capture up to ~150 lb/ac when residual N is high; OSU research in the PNW measured ~60 lb N/ac uptake with a rapeseed cover (2.5 ton DM/ac at 1.2% N). This N is temporarily held in biomass and released after termination as residue mineralizes. SARE—Cover Crops for Sustainable Crop Rotations, OSU Extension PNW 636, Table 5
• Biomass Production: Brassicas (including rapeseed) can produce up to ~8,000 lb dry matter/ac depending on planting date and fertility; field studies report 5,620–7,315 lb/ac for rapeseed, while Ohio state trials commonly see 1,000–2,500 lb/ac due to later planting windows—illustrating the importance of early seeding for biomass. SARE—Brassicas and Mustards, HortTechnology biomass summary, Ohio State—Ohioline

• Seeding Rate:
• Drilled: 5–10 lb/ac when seeded alone (bulk seed); for PLS-focused or mix scenarios, states may recommend lower rates (e.g., 1–4 lb/ac PLS in Ohio). SARE—Brassicas and Mustards, Ohio State—Ohioline
• Broadcast: 8–14 lb/ac when seeded alone; with shallow incorporation some states guide 2–4 lb/ac PLS, and 3–4 lb/ac if broadcasting without incorporation (expect more stand variability). SARE—Brassicas and Mustards, Ohio State—Ohioline
• Notes: Broadcasting generally requires higher rates than drilling to offset poorer seed-to-soil contact. Mississippi State Extension—Seeding Methods
• Seeding Depth:
• Target 0.25–0.5 inches; do not exceed ~0.75 inch. SARE—Brassicas and Mustards, Iowa State Extension
• Soil Type and pH:
• Best on well-drained loams to clay loams; avoid ponded or poorly drained soils (risk of root disease and winterkill). University of Missouri Extension
• pH tolerance roughly 5.5–8.0, with optimum performance near 6.0–7.0. Ohio State—Ohioline, Iowa State Extension, K-State eUpdate
• Planting Time (U.S. regional guidance):
• Northeast/Mid-Atlantic: Latest fall seeding dates for rapeseed/canola typically range from Sept 1 (northern zones) to Sept 15 (southeast PA zones) to ensure rosette establishment. Penn State Agronomy Guide—dates table
• Upper Midwest/Corn Belt: July through October fits most corn–soy systems; earlier fall planting increases biomass and winter survival. Iowa State Extension
• Central/Southern Great Plains: Plant roughly six weeks before the area’s first killing frost (≤25°F) so plants reach 3–6 true leaves (rosette) before winter; in Oklahoma that commonly means Sept 10–Oct 10. Oklahoma State Extension—PSS‑2131, OSU—Managing Winter Canola
• General brassica guidance for New England: establish ~4 weeks before the average first 28°F freeze; some winter rapeseed types tolerate temps near 10°F when well established. UMass Extension
• Termination:
• Winterkill is variable by cultivar and climate; do not assume winterkill. For covers that survive, terminate 2–3 weeks before planting the cash crop to avoid interference/allelopathy. Ohio State—Ohioline
• Chemical: Rapeseed can be harder to kill with glyphosate; SARE recommends at least 1 qt/ac glyphosate and notes multiple applications may be needed. SARE—Brassicas and Mustards
• Mechanical: Flail mowing and incorporation before full flower works; rolling/crimping is less reliable unless flowering. SARE—Brassicas and Mustards
• Rotational Considerations:
• Avoid short rotations with brassica cash crops (canola, cabbage, broccoli, mustard, etc.). To lower blackleg and clubroot risk, extension and industry guidance recommend at least a 2-year (preferably 3-year) break between canola/rapeseed crops. NDSU Extension—Clubroot, Canola Council—Rotation and Risk
• Manage volunteers: Volunteer canola/rapeseed can persist; following with a cereal facilitates broadleaf herbicide options. NDSU Extension—Canola Production
• Herbicide carryover: Rapeseed/canola is sensitive to several residual herbicides (e.g., some sulfonylureas, imidazolinones); check labels and state guidance for plant-back intervals (often months to >1 year). OSU—PSS‑2131
• Water Requirements and Drought Tolerance:
• Rapeseed has moderate drought tolerance once established (deep taproot), but is less drought tolerant than small-grain cereals; establishment requires adequate surface moisture and it performs poorly on waterlogged soils. NDSU Extension—Canola Production, University of Missouri Extension

Practical notes to hit your goals:

• For maximum erosion control and N scavenging, seed as early as your window allows (late summer to very early fall), target a uniform, shallow planting depth, and favor drilling over broadcasting when possible. SARE brassica chapter, MSU Extension on seeding methods
• Expect stronger compaction relief when the soil is moist during fall growth; deep taproots improve infiltration and create macropores that benefit the next crop. SARE brassica chapter, Plant & Soil (Chen & Weil)
• Use incorporation (tillage) immediately after mowing if you are targeting biofumigation for soil pests; pest suppression is variable and improves when tissue is finely chopped and quickly incorporated. SARE brassica chapter, OSU Extension biofumigation

Numbers at a glance (authoritative ranges):

• Seeding rates alone: drill 5–10 lb/ac; broadcast 8–14 lb/ac. In some state guides using PLS or when in mixes: drill 1–4 lb/ac; broadcast 2–4 lb/ac (3–4 without incorporation). SARE, Ohio State—Ohioline
• Seeding depth: 0.25–0.5 in (not deeper than ~0.75 in). SARE, Iowa State
• pH and soils: pH 5.5–8.0 workable; best near 6.0–7.0; avoid wet soils. Ohio State, UMO & K-State, https://eupdate.agronomy.ksu.edu/article_new/winter-canola-planting-considerations-510-1
• Planting windows: NE/PA latest dates Sept 1–15 by zone; IA July–Oct; Southern Great Plains ≈6 weeks before first killing frost. Penn State, Iowa State, OSU
• Termination: 2–3 weeks pre-plant; rapeseed may require ≥1 qt/ac glyphosate and/or multiple passes; mow/incorporate before full bloom. SARE, Ohio State
• N scavenging: typical non-legumes 30–50 lb N/ac (up to ~150 lb/ac with high residual N); measured rapeseed ~60 lb N/ac in PNW trials. SARE, OSU PNW 636
• Biomass: commonly 1,000–2,500 lb/ac in late plantings (OH); up to 8,000 lb/ac with early planting and good fertility. Ohio State, SARE

Tip: If you grow brassicas or canola in your cash-crop rotation, keep at least a 2-year (preferably 3-year) break between any brassica cash crop and rapeseed covers, manage volunteers, and avoid brassicas in cover mixes on known clubroot fields. NDSU—Clubroot, Canola Council