Phacelia

INTRODUCTION

Phacelia (Phacelia tanacetifolia), also called lacy phacelia or scorpionweed, is a fast-growing, cool‑season, non‑legume broadleaf used as a cover crop and insectary. Growers choose it for rapid canopy, strong nitrate scavenging, excellent pollinator/beneficial attraction, and easy winter‑kill in colder regions. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf); [UC Davis SAREP] (https://sarep.ucdavis.edu/covercrop/phacelia)

Benefits of Phacelia as a Cover Crop

• Erosion Control: Dense, living cover protects soil from raindrop impact and slows runoff. In Iowa field trials, small‑grain cover crops reduced interrill erosion 26–54% and rill erosion 65–90% versus no cover; phacelia performs a similar soil‑cover function when it achieves comparable canopy. [Iowa State Univ. Extension] (https://crops.extension.iastate.edu/files/inline-files/PM1999._covercrops.pdf); [UMN Extension, how cover crops reduce erosion and runoff] (https://extension.umn.edu/soil-and-water/cover-crops)
• Weed Suppression: Phacelia establishes and flowers quickly (about 45–60 days after emergence), creating a competitive canopy; OSU on‑farm work recorded 60–90% canopy cover with buckwheat and phacelia in eight weeks. For stronger suppression, SARE advises using the higher end of phacelia seeding rates. [USDA‑NRCS MT‑113] (https://www.govinfo.gov/content/pkg/GOVPUB-A57-PURL-gpo76667/pdf/GOVPUB-A57-PURL-gpo76667.pdf); [OSU Extension, 2026 impact story] (https://extension.oregonstate.edu/impact/osu-tests-summer-cover-crops-northern-oregon-coast-farms); [SARE California Implementation Guide] (https://www.sare.org/wp-content/uploads/Cover-Crop-in-Organic-Systems-California-Implementation-Guide.pdf)
• Soil Structure Improvement: Taproot with fibrous laterals reaches about 10–30 inches, helping open pore space and improve tilth; phacelia readily associates with arbuscular mycorrhizae. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf); [USDA‑NRCS MT‑113] (https://www.govinfo.gov/content/pkg/GOVPUB-A57-PURL-gpo76667/pdf/GOVPUB-A57-PURL-gpo76667.pdf)
• Water Management: Living roots enhance infiltration and reduce surface sealing; in California vegetable systems, cover‑cropped soils (including phacelia) were drier by late March—highlighting the need for timely termination in dry climates. [UMN Extension—Infiltration benefits of cover crops] (https://extension.umn.edu/soil-and-water/cover-crops); [Wyland et al. 1996, UC ANR PDF] (https://ucanr.edu/sites/default/files/2010-08/44275.pdf)
• Disease/Pest Break: As a different plant family from common cash crops, it can interrupt some pest/disease cycles. It attracts hoverflies whose larvae prey on aphids, but flowers can attract lygus—avoid near susceptible crops like strawberries. Also note it can host Sclerotinia minor and Rhizoctonia solani; avoid rotating with crops sensitive to these pathogens. [SARE California Implementation Guide] (https://www.sare.org/wp-content/uploads/Cover-Crop-in-Organic-Systems-California-Implementation-Guide.pdf); [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf)
• Nitrogen Management (non‑legume scavenger): Excellent at capturing residual nitrate; in a California on‑farm study, winter cover crops (including phacelia) reduced nitrate leaching 65–70% versus fallow. Phacelia residue decomposes quickly (C:N roughly 10:1–15:1), making N available to the next crop soon after termination. [Wyland et al. 1996, UC ANR PDF] (https://ucanr.edu/sites/default/files/2010-08/44275.pdf); [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf); [USDA‑NRCS/ISU Phacelia Fact Sheet, 2025] (https://www.nrcs.usda.gov/sites/default/files/2025-07/Phacelia%20Cover%20Crop%20Fact%20Sheet.pdf)
• Biomass Production: Very environment‑dependent. Montana NRCS trials reported 1,492–5,144 lb/ac dry matter (mean ~3,570–3,876 lb/ac) depending on seeding date; UC SAREP reports 3,300–6,000 lb/ac in coastal California; Iowa NRCS/ISU notes 200–1,000 lb/ac under shorter seasons. [USDA‑NRCS MT‑113] (https://www.govinfo.gov/content/pkg/GOVPUB-A57-PURL-gpo76667/pdf/GOVPUB-A57-PURL-gpo76667.pdf); [UC Davis SAREP] (https://sarep.ucdavis.edu/covercrop/phacelia); [USDA‑NRCS/ISU Phacelia Fact Sheet, 2025] (https://www.nrcs.usda.gov/sites/default/files/2025-07/Phacelia%20Cover%20Crop%20Fact%20Sheet.pdf)

Planting and Management Advice

• Seeding Rate (lbs/ac):
• Drill: 3–5 lbs/ac PLS (Midwest guidance) for basic cover; use higher rates for thicker canopy. In organic/weed‑suppression scenarios, 7–12 lbs/ac drilled is common. [USDA‑NRCS/ISU Phacelia Fact Sheet, 2025] (https://www.nrcs.usda.gov/sites/default/files/2025-07/Phacelia%20Cover%20Crop%20Fact%20Sheet.pdf); [SARE California Implementation Guide] (https://www.sare.org/wp-content/uploads/Cover-Crop-in-Organic-Systems-California-Implementation-Guide.pdf)
• Broadcast: 4–6 lbs/ac PLS (Midwest); 11–18 lbs/ac where a heavier rate is desired for uniform cover—firm with a cultipacker. [USDA‑NRCS/ISU Phacelia Fact Sheet, 2025] (https://www.nrcs.usda.gov/sites/default/files/2025-07/Phacelia%20Cover%20Crop%20Fact%20Sheet.pdf); [SARE California Implementation Guide] (https://www.sare.org/wp-content/uploads/Cover-Crop-in-Organic-Systems-California-Implementation-Guide.pdf)
• Note: Broadcast or aerial seeding generally requires higher rates than drilling to compensate for poorer seed‑to‑soil contact. [SARE—Managing Cover Crops Profitably] (https://www.sare.org/publications/managing-cover-crops-profitably/managing-cover-crops-in-conservation-tillage-systems/)
• Seeding Depth: 0.25 inch (¼ in). Shallow placement improves emergence; firm seedbed helps small seed establish. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf); [SARE California Implementation Guide] (https://www.sare.org/wp-content/uploads/Cover-Crop-in-Organic-Systems-California-Implementation-Guide.pdf)
• Soil Type and pH: Performs best on open, fertile, well‑drained sandy or gravelly soils; does poorly on waterlogged ground. Adapted to pH ~6.4–8.5. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf); [USDA‑NRCS MT‑113] (https://www.govinfo.gov/content/pkg/GOVPUB-A57-PURL-gpo76667/pdf/GOVPUB-A57-PURL-gpo76667.pdf)
• Planting Time (seasonal windows by region):
• West Coast/Mediterranean climates: Seed with first fall rains; grows fall–spring; incorporate or terminate in early to mid‑spring before soil moisture becomes limiting. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf)
• Northern/Central U.S. (Midwest/Upper Plains): Drill from early May through mid‑September (allow 6–8 weeks of growth before hard frost for fall plantings); winterkills in the mid‑teens (°F). [USDA‑NRCS/ISU Phacelia Fact Sheet, 2025] (https://www.nrcs.usda.gov/sites/default/files/2025-07/Phacelia%20Cover%20Crop%20Fact%20Sheet.pdf); [SARE California Implementation Guide] (https://www.sare.org/wp-content/uploads/Cover-Crop-in-Organic-Systems-California-Implementation-Guide.pdf)
• General temperature guidance: Germinates well between about 40–68°F; growth slows in heat. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf)
• Termination:
• Cold kill: Winter‑kills at roughly 18°F, providing a low‑risk, self‑terminating option in colder regions. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf)
• Mechanical: Mow/flail before seed set to prevent volunteers; residue decomposes quickly (low C:N). Rolling/crimping is less reliable on young, succulent stands; if used, do so at full bloom for better results. [USDA‑NRCS MT‑113] (https://www.govinfo.gov/content/pkg/GOVPUB-A57-PURL-gpo76667/pdf/GOVPUB-A57-PURL-gpo76667.pdf); [SARE—Managing Cover Crops Profitably] (https://www.sare.org/publications/managing-cover-crops-profitably/managing-cover-crops-in-conservation-tillage-systems/)
• Chemical: Where permitted, non‑selective herbicides labeled for broadleaf cover crops can terminate phacelia; always follow label and local guidance. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf)
• Rotational Considerations:
• Do not rotate immediately into susceptible vegetables/cash crops where Sclerotinia minor or Rhizoctonia solani are concerns; phacelia can host these pathogens. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf)
• Flowers can attract lygus; avoid planting adjacent to strawberries and other highly susceptible crops. [SARE California Implementation Guide] (https://www.sare.org/wp-content/uploads/Cover-Crop-in-Organic-Systems-California-Implementation-Guide.pdf)
• Useful pest break in rotations lacking Hydrophyllaceae crops; reported to suppress sugar beet cyst nematode in research cited by NRCS. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf)
• Water Requirements and Drought Tolerance:
• Performs with 7–18 inches of annual precipitation/irrigation; fairly drought tolerant relative to many broadleaf cover crops (rated “Very Good” for drought tolerance in Upper Midwest guidance). Terminate early in dry spring conditions to conserve soil moisture for the cash crop. [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf); [USDA‑NRCS/ISU Phacelia Fact Sheet, 2025] (https://www.nrcs.usda.gov/sites/default/files/2025-07/Phacelia%20Cover%20Crop%20Fact%20Sheet.pdf); [Wyland et al. 1996, UC ANR PDF] (https://ucanr.edu/sites/default/files/2010-08/44275.pdf)

Practical Notes

• For nitrate capture and erosion control, plant as early as feasible in fall (West) or at least 6–8 weeks before first hard frost (Midwest/North). Earlier planting increases biomass and canopy, which strongly drive benefits. [SARE California Implementation Guide] (https://www.sare.org/wp-content/uploads/Cover-Crop-in-Organic-Systems-California-Implementation-Guide.pdf); [USDA‑NRCS/ISU Phacelia Fact Sheet, 2025] (https://www.nrcs.usda.gov/sites/default/files/2025-07/Phacelia%20Cover%20Crop%20Fact%20Sheet.pdf)
• Expect rapid residue breakdown (low C:N ~10–15:1), quick N release, and relatively low lasting mulch—consider mixing with a small grain if longer‑lasting residue or stronger erosion control into spring is a goal. [USDA‑NRCS/ISU Phacelia Fact Sheet, 2025] (https://www.nrcs.usda.gov/sites/default/files/2025-07/Phacelia%20Cover%20Crop%20Fact%20Sheet.pdf); [USDA‑NRCS Plant Guide] (https://www.nrcs.usda.gov/plantmaterials/capmcpg13361.pdf)
• Typical biomass benchmarks help set expectations: ~1,500–5,100 lb/ac in interior West spring seedings; ~3,300–6,000 lb/ac in coastal CA; ~200–1,000 lb/ac in short‑season Upper Midwest plantings. [USDA‑NRCS MT‑113] (https://www.govinfo.gov/content/pkg/GOVPUB-A57-PURL-gpo76667/pdf/GOVPUB-A57-PURL-gpo76667.pdf); [UC Davis SAREP] (https://sarep.ucdavis.edu/covercrop/phacelia); [USDA‑NRCS/ISU Phacelia Fact Sheet, 2025] (https://www.nrcs.usda.gov/sites/default/files/2025-07/Phacelia%20Cover%20Crop%20Fact%20Sheet.pdf)

This guidance pulls from USDA‑NRCS plant materials documents, university extension, SARE, and peer‑reviewed research hosted by UC ANR to provide region‑specific, actionable numbers for phacelia cover cropping.