What Is Green Manure?
Green manure is a cover crop grown not for harvest but to be cut down and incorporated into the soil — adding organic matter, fixing nitrogen, suppressing weeds, and protecting soil structure during the off-season. The term comes from turning under young, green growth before it seeds, feeding the soil the way animal manure does.
Why Cover Crops Matter
Bare soil is biologically impoverished soil. Without plant roots and organic matter inputs, beneficial microbes decline, structure deteriorates, and weed seeds have clear conditions to germinate. A cover crop planted after the main crop is harvested keeps soil alive and improving rather than declining through the dormant season.
Top Cover Crop Choices by Goal
Nitrogen Fixation: Legumes
Legumes (clover, vetch, peas, beans) host rhizobacteria that pull nitrogen from the air and convert it to plant-available form in root nodules. When incorporated, this nitrogen feeds the following crop. Plant hairy vetch or winter peas in fall for spring incorporation; plant cowpeas in warmer weather for fall incorporation.
Organic Matter Addition: Grasses and Grains
Winter rye, oats, and buckwheat produce high volumes of biomass that add significant organic matter when incorporated. Winter rye is the most cold-hardy and germinates in soils too cold for most crops. Buckwheat is a warm-weather option that also suppresses weeds aggressively and attracts beneficial insects.
Compaction Breaking: Daikon Radish
Tillage radishes grow massive taproots that penetrate compacted layers 12-24 inches deep. When they die over winter, the root channels remain — creating passages for subsequent crop roots and improving drainage dramatically.
Timing and Termination
Plant fall cover crops 4-6 weeks before first hard frost to ensure establishment. Terminate in spring 2-3 weeks before planting, giving biomass time to break down. Terminate before crops set seed — especially critical for aggressive self-seeders like winter rye.
Frequently Asked Questions
Can I use cover crops in a raised bed?
Yes. Plant a fall cover crop after summer crops are removed, then cut down and incorporate in spring 3-4 weeks before planting. A hairy vetch plus oats mix works well. The bed improves year over year.
Do cover crops actually fix enough nitrogen to matter?
A good stand of hairy vetch can fix 60-150 lbs of nitrogen per acre — enough to partially or fully meet the following crop nitrogen needs. Results vary with soil pH, moisture, and inoculant availability.
What is a legume inoculant?
Legume inoculants are commercial preparations of rhizobacteria that enable nitrogen fixation. They are especially important in new beds where the bacteria are not yet established. Coat seeds before planting according to package directions.
Do I have to till in the cover crop?
No. The no-till approach leaves crop residue on the soil surface as mulch, planting directly through it. This preserves soil structure and reduces labor. Cut flush with the soil and cover with cardboard for garden-scale no-till.
How do cover crops suppress weeds?
By occupying the space where weeds would germinate — dense canopy shades out weed seedlings. Buckwheat is notably effective because it germinates and establishes quickly. Winter rye produces allelopathic compounds that inhibit many weed seeds even after incorporation.