Growing Soil: How Cover Crops Can Boost Your Garden

by Brett Kerley

As gardeners, we usually plant for food, beauty, or pollinators. But what if we planted for the soil itself? That’s the idea behind cover crops. They protect, replenish, and rejuvenate the garden when it’s not actively growing vegetables or flowers.

In Edmonton’s short-season, Zone 3-4 climate, cover crops offer a low-cost, high-impact way to improve garden soil, especially in newer neighbourhoods or compacted plots. They’re ideal for raised beds, backyard gardens, community garden plots, and even large containers.

Cover crops—also called green manure—are plants grown specifically to cover bare soil and build soil fertility. Unlike vegetables or ornamentals, these plants are not harvested. Instead, they’re turned back into the earth to feed the soil, prevent erosion, suppress weeds, and increase organic matter.

They may be used:

• In the fall after harvesting crops
  
• In spring before planting
  
• Between plantings in summer
• As long-term living mulch in pathways or around perennials

Tomatoes with Crimson Clover Cover Crop

Zone 3/4 gardening comes with challenges:

• Short growing season (110–140 frost-free days)
• Freeze-thaw cycles that compact and damage soil
• Erosion risk during snowmelt and spring rains
• Compacted, lifeless, or disturbed soils in new developments
• Weed pressure from exposed soil in spring and fall

Cover crops help resolve these challenges with multiple benefits:

Improve Soil Structure

The roots of cover crops loosen compacted clay and sandy soil. This improves aeration, drainage, and water retention—especially important in raised beds and dense subsoils.

Build Organic Matter

When chopped and returned to the soil, cover crops feed soil organisms, increase humus, and improve long-term fertility and texture.

Suppress Weeds

A fast-growing cover crop shades out weed seedlings, acting as a natural, living mulch.

Provide Natural Fertilizer

Leguminous crops (like clover and peas) fix nitrogen from the air into the soil through root nodules, reducing the need for added fertilizers.

Prevent Erosion & Leaching

By covering the ground during heavy rain or spring melt, cover crops protect your topsoil and hold nutrients in place.

Fall Rye (Secale cereale)

• Zone: Hardy to Zone 2
• Height: 24–48 in (60–120 cm)
• When to Plant: Late August to mid-September
• Cold Hardy? Overwinters reliably in Edmonton
• Benefits: Dense roots break up clay, suppress weeds, survives winter and regrows early
• Turn-In Tip: Mow or dig in by mid-May, before flowering for easier incorporation
• Ideal For: Overwintering cover for spring beds, fallow community plots

Field Peas (Pisum sativum subsp. arvense)

• Zone: Hardy to Zone 3
• Height: 18–36 in (45–90 cm) (climbing; needs support if grown alone)
• When to Plant: Early spring or late summer
• Cold Hardy? Yes – grows well in cool soil
• Benefits: Nitrogen fixer, adds organic matter, breaks up soil
• Tip: Combine with oats for a balanced root and shoot mix
• Ideal For: Soil fertility boosts before heavy feeders (tomatoes, squash)

1. Clear the Bed: Remove finished crops, debris, and large weeds. Rake smooth.
2. Broadcast Seeds: Sow seeds by hand or use a small spreader. Rake lightly to cover with soil (~¼ to ½ inch deep).
3. Water In: Gently water until the soil is moist but not soggy. Keep soil damp during germination (4–10 days).
4. Grow for 4–8 Weeks: Let your cover crop mature to about 6–12 inches (15–30 cm) tall, or flower stage for buckwheat and vetch.
5. Cut or Mow Before Seed Sets: Cut the crop down using garden shears, pruners, or a string trimmer. For winter-killed crops, skip this step.
6. Incorporate or Mulch: Turn green matter into soil or leave it as mulch. Wait 2–3 weeks before planting seeds.

• Mix & Match: Try pea/oat combos or buckwheat/clover mixes for texture, nutrients, and balance.
• Use containers: Old pots or raised beds can grow quick oats or peas in fall.
• Living mulch: Clover is ideal between stepping stones or under squash vines.
• Pathways: Sow white clover in walking areas to reduce compaction and fix nitrogen.

Plots often sit empty for 6+ months. Sow rye or clover after harvest to:

• Stop spring weeds
• Improve next year’s yield
• Keep your plot looking lush and maintained

Check with your garden coordinator about digging crops into shared soil.

• Don’t let rye or buckwheat go to seed unless you want volunteers!
• Don’t plant new seeds immediately after turning in green material—wait 2–3 weeks for decomposition.
• Don’t skip watering during germination, especially in dry autumns.

Many EHS members are turning to cover cropping—especially in backyard raised beds and community plots. At the 2025 EHS Garden Tour, several members shared how annual rotations of peas, oats, and clover have improved their harvests, loosened soil, and reduced their need for compost over time.

Cover crops aren’t flashy. They don’t bloom in technicolour or fill your basket with food. But they quietly and powerfully build your soil, restore fertility, and support sustainable gardening—especially important as we adapt to climate change, soil degradation, and water stress.

Try one this season. Your future self (and your veggies) will thank you.