Cover Crops for Portland Gardens

The idea of planting crops to feed the soil is ancient. As early as 6000 B.C., farmers in Greece, Rome, China, and the Middle East grew legumes between harvests to restore fertility. By the 1700s, Northern European farmers were experimenting with lupines to improve poor, sandy soils, and by the mid-1800s, these European cover crops had become a part of farming in the US.

Modern gardeners are rediscovering what our ancestors knew: keeping living roots in the soil protects it from erosion, feeds microbes, adds nutrients, and creates a healthier foundation for the next season’s crops. One of the best ways to keep your garden thriving year after year is to plant cover crops in the fall. They protect bare soil through winter, add fertility, and reduce weeds in the spring. In a diverse food garden, no single cover crop fits every space.

Use the following guide to find the right mix for your gardens.

Under Fruit Trees

Choose cover crops that can grow under partial shade and won’t make tending or harvesting difficult.

• Crimson clover (Trifolium incarnatum) — Nitrogen-fixing and produces bright crimson blooms in spring. Taller (12–20"), so best in open spots under the canopy but away from the trunk. (chop-and-drop)

• Annual ryegrass (Lolium multiflorum) — A quick, dense grass that knits the soil, prevents erosion, and suppresses weeds. (mow)

Tip: sow lightly near the trunk and heavier toward the dripline where the soil is bare. Around trees that must be harvested with a ladder, sow a foot-wide ring of ryegrass or spread arborist chips outside the canopy as a path around the tree.

In Open Beds

Where there’s room to sprawl, taller cover crops are perfect because they generate lots of biomass that becomes mulch and compost in place.

• Fava beans (Vicia faba) — Fix nitrogen and produce edible beans in spring. (chop-and-drop)

• Field peas (Pisum sativum) with oats (Avena sativa) — Peas add nitrogen, oats winter-kill leaving mulch. (chop-and-drop)

• Winter rye (Secale cereale) — Very cold-hardy, grows tall, and suppresses weeds through natural allelopathy. (best for crimping)

Around Shrubs

Around blueberries, pomegranate, beautyberry, and other fruiting shrubs, low-growing covers work best, suppressing weeds without smothering shrubs.

• Native clovers (Trifolium microcephalum, etc.) — Annuals that provide living mulch and nitrogen. (light shear if needed)

• Achillea millefolium var. ‘Yaak’ (dwarf yarrow) — Forms a 6–12" perennial mat with pollinator-friendly flowers. (shear after bloom)

• Thymus serpyllum (creeping thyme) — A 2–4" fragrant groundcover that thrives in full sun, tolerates light foot traffic, and smothers weeds. (living mulch, no mowing)

New Garden Beds or Compacted Areas

When soil is hard or you’re preparing a new growing area, nothing beats nature's soil-driller, daikon radish (Raphanus sativus var. niger). Daikon sends down deep taproots that break up compacted soil and scavenge nutrients. Winter frost kills them, the roots decompose in place, leaving channels for air and water. Portland winters are occasionally mild enough for daikon to survive, and in the spring, they can be harvested or left to grow even larger. (usually frost-killed, no mowing)

High-Traffic Spots

Paths and edges require tough plants that can withstand the impact of feet and wheels.

• Annual ryegrass (Lolium multiflorum) — Knits the soil together. (mow)

• Low-growing perennials like yarrow (Achillea ‘Yaak’) or creeping thyme (Thymus serpyllum) stabilize soil and outcompete weeds. (shear or leave as living mulch)

Special Spots

• Douglas’ meadowfoam (Limnanthes douglasii) — A cheerful spring bloomer, native to the Pacific Northwest, that supports early pollinators and reseeds readily. (dies back naturally)

• Western buttercup (Ranunculus occidentalis) — Suited to damp spots, where it can compete with invasive creeping buttercup (Ranunculus repens). (leave in place, pull, or light shear after bloom)

Bonus Cover Crop: Calendula

If you want something practical and pretty, sow calendula (Calendula officinalis) this fall. Unlike classic cover crops, calendula isn’t a nitrogen fixer, but it earns its place in the garden:

• Cold-tolerant: Survives Portland winters, often blooming right through frosts.

• Soil cover: Protects bare soil with leafy rosettes, shading out weeds.

• Pollinator support: Produces some of the earliest flowers for bees in late winter and spring.

• Self-seeding: Easy to naturalize in veggie beds.

Spring management: Chop-and-drop calendula before it sets seed if you want mulch, or let some bloom to enjoy cut flowers and pollinator visits.

How Much Seed Do You Need?

Figuring out how much seed you actually need for cover cropping requires:

• Knowing the seed's recommended sowing rate, usually provided in pounds per acre, grams per square foot, or ounces per thousand square feet. Sowing too few seeds causes poor coverage; sowing too many can lead to poor growth or even no germination (ask me how I know).

• Calculating how many grams or ounces are needed for a 4′ x 8′ bed or the space under a fruit tree canopy, based on the sowing rate.

  
  

Once you have that figured out, you can weigh the required grams or ounces of seed.

That's a lot of math! I measure and pack seeds in convenient sizes for 100 square feet or 25 square feet. Select the packet that matches your garden bed or orchard tree, mix with sand, and sow.

Why Plant Now?

Fall is the ideal time in Portland. The soil is still warm enough for quick germination, and the autumn rains keep seedlings watered. A few weeks from now, you’ll have a lush green carpet protecting your garden all winter.

Protecting with Tulle

New seedlings are irresistible to birds, squirrels, and slugs. I cover all newly soil with tulle. This lightweight fabric, the same kind used in wedding veils, keeps pests out while letting sunlight, rain, and air through. I first saw the idea from Robbie & Gary Gardening Easy on YouTube, and now I’m hooked. It’s inexpensive, reusable, and easy to drape over hoops or stakes. I have tulle in forest green and chartreuse and they brighten up the beds in winter.

What to Do with Cover Crops in Spring and Summer

Planting cover crops in fall is just the first step — how you handle them the following spring determines whether they become food for your soil or a nuisance in your beds. Here’s a guide to what to chop, mow, or manage differently:

Chop-and-Drop Mulch

Some cover crops are perfect for cutting down and leaving in place as mulch. This creates an in-situ compost layer that feeds soil microbes and suppresses weeds.

• Fava beans (Vicia faba) — Harvest beans for fresh eating. Let 2-3 plants go to seed and save seeds for planting in the fall. Cut most plants at ground level before they set too much seed. Leave the roots (with nitrogen nodules) in place. 

• Field peas (Pisum sativum) + oats (Avena sativa) — Peas die once it warms up, and oats usually winter-kill. Chop any remaining stems and leave them on the soil.

• Crimson clover (Trifolium incarnatum) — Chop at flowering (before it sets seed). The stems and leaves decompose quickly into mulch.

Mow or Crimp for a Living Mulch Mat

Grasses and cereals can be mowed down or crimped to form a weed-suppressing mulch.

• Annual ryegrass (Lolium multiflorum) — Mow in spring before it goes to seed. It breaks down fast and is easy to manage.

• Winter rye (Secale cereale) — Mow or crimp at flowering to kill it. It leaves a thick, straw-like mat that’s great for suppressing weeds in summer crops like squash or tomatoes.

Pull or Let Frost Take Them

Some crops aren’t ideal for mowing but play a role in soil improvement.

• Daikon radish (Raphanus sativus var. niger) — Usually winter-kills in Portland. The rotting roots leave behind natural “soil channels.” If they survive, pull them up and compost, or slice at the soil line.

• Meadowfoam (Limnanthes douglasii) — Dies back naturally after flowering; leave residues as mulch or rake them into compost.

• Western buttercup (Ranunculus occidentalis) — Allow to bloom, then pull or mow lightly if needed to prevent reseeding.

Shear Back Perennial Living Mulches

Low perennials like thyme and yarrow don’t need to be replaced each year.

• Achillea millefolium var. ‘Yaak’ (dwarf yarrow) — Can be mowed or trimmed after flowering to refresh growth.

• Thymus serpyllum (creeping thyme) — A true living mulch. Shear lightly if it gets too woody or overgrows paths.

• Native clovers (Trifolium microcephalum, etc.) — Allow to reseed and spread, or cut lightly after bloom if you want to limit seeding.

Key takeaway:

• Chop-and-drop legumes (favas, peas, crimson clover) to feed soil.

• Mow or crimp grasses (rye, oats, ryegrass) for mulch.

• Let frost or natural cycles handle daikon, meadowfoam, buttercups.

• Trim perennials like thyme and yarrow as needed — they’re long-term partners, not seasonal crops.

Mow vs. Crimp: What’s the Difference?

Cover crops like rye and ryegrass can be “terminated” in spring by either mowing or crimping. Both methods leave biomass on the soil as mulch, but they work a little differently:

Mowing 

• Cuts stems cleanly at the base.

• Works at almost any stage of growth.

• Plants may regrow if cut too early (especially ryegrass).

• Creates a looser mulch layer that breaks down faster.

Crimping 

• Bends and crushes stems at the base instead of cutting them.

• Only effective once grasses are flowering (late April–May in Portland).

• Plants die in place, leaving a thick, woven mat.

• Mulch layer suppresses weeds longer — ideal under summer crops like squash or tomatoes.

Rule of thumb:

• Use mowing if you want quick turnover or fresh mulch for the compost pile.

• Use crimping if you want a long-lasting weed barrier right where the plants grew.