Cover Crops

by Julia Frisbie

posted August 19, 2021

Mid to late August is the time to plant overwintering cover crops. Also known as living mulches and green manures, the goal when planting a cover crop is to cover the soil between main crops. By now you know that I’m really into mulching, and always try to keep my soil covered. Cover crops have lots of benefits, especially over the winter season:

  • They prevent erosion.
  • They suppress weeds (like bittercress, the winter germination specialist).
  • They feed the “underground herd” (a term borrowed from Gabe Brown’s book Dirt to Soil) of microbes and mycorrhizae by keeping living roots in the ground year-round.
  • They provide habitat and food for above-ground critters who share space with us in winter.
  • They add organic matter to the soil.

As if all this weren’t enough of a gift, many species offer additional benefits. Some are insectaries. Some can help break up compacted soil (especially brassicas, like mustards and daikon radishes). Some are really amazing dynamic accumulators, which means they’re especially efficient at building biomass. Many of the most popular cover crop species can add nitrogen to the soil (like legumes). Cover crops teach us that each individual has a role to play in supporting the health of the whole. 

To compare common cover crop species, check out Gaia’s Garden by Toby Hemenway. It’s another book that I finally purchased after borrowing from our local library three times… so I know the library has it! I love the author’s way of describing plant functions in tables. Here’s the one on winter cover crops:



So, what works best on Fidalgo island? I haven’t experimented with grasses and grains, but I’ve tried several types of clover (all clovers fix nitrogen!) and lots of wildflowers. Here are a few favorites.

  • Crimson clover. It’s fuzzy and soft with lovely, deep magenta booms. The only downside is that it doesn’t suppress weeds as readily as other clovers because it’s tall and skinny rather than spreading.
  • Microclover. It’s adorable and very low-growing, but it takes a long time to establish because it’s so small.
  • New Zealand White clover. This is my clover of choice for weed suppression. It out-competes weeds and becomes a perennial, standing up to foot traffic in my yard year after year.
  • Phacelia. Its common name is “bees’ friend” for a reason. It can grow four feet tall and gives me a rash when I brush against it, so I don’t plant it next to paths. This plant is an annual, but if you let it go to seed once, you’ll have it forever.
  • Poppies. I love looking at a sea of bright color. They require so little care, and seed is so cheap, that if you have a big blank spot in your yard you have nothing to lose by throwing poppy seeds at it. I’ve grown both Eschscholzia californica and Papaver rhoeas with good success. Like phacelia, poppies are annuals, but they reseed themselves freely.

I’d encourage you to plant as diverse a cover crop mix as you can stand to look at, because different species fill different ecological niches. For example, in my last article I noted that Jean-Martin Fortier of The Market Gardener plants an oat/pea mix as green manure in late August and early September. Peas fix the nitrogen, but can be a bit slow to start. Oats grow quickly to suppress weeds, creating a perfect environment for the peas to do their thing. These two offer a greater benefit together than they do apart. Gabe Brown takes it farther by planting dozens of species at once in the same place. Cover crops teach us that the whole can be greater than the sum of its parts. 

Cover crops can enrich existing garden beds, but they can also be used to help prepare new ones. Fall is the best time to start new garden beds, so if you’re thinking of sheet mulching your grass into oblivion, now is the time to start planning. A lush cover crop is a wonderful way to tuck all your new compost/soil into place over the winter months.

Whatever you choose, mix the seeds together in a substrate like sand before broadcasting onto damp soil and tamping down. Then, just let nature do its thing. Some, like the clover, will germinate right away. Some, like the poppies, will wait until spring to get started. Not every plant will thrive in every spot… but by sowing a diverse mix, you will have hedged against some of them failing, so you’ll still end up with your soil covered and enriched over the winter months. Cover crops teach us that we can’t afford monocultures. Variety is protective against an unknowable future.