Cover Crops in Seattle: What to Sow Before October
A September look at which cover crop seed belongs in which bed, and why the next two weeks decide whether any of it works.
Last week I put up the feeding plan I put up last week and said September is the highest value month of the year for anything you do to soil. Then I gave the cover crop one paragraph, which was not enough, so here is the rest of it.
Cover crops in Seattle go in between the middle of September and the first week of October, and the seed you buy depends on which bed it is going into and what you plan to grow there next spring. That second part is the one people skip. They buy whatever bag says cover crop mix, scatter it everywhere, and then stand in front of a chest high wall of rye in April wondering how to get it out before the tomatoes go in.
What bare soil costs you over a winter here
A bed that sits open from October to March takes about five months of rain straight onto the surface. Three things happen, and none of them are dramatic enough to notice until spring.
The nutrients you paid for leave. Water moving down through loose raised bed soil carries nitrogen and boron with it, and a bed with nothing growing in it has no roots to catch any of that on the way past.
The surface caps. Rain hitting bare soil breaks the crumb structure at the top, and by February you have a dense, slightly grey lid over the bed. It is the same soil you were proud of in July. It just got hit ten thousand times.
And the bittercress arrives. Hairy bittercress germinates in the fall, sits through the winter looking harmless, and starts firing seed at you in February before you have your boots on. An empty bed is an invitation. A bed with something growing in it is not.
This is most of what people mean when they tell me about tired soil in raised beds. The soil is not tired, it spent the winter unemployed.
When to sow cover crops in Seattle
The window is narrower than the seed packets suggest, because the packets are written for the whole country.
Mid September is the good moment for crimson clover. It wants the first half of the month to get going, so if you are reading this on the 15th, you are at the back edge of it. Sow it this week and it will still make a stand.
Late September through the first week of October belongs to cereal rye, winter wheat and fava beans. All three germinate in cool soil, which the clover will not do once we get into October. Fava beans in particular are happiest going in during October, because they grow faster in cool weather than any other legume we can use here.
After about the 10th of October, stop. A cover crop sown too late either does not come up or comes up so thin that it protects nothing, and then you have paid for seed and got a bed of mud with a green haze on it. Beds that are still full of kale and leeks in November were never cover crop beds anyway. Mulch those with leaves or compost instead and call it done.
Which seed for which bed
This is the decision that matters, and it takes about two minutes if you know what is going in the bed next spring.
Crimson clover. A legume, so it makes its own nitrogen. Easy to cut down, easy to turn under, does not leave you fighting anything. The right choice for a bed that gets a heavy feeder next year, so brassicas, squash or corn. It is poor at crowding out weeds, so it wants a clean bed.
Fava or bell beans. The other nitrogen source, and the one to buy if you are sowing in October. Thick stems and deep roots, good for a bed that feels compacted. They need chopping before they go under, so plan for ten minutes with the pruners rather than a quick pass with a rake.
Austrian winter peas. Fast, soft, pleasant to cut down, and the shoots are edible in early spring if you want a bonus. They lose to winter weeds in a mild year, so put them in a bed that is genuinely clean.
Cereal rye. The workhorse. Germinates in cold soil, holds the surface, and its roots catch the nitrogen that is otherwise heading for Puget Sound. It is also the one that gets away from you. Grown on its own it can tie up nitrogen for the following crop, and by late April it is tall and tough. Never put rye in a bed you want to plant in March.
A rye and vetch or rye and pea mix. The grass holds the soil and scavenges, the legume adds nitrogen, and the legume uses the rye stems to climb in spring. This is the reliable default when you are not sure yet what is going in the bed.
Amounts, because everyone buys far too much. Per 100 square feet you want roughly a quarter cup of crimson clover, or one cup of rye, favas or winter peas. A standard four by eight foot bed is 32 square feet, so that is about one tablespoon of clover, or a third of a cup of the bigger seed. The shallow beds on my south side are smaller than that and take almost nothing.
One thing worth doing with any legume the first time you grow it: buy seed that comes pre-inoculated, or buy the inoculant separately and mix it in. The bacteria that let a legume fix nitrogen are not always present in a new bed. You can check in spring by pulling one plant and slicing a root nodule with your thumbnail. Pink inside means it is working. White or green means it is just a plant.
To sow, clear the bed, rake it level, scatter the seed by hand, rake lightly so most of it ends up just under the surface, and water once if the rain has not arrived. Seed sitting on top is bird food.
The part that goes wrong in spring
Cover crops fail in April, not in September, and almost always for the same reason. People let them get big because they look good, then cut them down the same weekend they want to plant.
Cut it down before it flowers. Crimson clover at first bloom, favas before pods form, rye while it is still soft enough to cut with shears. Then wait at least two weeks before you plant into that bed, and closer to three or four if there is a lot of material. The soil biology that breaks the residue down uses nitrogen to do it, and for those two weeks it is competing with your seedlings. Young, soft growth breaks down fast. Woody spring rye does not.
You can chop it and leave it on the surface as a mulch, or turn it in shallowly. Both work. Turning it in works faster, leaving it on top disturbs less. I do both, depending on how much time I have and which bed is first in line.
If you do one thing this week
Look at which beds are coming empty in the next two weeks, and buy seed for those beds specifically. Crimson clover for anything getting a heavy feeder next year, rye or a rye mix for the beds you will not touch until late May, favas for whatever comes empty in October. Everything still cropping gets mulched, not seeded.
That is a few dollars of seed and one afternoon, and it does more for next spring's soil than anything you can buy in April.
If you want the whole feeding picture rather than just the September piece, the PNW Organic Feeding Calendar lays out all seventy two crops month by month, with amounts scaled to your bed size and the stop month for each one. It opens in any browser and it is yours once you download it.
And the next Walk My Garden With Me is Saturday, October 10, 1:00 to 2:30, here in Phinney Ridge. Free, capped at twelve people, nothing to buy at the end. By then the cover crops will be up and you can see what a three week old stand actually looks like, which is less impressive than you expect and exactly the point.
Happy fall gardening. I am here for you.
Jackie
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