Leaf Mulch in Seattle: How to Cover Your Beds for Winter
An October guide to the free mulch falling on every Seattle street: which leaves to use, how thick to spread them, and where to keep them away from.
Three times this fall I told you to cover an empty bed with fallen leaves: in the cover crop post, in the October planting list, and again last week. Every time it got one sentence. That is the same thing I did to cover crops in the feeding plan, so here is the rest of it.
Leaf mulch in Seattle is the answer for every bed that is still bare today. The cover crop window closed around October 10, and seed sown now comes up too thin to protect anything. Leaves are what you use instead, and the timing could not be better, because the big leaf drop here runs from late October through November. The trees are giving it away for free. You only have to carry it from the sidewalk to the bed.
What leaf mulch does over a Seattle winter
A bare bed takes five months of rain straight onto the surface, and I explained what that costs you in cover crops in Seattle: the nutrients wash down, the surface caps into a grey lid, and the bittercress moves in. A few inches of leaves stops most of that. The rain lands on the leaves instead of the soil, the surface stays loose, and weed seeds that need light to germinate stay in the dark.
Underneath, the worms do the work. They pull leaf pieces down into the bed all winter, and by March the bottom layer has turned soft and dark, close to compost. That is organic matter you did not have to buy, bag, or haul home from the garden center.
I want to be honest about one thing. Leaves are not as good as a cover crop. A cover crop has living roots that catch nitrogen before the rain carries it off, and a layer of leaves can't do that. But leaves protect the surface, feed the soil life, and cost nothing, which makes them the best second choice there is.
Which leaves to use, and which to skip
Most leaves from Seattle yards and street trees are fine. Maple, birch, cherry, apple, plum, ash and linden all break down well and can go onto the beds once they are shredded. A few need more thought:
Big leaves need shredding first. Bigleaf maple leaves can be bigger than a dinner plate, and whole they mat into a wet sheet that the rain runs off instead of through.
Oak leaves are slow. They are tough and take longer to break down, so mix them with other leaves rather than using them on their own.
Skip black walnut. Its leaves carry a compound that stunts tomatoes, peppers and potatoes. Those go in the yard waste cart.
Skip anything that was sick. Leaves with apple scab, rust or powdery mildew carry the problem straight into next year.
Skip the gutter. Leaves swept up along a busy street come with grit, oil and whatever the cars left behind.
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How to mulch a bed with leaves
Shred first, always. I don't own a lawn mower, so I cut mine into pieces with a pair of large shears, and that works fine. If you have a mower, run it over the pile a couple of times, or fill a trash can halfway and work a string trimmer down into it (safety glasses on for this one). Shredded leaves stay where you put them, let the rain through, and mostly break down in one winter. Whole leaves blow into the neighbor's yard with the first windstorm, and the ones that stay behind glue themselves together.
Then go bed by bed:
Empty beds: 3 to 4 inches. It looks like a lot, but it settles to about half by February.
Garlic: plant first, then cover with 3 to 4 inches. The shoots push through on their own in late winter.
Beds with crops still growing: 1 to 2 inches between the plants, kept a hand's width away from every stem.
Don't dig the leaves in. Raw leaves are mostly carbon, and the soil life that breaks them down borrows nitrogen from the soil to do it. On top of the bed that hardly matters, but mixed into the root zone it does. If your soil test came back low in nitrogen, like mine did this year, that is one more reason to leave them on top.
A thin layer of compost under the leaves is fine, and honestly it is the only feeding a bed needs this time of year. I explained why the fertilizer can wait in when to fertilize vegetables in Seattle, and when to stop. If it isn't raining when you spread the leaves, water them once so they settle and stop moving around.
The part that goes wrong: slugs and spring
Leaf mulch causes two problems here, and both are easy to avoid once you know about them.
The first one is slugs. A damp layer of leaves is exactly where a slug wants to spend the day, and if that layer touches a young cabbage, the slugs don't even have to commute. I already have slug and caterpillar damage on some of my young cabbages and kohlrabi this month, which I wrote about in six questions I ask my Seattle garden in October, so the last thing those plants need is a leaf pile pushed up against their stems. Keep the layer thin around young transplants and pull it back a hand's width. Once the plants are bigger and tougher, you can fill in closer.
The second one is spring. The same layer that protects the soil all winter also keeps it cold and wet, and Seattle soil is slow enough to warm up without any help. About two weeks before you plant a bed in March or April, rake the leaves to the side or onto the paths and let the sun reach the soil. Whatever hasn't broken down goes into the compost, or back around the plants once they are up and growing.
If you do one thing this week
Stop putting leaves in the yard waste cart. Rake them onto a tarp or into a corner, shred them, and then look at your beds one at a time. Bare beds get three to four inches, the garlic gets covered right after planting (the timing is in what to plant in October in Seattle), and the young brassicas get a thin layer kept away from their stems. If you end up with more leaves than beds, keep the rest in bags or a wire bin. Next summer your compost pile will be begging for browns, and the trees won't be dropping any.
That is one afternoon and no money, and your soil comes out of winter better than it went in.
Want someone to walk your beds with you?
If you'd like help deciding what each of your beds needs this winter, that is exactly what a Kitchen Garden Consultation is for. I come to your garden for an hour, we walk it together, and within three business days you get a written plan for the season ahead. It is $175.
And this Saturday, October 17, from 1:00 to 2:30 pm, Walk My Garden With Me is here in Phinney Ridge. It is free, small on purpose, and there is no pitch at the end. Save your spot here.
Happy fall gardening. Let me know if you need help, I'm here for you.
Jackie
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