Mulch. I know I hammer away at the mulch topic on this blog. But I truly think it’s that important. And this time of the year, we have a ready supply of something that makes fabulous mulch. Please read on.
Maintaining a thick layer of mulch is one of the best ways to keep the weeds at bay in our region. Make it ‘fresh’ mulch as opposed to the beautiful, black, partially-composted stuff. That stuff has begun the composting process, and as a result has some nitrogen released and available for feeding established plants and germinating seeds. ‘Fresh’ mulch, like fresh bark, arborist chips or freshly fallen leaves, still has its nitrogen locked up tightly in its cellular structure making it unavailable for uptake by plants. When you use these ‘fresh’ woody mulches, they DO eventually compost. But they start doing so where the fresh leaves touch the existing soil or compost layer. You should maintain at least 2-3″ of ‘fresh’ mulch at all times. Below that there will be a layer of active composting. And below that the worms, microbes and other soil critters will move the composted leaves more deeply into the soil, thus enriching the topsoil.
I realize we (Master Gardeners, garden designers, and other gardening professionals) used to tell people they could use compost as mulch. But in areas like here in Clark County, WA, where as much as 50% of weeds distribute their seeds on the air currents, you don’t want compost (growing medium) exposed on top of the soil. I’m going to say it: WE WERE WRONG! This is why I love science. We learn new stuff as we get smarter and acquire better tools for observation and measurement. And our theories, rules and practices change accordingly.
In our region, to avoid weeds, we need to cover the bare soil and compost layer with fresh woody mulch. As the ‘fresh’ mulch turns dark, apply more fresh. That seems like the usual chore (and expense) of applying more bark mulch every year. But there is a very elegant solution. Use your fall leaves as mulch! They are high-carbon, and they are ‘fresh’. Over the course of a year they turn into fabulous compost to feed your plants. And best of all, Mother Nature renews the supply for you — FOR FREE! — every year. The only thing you have to do is make sure you plant enough deciduous trees and shrubs to supply the amount of leaves your garden needs, and then let the fallen leaves lay each autumn. You will likely need at least 4″ of freshly fallen leaves that will then compact down to be closer to 2″. And it’s recommended to have 1/3 – 1/2 of your garden be deciduous plants to supply enough leaves.
This is a new aesthetic. Fall leaves, fresh bark, or arborist chips as mulch. We designers have been orchestrating ‘tidy’ gardens for a very long time. But guess what? Messy gardens are easier. And messy gardens are more appealing to birds and other wildlife. It’s fall. The leaves are falling. What will you be doing with YOUR fall leaves? Please consider letting they lay on your garden beds.
Note: There are some leaves that aren’t suitable for letting lay in the ornamental garden (or on the lawn). Thick leathery ones like Southern magnolia. Huge ones like Sycamore or Big Leaf Maple. Or ones that could possibly have foliar diseases like many of our cherry and apple species trees.