Pro tips for mulching your perfect Pacific Northwest garden

LOCAL GARDEN GURU Christina Pfeiffer maintains that proper mulching is the secret to beautiful gardens. Pfeiffer, a professional horticulturist and popular garden educator, recommends mulching the soil like nature does. Most home landscapes are a mix of trees, shrubs and perennials, conditions most like a woodland, where coarse materials, like fallen leaves and twigs, gradually decompose into a fine-textured, biologically rich layer of humus soil.

“A fine textured surface material [like compost or fine bark] might look pretty at first, but it’s also friendly to weed seed germination,” Pfeiffer says. Furthermore, compost can crust over and prevent water and oxygen from penetrating the soil. However, compost is an excellent amendment when worked in or layered on the surface of the soil beneath a coarse mulch.

Coarse textured materials, such as wood chips that are at least ½-inch in diameter, applied 2 to 4 inches deep, will effectively shade out seed germination and suppress weeds. Chunky materials won’t crust and, in fact, will form channels that help direct moisture into the soil. A coarse layer, about 3 to 4 inches, of organic mulch helps to reduce storm runoff and prevents soil compaction during the rainy season.

Now is a good time to examine your soil, lightly stir the surface with a hoe or garden fork to break up any crusting, and lay down coarse mulch. Arborist wood chips are a good source of coarse mulch, but not everyone has room for a huge load. Of course, you can share with neighbors, but for most of us, commercially bagged chips, or a more modest bulk delivery from a local soil yard, is a good strategy. Pro tip: Taper mulch around the trunks of trees and shrubs to prevent damage to the wood.

Leaf mold, which is simply partially decomposed leaves, is another valuable addition to garden soil. Pfeiffer says, “Leaf mold is the best mulch you cannot buy.” Fortunately, we’re all about to get free access to untold leaves.

Going back to the woodland analogy, Pfeiffer suggests leaving leaves where they fall in beds around your deciduous shrubs and trees. As the leaves break down over time, they release nutrients and add organic matter to the soil. “We’ve really got it backward when we scour garden beds of every last leaf that hits the ground,” she says. “Nature lays down coarse textured mulch in the fall, and so should we!”

All leaves, but especially large leaves, those with a leathery texture and conifer needles, will break down more quickly when shredded. A couple of swipes with a lawn mower is the easiest method for most homeowners. Other options include backpack leaf vacuums that have integrated shredders, or a small multipurpose garden shredder that also will handle lightweight woody debris. Shredding turns a giant pile of leaves into a more manageable volume.

Gardeners with an especially robust quantity of fallen leaves can fill a wire cylinder or a dedicated compost bin to enclose a pile (minimum bin size should be at least 3 by 3 feet). Smaller containers, like an extra garbage can or garden waste bags, will work, provided you poke holes for drainage and air flow. Wet the pile — nature takes care of that during the rainy season — and let time (and fungi) get to work. Leaves gathered and shredded this fall, if kept moist over winter, should be garden-ready next April or May, just in time for spring planting. Fresh leaves break down more quickly than older leaves that have dried out, so don’t put off raking.

Coarsely mulched soil that’s rich in organic matter, like leaf mold, supports good soil biology, consisting of beneficial fungi and larger soil organisms such as earthworms, millipedes and ground beetles. Put it all together, and you’ve got the foundation of a beautiful garden.

Christina Pfeiffer is the co-author of “Pacific Northwest Month-by-Month Gardening” and regularly presents seasonal seminars through the University of Washington Botanic Gardens. Find out more at christinapfeiffer.net.