In an intact ecosystem, nature protects bare soil with native plants (or decaying plant matter) that offer a protective umbrella aboveground and keep soil in place with their roots. In disturbed areas, nature can’t count on the indigenous plants that fell under the plow 200 years ago or were destroyed more recently, so it works with what’s left: Weedy plants brought in, intentionally or not, from other ecosystems or other continents, some of which are so invasive that they destroy wildlife habitat. That’s not a good thing, so we either pull the weeds and leave the soil bare (which can cause soil degradation and exposes more weed seeds to light), or spend many hours every year spreading wood chips, bark dust, rocks, or — heaven forbid — nondecomposable plastic sheeting or rubber mulch to try to keep them at bay. But there’s a much better way that’s good for biodiversity and your back.
Before I get to that, keep in mind that some mulch applications can be beneficial—as in compost applied to veggie gardens that need lots of nutrient-rich organic matter and help retaining moisture, or a couple of inches of aged wood chips to help new trees and shrubs get established. Although a layer of wood chips won’t control erosion on slopes or provide wildlife habitat, it also won’t destroy your soil if it’s not tilled in, so it has its place. Let’s say you’ve planted some new trees. Wood chips spread circumferentially around the trees a little past the drip line will suppress weeds, allow water infiltration, minimize water loss (when applied while the soil is still moist), and encourage microbial soil life (but be sure to keep all mulches at least six inches away from trunks or main stems to prevent rot). However, applying additional mulch in the following years in the same place provides no benefits since the trees’ feeder roots will have grown way past the circle of mulch, in some cases eventually reaching double the diameter of the tree’s canopy (the majority of trees don’t have tap roots). As your plants become established and if they’ve been spaced appropriately and leaf litter is allowed to accumulate, there won’t be a need for any kind of trucked-in or bagged mulch. And, you won’t have to worry about whether your mulch application entombs bees and other insects that nest in the ground.
A low-nitrogen compost, such as leaf compost, ought to be used in place of wood chips, especially in areas where soil is degraded, such as soil that was once trapped under concrete. But remember that thick applications of compost can smother beneficial insects and get in the way of ground cover plants on-the-go, so using whole, unchopped leaves is best for eco-functional, “real” gardens.
The worst offenders
Fine wood chips or bark “dust” tend to compress into a dense, impermeable mat that prevents rainwater from soaking in and may even blow away soon after application. Rock or gravel mulch is devoid of life, adds absolutely nothing to the soil, makes it impossible to add organic matter later on, and in sunny spots will either reflect or absorb heat (depending on the lightness or darkness of the rocks)—not a good thing to do to plants, most of which can’t take the heat. Rock or gravel mulch also make it harder to get rid of weeds and while it might be okay in rock gardens, it is not beneficial for most other plantings and does not prevent weeds. Lastly, any thoughts of using plastic “weed liners” or “landscape fabric” should be quickly consigned to oblivion since they prevent moisture from reaching plants’ roots and soil life, and contribute to the enormous glut of extremely problematic plastic on this planet.
What do wood chips, rocks and plastic have in common? They’re unnatural. Sure, they may keep weeds down temporarily, but they also smother beneficial arthropods that live in or on the soil and make it impossible for ground feeding birds, who instinctively rummage through fallen leaves, to find food. They can never create habitat that plants and animals need. When in doubt, ask “What would Nature do?” Her answer certainly wouldn’t be to finely grind up trees or roll out the plastic.
A living mulch
To add wildlife habitat and connectivity, increase diversity, protect the soil, sequester carbon, mitigate storm water, keep weeds down, and possibly control erosion (depending on the plant species), think green—that is, living, growing plants. What may first come to mind are low, ground-hugging plants, but taller plants also contribute benefits as well. A densely layered, fully planted garden — from ground cover and small shrubs, to tall shrubs and trees — will shade out weed seedlings and minimize the soil nutrients they need, weakening their chances at prospering. It will also be much better at carbon sequestration than lawn or garden beds made up mostly of mulch.
Arrange big trees and the understory — shrubs and perennials — in a layered effect, to create connections and conditions that help to cover the soil. When plants touch one another and overlap a bit, or — in the case of ground covers — cling to the ground and spread (a lot or a little, depending on your needs and the size of your yard) we mimic nature and lessen maintenance tasks. A living mulch looks much better, too. And simply allowing leaves and other dead plant material to stay on bare soil will add nutrients and organic matter to the soil as they decompose.
Placement is important
Plants are often placed too far apart or are placed appropriately but then sheared into odd shapes, leaving the soil bare. Or they’re placed so that when they reach mature size they grow into walkways or houses and the blame is placed on the plant: The “it’s overgrown” quip often results in plants butchered beyond recognition and loss of habitat. The best placement allows plants to assume their natural shapes and habits and lets them touch and overlap a little, both above and below ground. When we arrange plants so that their roots occupy most of the soil, it becomes more difficult for new weeds to take hold. One caveat! Do leave some soil bare, particularly in open areas, because 70 percent of native bees nest in the ground — they burrow into bare earth like ants do — and they cannot nest in thick layers of loose mulch or thick, lush ground cover. So, don’t cover every square inch of your property; everything in moderation.
Planning for change
When planning your garden and before you draw up a planting plan, it helps to do a birds-eye-view sketch, drawn to scale, with just general plant material or plant groupings. One example: A large tree to shade the southwest side of your house, shrubs that can handle partly shady conditions beneath the tree, and woodland perennials/ground cover plants to blanket the soil. Then choose the plant species that fit the conditions and size constraints. It’s essential to research mature widths as you choose plants (especially shrubs), so that their placement won’t be too close or too far apart. Check at least two sources to be sure and don’t always rely on plant tags, which may or may not be correct (I find that most shrubs get bigger than tags say). For continuous cover, place them a little closer than their mature width apart, giving shrubs and trees enough space so that they don’t infringe on walkways and neighboring properties and such. Try to choose plants that occur in natural communities within your area so that they will be able to communicate through the soil, as well as air, to trade nutrients and secrets that helped them survive together for thousands of years. And while sun-loving plants should be arranged so that they don’t shade each other out, it’s okay to let plants compete a little. For ground cover plants that need shade, allow your larger plants (that will eventually supply shade) to grow a few years before adding the ground cover. Or, start with ground cover that likes some sun, and then replace it later on when you’ve got enough shade. The latter approach works best with slow growing trees like Oregon white oak; the former with speedy growers such as Douglas-fir.
Gardens are ever-changing, just as Nature is, so it should be no surprise when they reject the status quo and slowly transform and shift over time: There may be early successional plants (“pioneer species”) that establish quickly and help to create a quick green mulch that competes with early weeds. After a few years they may give way to the next succession of plants that come later. Plants that move, either by self-sowing or via underground roots, are usually trekking to a place that suits them, and we can learn from their relocations. For example, if you’ve planted a sun-loving perennial in a partly shady spot, you won’t need to think too hard about why it’s sown itself in a sunny pathway. Of course, there will be times when you’ll need to do some editing so that your design continues to please and function well.
Native ground cover to consider
Below are a few low ground cover type plants (those that will spread or self-sow in the right conditions) for the Pacific Northwest (west of the Cascades) in sunny to partly sunny spots and shadier areas. Besides light needs, aways check moisture requirements and find out whether it is native to your specific area. Consider growing several species in the same area so that they mingle into a tapestry that creates texture and prolongs bloom time. Some (*) are quite assertive in certain conditions, so may not be best for small properties. Also keep in mind that most smaller plants will self sow and fill in spaces eventually, such as columbine (Aquilegia formosa) and fringe cup (Tellima grandiflora). Finally, don’t forget about moss in mostly shady places—it’s great on compacted soil and rocks, provides wildlife habitat and nesting material for some birds, sequesters carbon, helps control erosion, and doesn’t need mowing like lawn does.
For mostly sunny sites:
Arctostaphylos uva ursi (kinnikinnick)
Campanula rotundifolia (common harebell) *
Carex obnupta (slough sedge)
Ceanothus prostratus (prostrate ceanothus)
Erigeron glaucus (seaside daisy)
Penstemon cardwellii (Cardwell’s penstemon)
Sedum oreganum or S. spathulifolium (sedum)
Sisyrinchium idahoense (blue-eyed grass)
Viola adunca (early blue violet)
For shadier, woodland sites:
Achlys triphylla (vanilla leaf)
Asarum caudatum (western wild ginger)
Dicentra formosa (western bleeding heart) *
Mahonia nervosa (Cascade Oregon grape)
Maianthemum stellatum (starry false Solomon’s seal)
Maianthemum dilatatum (false lily of the valley) *
Oxalis oregana (wood sorrel) *
Vancouveria hexandra (inside-out flower)
Viola glabella (stream violet)