Using Perennial Plants As a Living Green Mulch
Tips for replacing bark with plants
A dead mulch or a live one? For Larry Weaner, founder of the much sought-after ecological landscape design and installation firm Larry Weaner Landscape Associates, the choice is clear. During a conversation on my Growing Greener podcast, he told me that surrounding shrubs and trees with low-growing native perennials provides all the benefits of tucking them in with a blanket of organic non-living mulch, and a number of others besides. Plus, he added, once established, the living mulch, or green mulch, is free of the annual labor required by a conventional one.
The secret to success in creating a living mulch lies in a judicious choice of plants. Larry, whose materials of choice are plant species indigenous to a landscape’s region, prefers to use low-growing native perennials. In part, this reflects Larry’s commitment to creating landscapes that benefit wildlife and reinforce local ecosystems, both of which are services that native plants perform naturally and which exotic plants mostly do not. However, this preference also derives from Larry’s decades of experience that locally indigenous natives will generally perform better in the long term with less maintenance than imported, exotic plants.
Of course, there are super-robust exotics such as Japanese pachysandra (Pachysandra terminalis) and Vinca minor (commonly known as periwinkle) that will quickly spread to cover the soil and prove almost care free. Unfortunately, their vigor doesn’t stop at the property line; such imports tend to invade adjacent natural areas, displacing the native flora. Happily, native plant communities offer many alternatives for the garden.
The difference between green mulch and groundcovers
Unlike traditional groundcovers, which are commonly planted as stands of single species, a native green mulch should be diverse.
In other words, don’t plant just black-eyed Susans (Rudbeckia hirta) in that sunny soil around the base of your shrubs or trees. Instead, intermingle them with other plants that commonly join them in the wild, such as bee balm (Monarda fistulosa), purple prairie clover (Dalea purpurea) and blazing star (Liatris spicata).
Why? By boosting diversity, you greatly increase the types of wildlife your planting will benefit. By providing flowers that bloom at different seasons, you also provide an extended buffet for pollinators, while prolonging the treat for your eyes.
The advantages of living mulches over traditional mulches
According to Larry Weaner, a native living mulch provides all the benefits we associate with traditional organic mulches. The complex of living plants shades the soil, cooling it, inhibiting the germination of weed seeds and conserving its moisture.
The plants’ roots also open channels down into the soil, enhancing the absorption of any water that falls on the area. In contrast, some conventional mulches, in particular shredded bark, tend to pack down with time so they actually shed water, keeping the soil beneath them dry.
A living mulch also injects organic matter deep into the soil as new roots replace older ones, which then decay.
Getting started with green mulches in sun and shade
Planting and establishing perennials is a greater chore than spreading conventional mulch, but you will recoup the labor over time: Unlike the shredded bark or wood that must be refreshed annually, the perennials, if well chosen for the conditions, will naturally renew themselves with seedlings or offsets from year to year.
In open, sunny spots, you can install a living native mulch economically by sowing a low-growing meadow seed mix of compact grasses and flowers. But because the seeds of native woodland perennials typically have short life spans, they are usually impossible to start in this manner. Generally, a living mulch of woodland perennials must be started by purchasing plants. This can get very expensive if the planting needs to cover a large area.
To reduce the cost, Larry Weaner suggests installing colonies of the plants at intervals throughout the area. Over time, they will spread by roots or stems or by dropping their seeds, which in a natural setting have the chance to germinate and grow while still fresh and viable.
Choosing plants for a living mulch
Larry Weaner recommends looking at natural plant communities for direction in choosing green mulches and companions, because the species growing together in the wild have evolved to share resources.
“If I look at water,” he says, “a plant that has surface, fibrous roots literally in the same space as a tap-rooted plant—neither of those is competing with each other for water because they’re drawing water at two completely different levels. Nature has worked that out.”
For the green mulch layer, Weaner suggests finding low, spreading plants that are native to your area and to favor those with rhizomatous root systems. This means the plant sends roots outward and stems grow up from those roots, providing quick cover and good weed suppression. Weaner recommends making rhizomatous plants total 50 to 100 percent of the ground-level composition in the garden.
The plant palette that is appropriate for one area is not going to be suited to the ecosystem and conditions you will find somewhere else. Fortunately, the National Wildlife Federation offers a free native plant finder tool online. Type in your ZIP code and it supplies a list of the flowers and grasses native to your area that are most supportive of caterpillars, which not only mature into butterflies and moths but also serve as food for most songbird chicks. This will give you a start on developing your own recipe for a locally adapted living mulch.
Image credit: Plant Image Library/CC BY-SA 2.0 Deed
