Mulling about Mulches

By Laura Gardner, Master Gardener

At the beginning of February, the snow had melted on the grounds of the native plant garden at the Peterborough Public Library. This garden is an ongoing project between the library and the Peterborough and Area Master Gardeners. I noticed a few weeds in areas that did not have a thick enough layer of wood mulch. Ideally, they would have been removed in the fall, but I didn’t get that opportunity. Being situated downtown, the library garden is open to many types of plants that arrive via wind-blown seeds. As the intentional plantings continue to grow and fill in the empty spaces, weeds will become less of a concern.

However, it may be a good idea to consider incorporating living green mulch such as low sedges and reducing or eliminating the wood mulch in the garden to minimize weeds and to develop a more natural aesthetic that mimics nature. Benjamin Vogt of Monarch Gardens defines a green mulch as a lower plant layer that covers more than 90% of the ground. Vogt talks about how sedges, short grasses, and other low-lying forbs can be used as an initial base layer in a garden with taller plants interspersed throughout. They can also be used to plug in holes in between existing plantings. Vogt also mentions that wood mulch can inhibit the growth and spread of the plantings—which reduces the overall natural aesthetic.

Incorporating green mulch does have an initial cost but over time will cost less than wood mulch as the latter needs to be constantly replenished as it breaks down. Native plant nurseries may have smaller plugs that can be purchased in bulk at a lower cost than larger potted plants. In addition, sedges and grasses can be divided and replanted. Wood mulch is also not as environmentally friendly as it needs to be transported to the site.

Green mulch offers the same benefits as wood mulch—the ability to provide organic matter to the soil when it dies back in the fall; it helps to regulate soil temperature and moisture levels; it shields weed seeds from the light exposure they require to germinate, and it prevents soil erosion. One thing I have noticed about a particular area in the garden is that despite the heavy application of wood mulch, invasive perennial weeds like Canada Thistle (Cirsium arvense) and Field Bindweed (Convolvulis arvensis) still have the strength to poke through. The long-term and more practical solution to managing these perennial weeds here is to encourage dense plantings of green mulch and the spread of existing plantings. These weed species thrive in the sun but will weaken if shaded out by other plants.

Green mulch provides support to various wildlife species. For example, unlike traditional turf grass, sedges grow with a space around them, thus providing access for ground-nesting bees. If wood mulch is too heavily applied in the garden, it inhibits pollinator lifecycles. Sedges and grasses also serve as host plants for some Lepidoptera species and produce seeds that are valued by birds.

When considering sedges, note that many prefer at least partial shade and moist soil conditions and so the right ones need to be selected. For the library garden, the area that is sunny and dry could accommodate Short-Beaked Sedge (Carex brevior), Ivory Sedge or Bristle-Leaved Sedge (Carex eburnea); while the partially shaded but dry areas could be home to Pennsylvania Sedge (Carex pensylvanica). A low-lying grass that could be used is Blue Grama (Bouteloua gracilis). When sourcing these, it is best to check area native plant nursery catalogues this spring for available stock. The ones to check locally are Natural Themes in Frankford, Grow Wild in Omemee, GreenUp Ecology Park Nursery in Peterborough, and Native Plants in Claremont. Ontario Native Plants is an online mail order provider. For hard-to-find sedges, Prairie Moon in Minnesota has a wide variety available to ship to Canada as seed.