This question and answer are part of our Ask CSU Extension Anything (About Native Plants) series. We appreciate CSU Extension for answering these questions to expand our community’s knowledge of native plant landscaping. Send us your questions to the Ask CSU email account.
FULL QUESTION: I’ve been hearing/reading a lot about using a combination of native grasses and sedges as a “green mulch” (with flowering plants added in a few masses throughout) as a way to keep down on weeds without having to re-apply wood chips every few years. Could you recommend a few good native grasses and or sedges that might fit the bill? I have clay soil (of course) and areas that get full sun as well as areas that get mostly shade. Drought tolerant plants would be a plus 🙂
ANSWER: Also known as “matrix planting,” the technique of using dense plant cover to exclude weeds is becoming more popular. In essence, instead of a landscape where plants are grown and admired as individuals, gardens are more like meadows, with no exposed mulch or bare soil between plants.
The goal of weed exclusion can be met by: 1) planting desired species more closely together; 2) using a “layering” approach, where ground covering plants are used as a visual foil for larger specimens; or 3) a combination of these strategies.
Green mulching or matrix planting relies on desirable plants filling all the available niches in the landscape, thereby effectively excluding weeds. In seasonally dry environments like much of Colorado, the very driest places might not support complete plant cover since competition for water will limit the number of plants that can grow in a given area. For a good idea of what the potential plant cover is, look for nearby natural or waste places and see what the cover is like. If plants are dense, growing leaf tip to leaf tip, a matrix planting/meadow/green mulch strategy is possible. If plants are more widely spaced (imagine the Sonoran desert) then you can still fill all the niches with plants, but you’ll have bare soil too. Gardens in these situations would still need to rely on wood or gravel mulch to cover the bare soil. Most places in Colorado are wet enough for complete or nearly complete plant cover with the right plants.
Choosing the mix of species that will make up your weed-resistant garden relies on matching their life strategies. Extremely competitive plants will exclude the weeds, but have the potential to start excluding your other desirable plants too! If you’re planning to use grasses, look for varieties that are 1) weakly rhizomatous or clumping or 2) if rhizomatous, then intolerant of shade (e.g. buffalo grass). This enables larger specimen plants to overtop and outcompete the groundcover and persist in the landscape.
A few options for native plants that may work as green mulch ground covers in your landscape are below. (A lot of native plants could potentially work for this, but are not generally available in the trade. Dryland sedges, for example Carex inops, are likely ideal, but are commercially unavailable).
Native Grasses and Sedges:
Purple Three-Awn (Aristida purpurea)
Buffalo Grass (Buchloe dactyloides)
Blue Grama (Bouteloua gracilis) Note: avoid using robust cultivars like ‘Blonde Ambition’
*Field Sedge (Carex praegracilis)
*Appalachian Sedge (Carex appalachica) [experimental in CO, North American, but not regional native]
Arizona Fescue (Festuca arizonica)
Idaho Fescue (Fescue idahoensis)
*Pussytoes (Antennaria spp)
Fringed sage (Artemisia frigida)
Poppy Mallow (Callirhoe involucrata)
Sundrops (Oenothera [Calylophus] serrulata and O. hartwegii)
Prairie Zinnia (Zinnia grandiflora)
*indicates plants with at least some shade tolerance
Answer developed by: John Murgel (Extension County Specialist, Horticulture and Natural Resources, Douglas County) Colorado State University Extension.
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