Herbaceous Plants as Living Mulches

Median Garden with Fragaria 'Lipstick'

The cadence of the spring tree ring, shrub bed, and perennial garden maintenance season is well understood. The gardener and her crew cut back perennials left standing from the previous season before blowing out any leftover leaves and debris. They gather it up and haul it away. They cut new edges before working tirelessly over steaming wheelbarrows of mulch before applying a pre-emergence herbicide. It's the same process as last year, and the year before that. Come to think of it, it's the same process as the year before that one, and it's the one she learned from her predecessor. That mulch layer is getting rather thick, even though they blow much of it out with the leaves in the fall. Perhaps it's time to try something different. What if she and her team used plants, whether perennials, groundcovers, native plants, or shrubs, in the space occupied by mulch? The mulch beds require cultivation, raking, weeding, and spraying throughout the year. A garden of native and exotic ornamental plants would, too, so why not look to plants to serve as a living mulch?

In all branches of the cultivated landscape, annual applications of mulch have become inevitable. Every spring, landscape supply and contractor yards are stocked with mountains of blended bark, blended hardwood, and color-enhanced mulches. Tub grinders, trommels, trucks, and trailers around the region work year-round, dedicated to the cause of processing, sorting, selling (and re-selling) wood waste. While mulch is an incredible story of adaptation, recycling, and reuse, its manufacture is highly resource intensive, and it is only one of many ways to hold landscape soils in place. This is not about to be a treatise on the shortcomings of wood mulches; mulches remain valuable for their function (protecting developing and established root systems from fluctuations in temperature and moisture), for their aesthetics (helping create a sense of cleanliness and order in the cultivated landscape), and for their economic value. I will propose here that mulches should maintain their place in post-installation landscapes. We should reassess their current use as a driver of annual enhancement revenue or a quick coverup of a hasty weeding job (I have done both), and we should reach for more generative, ecosystem service-promoting alternatives in this era of sustainability. I hope you'll continue to use whatever practice works best for you and for your company or team, but I ask that you reach for your nursery catalog before you pick up your pitchfork (whether you're coming for me or heading for the mulch bin).

Geranium cultivars 'Karmina' and 'Biokovo'

Many woody and herbaceous alternatives to mulches have been tried and proved at Chicago region horticultural institutions. Jess Goehler, Curator of Collections at the Chicago Botanic Garden, has a long list of recommendations, including Ajuga: "They can be finicky because of their shallow roots but will maintain where enough moisture is available." Patrick Dahl, Plant Evaluator at the Botanic Garden, writes that the darker-leaved Ajuga cultivars handle dry spells better than others.

Both Jess and Kathryn Deery, Director of Horticulture at Lurie Garden and the Millennium Park Foundation, suggests that Geranium cultivars, particularly 'Biokovo', are effective living mulches. Kathryn uses Geranium in groups or blocks. She finds them useful as punctuation throughout a garden and for filling holes and gaps, and for covering the ground early in the season. She ensures good coverage by keeping tight spacing between plants, anywhere from 12" to 18" on center. This makes plantings look finished and full while helping keep down weed pressure during establishment. 'Biokovo', 'Karmina', and other perennial Geranium cultivars are standout performers in the display gardens at Midwest Groundcovers' St. Charles, Illinois nursery.

Geranium 'Biokovo'

Both gardeners cite the utility of Sesleria, whether autumnalis, 'Greenlee Hybrid', or albicans. Its early emergence as a cool season, clump-forming, cool season grass makes it an excellent unifying element in a planting design or as a companion to other plants. Sesleria autumnalis is used effectively in the Austin Eischeid-designed entrance garden at Midwest Groundcovers' St. Charles headquarters.

Sesleria autumnalis

Woody plants like Aronia Low Scape Mound® and Juniperus horizontalis cultivars are also performers at the Botanic Garden. Aronia Low Scape Mound® is adaptable to a range of challenging soil conditions and offers several seasons of interest with white flowers, purple-black fruit, and red fall foliage. Juniperus horizontalis are tolerant of direct sun and heat and their unique habits, whether the swirling branches of 'Hughes' or the shag carpet-like wiltonii ('Blue Rug') are of interest throughout the year. Either plant is well-suited to the front of a perennial border, along a pavement, or on a parking lot island.

Illinois natives Polemonium reptans, Phlox subulata, Penstemon hirsutus, Asarum canadense, Sedum ternatum, and Geum triflorum are also reliable spreaders that play well with other plants. For Kathryn, Asarum canadense and Sedum ternatum weave through large portions of Lurie Garden east of the water feature (the Dark Plate). She lets them spread as they please, and they act as groundcover with little impact on the overall design. Kathryn also acknowledges Geum triflorum as a significant design element in the west side of the garden (the Light Plate). She writes that Geum triflorum "shines in spring and early summer then quiets down later in the season. The foliage can get some interesting fall and winter color. When planted as a block, Geum knits together, which is effective for keeping weeds out." A particularly successful example of this use of Geum triflorum can be found in the Hocus Pocus Groundcovers Garden adjacent to the flagpoles at Midwest Groundcovers' St. Charles office.

Cerastium plumbago and Hydrangea paniculata

Both Jess and Kathryn are champions for Carex species in their gardens. Carex species are great for covering the ground early in the season to outcompete weeds and to provide habitat. Their varying leaf textures and habits also make them excellent design elements on their own. Carex species that stand out for their landscape value include C. appalachica, blanda, radiata, albicans, woodii, shortiana, muskingumensis, pensylvanica (of course), and more. Kathryn cites the work of Roy Diblik and at the research at the Mt. Cuba Center as ongoing sources of inspiration. You, dear reader, can view these Carex species and more in the Revitalized Median Garden (C. muskingumensis) at Midwest Groundcovers' St. Charles nursery and in the Carex Classroom at Midwest Natural Garden. Also thriving in the Revitalized Median Garden are several plantings of Ceratostigma plumbaginoides. Its low-growing, mounding form is slow to come up in the spring, but it's worth the wait. The brilliant blue blooms and fall color that turns from green to scarlet to maroon give it a lot of character.

This rabbit hole goes very deep, and with this article we've just begun to circle the edge. I hope you've found here a willingness to experiment with plant spacing, with plants unfamiliar to you, and with expectation management for your clients and customers. Opportunities abound where you can supplement existing plantings, plan new ones, remediate heavily mulched areas, and add provide residual value by replacing mulching labor with gardening labor. Thank you for reading.

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