Living Mulch

Mulching garden beds has been a consistent ‘must-do’ for many gardeners.  Mulching reduces moisture evaporation, suppresses weeds, adds nutrients to the soil, reduces erosion and in many cases improves moisture absorption. Wood and bark chips are the most common materials employed.  Recently I have seen and read more about the negative effects of wood chip mulches. In particular when these mulches are over applied and allowed to compact, they can actually retard rain water absorption.  Thickly applied mulch can retard the growth and expansion perennials.   In his book The Know Maintenance Perennial Garden, Roy Diblik discusses how unnatural wood chip mulch is.  In areas that get adequate rainfall to support a tight matrix of plants, a living, or green, mulch is the ideal.

For many sustainable and naturalistic gardens the goal is to create this ‘green mulch’ or ‘living mulch’ by filling the space between garden features or larger plants with more plants.  ‘Green mulch’ is living plant material that performs all the functions of wood mulch such as weed suppression and soil moisture and temperature moderation.  In addition properly selected green mulches offer additional benefits like supporting wildlife and insects with food and cover, lower maintenance (since they do not to be replenished or broken up on a regular basis).  This green mulch may be a single species or a variety of plants that form an interlocking matrix. This matrix is most like what you would encounter in a meadow or woodland setting.

An example of a classically mulched bed. 
Each plant is distinctly identifiable and  there are no
random plants to confuse the composition.

One problem with green mulches is that they can look weedy due to lack of readability, no clear design or cluttered appearance.  Many folks are more comfortable with a landscape or garden where the features are clear and recognizable.  Having beds with clean edges is a quick way to make a garden more legible.  This human desire for legibility is also seen in the preference of many for plants neatly separated by oceans of mulch. 

One means of creating a green mulch is to install plants closer together so that they quickly grow together to create a continuous green carpet of foliage. This is one of the themes of the book New Naturalism, by Kelly Norris, published in 2021.  The trick here is to create plant communities that a good match to your site conditions and that the plants play well together. 

An example of a green mulch.  Creeping phlox, Phlox subulata,
and wild strawberry, Fragaria virginiana, have grown together
 to form a 4-6" deep ground cover.  Shrubs and perennials,
like the wild geranium, Geranium maculatum, here,
 are able to grow through.

Another route to a green mulch is to plant lower grow species as a matrix between your feature plants.  The trick here is to keep your garden readable so that the matrix does not obscure your design intent.  By way of example consider what bindweed does to a garden.  While it grows quickly to fill all the voids in the garden it also grows up and over taller plants resulting in tangle of plant material that has no discernible form, no starting or ending points and little textural contrast. (Kudzu does this on a much larger scale, draping a woodland edge with green vines, effectively removing all contrast between the trees.)  Consider instead a perennial bed where the spaces between plants are filled with low growing violets.  The glossy round leaves of the violets don’t interfere with the forms and textures of taller perennials or shrubs.  A list of some of the lowest growing  Mid-Atlantic natives includes:  Pussytoes (Antennaria sp.), wild ginger (Asarum canadense), Green and Gold (Chrysogonum virginianum), wild strawberry (Fragaria virginiana), barren strawberry (Geum fragarioides), Meehan’s mint (Meehania cordata), partridgeberry (Mitchella repens), golden ragwort (Packera aurea), various species of phlox (Phlox divaracata, stolonifera or subulata),  foamflower (Tiarella cordifolia), common violet (Viola soriana),  and  short sedges like rosy and ivory sedge (Carex rosea  and  C. eburnea ).

Golden ragwort is excellent native ground cover.  It spreads rapidly
 in moist, partly sunny locations.  It also seems to suppress invasive
 weeds like Japanese stilt grass and garlic mustard.

 

Using spring ephemerals for this purpose is a very natural means of creating a matrix.  These perennials naturally flourish early in the year when tree and shrub canopies are open, then slowly go dormant as the canopy closes up.  They return again the following spring.  In the Mid-Atlantic you may find Dutchman’s breeches and squirrel corn (Dicentra sp.), mayapple (Podophyllum peltatum)  and Virginia bluebells (Mertensia virginiana)  as common examples of these.  A limitation with using ephemerals is that they do totally disappear after setting seed.  They are ideal in an established perennial or shrub bed where you just need something in early spring while the larger plants are leafing out.  

Ephemerals, like cutleaf toothwort, Cardamine concatenata (in bloom),
 and Dutchman's breeches, Dicentra cucullaria, fill out in early spring.
but disappear as the tree canopy fills in. 

Using short-lived species that survive primarily by reseeding is another means to establishing a sustainable living mulch.  By nature these are opportunistic gap fillers.  As the longer lived perennial and shrub layers get established these reseeders tend to be squeezed out as their preferred, open habitat disappears.  Yellow fumewort (Corydalis flavula), American pennyroyal (Hedeoma pulegiodes), common yellow woodsorrel (Oxalis stricta), and self-heal ( Prunella vulgaris) are some of the shorter species that can be used in this way.  Taller, showier species that can be used as temporary gap fillers include wild columbine (Aquilegia canadensis), Partridge pea  (Chamaecrista fasciculata), fleabanes (Erigeron annuus and E. philadelphicus), and black-eyed Susans (Rudbeckia hirta).

Wild columbine, growing about 2' high, is relatively
 short lived in gardens due to competition and rich soils.
It produces a lot of seed so that it can reappear in other suitable
 locations where seeds can make soil contact and
have access to sunlight. (unmulched areas).

Yellow wood sorrel is considered a weed in many cases.  However,
 this native plant satisfies the requirements of a living mulch quite well. 
It grows quickly and produces lots of seed to fill gaps, is relatively short lived,
and is easily displaced by larger plants. As a native it also supports wildlife,
 particularly bees and birds.  Its lax, floppy habit detracts from it appearance,
making it appropriate around much larger plants and shrubs
or where this flop is not a distraction.

Oxalis stricta is very similar to its European relative, O. europea. 
Each has the same common name, common yellow woodsorrel. 
The North American species can be distinguished by the horizontal
disposition of the seed stalks (indicated by arrow). 
O. europea has seed pods on ascending stalks.

So if you are willing to give up the repetitive chore of mulching your garden beds, consider having your plants do that job for you and establish a living mulch.