Plants as Mulch

autumn fern
Photo from: Grounded Design

As experienced gardeners, we all know that if there’s bare soil, a plant will take its place, usually an opportunistic plant with more aggressive or invasive attributes. In the past, professional landscapers have filled these areas with very attractive bark mulches to allow space for plants to grow and fill in the area as well as providing all the benefits of mulches. (i.e., decreased competition from weeds, retaining soil moisture, preventing erosion etc.). Some landscape designers and architects are looking at landscapes in a new way that considers the need for biodiversity that includes native plants species.


Thomas Rainer, a leader in the New Perennial Movement, according to The New York Times, spoke recently at the Landscape Designer Conference at the Landscape Ontario Congress in Toronto. In his landscapes, his objective is to create outdoor spaces that are beautiful, bio-diverse and low-maintenance.  He calls this a Designed Plant Community. Part of this is achieved by using plants as mulch and considering the sociability of plants. To identify plant sociability, Rainer rates plants on a scale of one to five; one, being the least aggressive, (i.e., staying in place), to five being the most aggressive. Here are some examples:


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Ranier’s team at Phyto Studio, arranges plants in three distinct layers which can also include a fourth, temporary layer.

First Layer: The first layer is used for Structural Plants. These plants are the backbone of the planting. They include large plants such as trees, dominant shrubs, and even tall perennials or grasses. The emphasis in this layer is about the plant form – often a distinct shape or silhouette. Generally they are also longer lived but may only become more dominant in the landscape after several years of growth, providing year-round structure. This group would include less than 15 per cent of the plant choices in the landscape. Some of Rainer’s examples include:

Second Layer: This layer is used for Seasonal Theme Plants. These plants will dominate the landscape for a particular period of the growing season. It might be because of seasonal flowering or a bold texture. Even after blooms fade, they continue to add to the landscape and don’t disappear or fade into the background. These plants are used in larger concentration – encompassing 25-40 per cent of the plants and act as companions to the structural plants above. In a woodland garden this might include the Autumn Fern, Dryopteris erythrosora ‘Brilliance’ (see photo). Plants should be of medium “longevity and vigour.” They play more of a role as filler plants to spill over and around the structural layer. Some examples include:

  • –  Coreopsis ‘Red Satin’
  • –  Achillea ‘Strawberry Seduction’
  • –  Asters, Symphyotrichum oblongifolium ‘October Skies’
  • –  Bee Balm Monarda ‘Bradburiana


Third Layer: This layer includes Ground-Covering Plants. Thomas Rainer calls this layer the “essence of the community.” Its function is to fill in the gaps of the seasonal layer, but really this layer is what holds the plant community together. Plants in this group are usually more aggressive and have a spreading behavior. They grow close to the ground and may have a bloom in early spring. As the garden develops, they will fade into the background later as the season progresses, but still cover the soil.  They can be selected for specific purposes such as storm-water management (i.e. pick semi-evergreen species with diverse root systems and plant with a very high concentration to absorb moisture); erosion control (i.e, choose evergreen or semi-evergreen with persistent foliage with self-seeding or aggressive root systems) or soil building (i.e, choose legume species) etc. Some examples of plants in this group include:

  • –  Sedges Carex spp.
  • –  Coral bells, Heuchera spp.
  • –  Ginger, Asarum canadense
  • –  Barren Strawberry, Waldsteinia fragarioides

Fourth Layer: This layer includes Filler Species that may be needed until the other layers become established. Eventually this layer will entirely disappear as other plants grow and fill in, although it may return if other species fail to thrive. Good plants in this layer are annuals, biennials and short- lived perennials. Examples include:

  • –  Cardinal Flower, Lobelia cardinalis in a rain garden
  • –  Foxtail Barley, Hordum jubatum
  • –  Columbine, Aquilegia canadensis
Inspired by Nature’s Layers: but not hung up on natives only

Finally, Thomas Rainer encouraged gardeners & landscape designers to embrace the stresses and soils of a particular site and resist amending to make it more fertile. He suggested that high fertility could result in decreased diversity. In his objective of low-
maintenance landscapes he also emphasized MANAGEMENT over MAINTENANCE.

I encourage you to explore these ideas further by reviewing any of the following, all of which are filled with incredible photos of Designed Plant Communities. They are a feast for the eyes and also wonderful habitat for a community of biodiversity.

Further Reading:

Janet Mackey

Member Halton Region Master Gardeners

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