By Andrew Bunting, PHS VP of Horticulture
Over the next several months across the country, gardens will slowly start to transition into winter dormancy. This period of the year can also be one of the most ornamental, with many late flowering plants like the asters and goldenrods coming into full flower. Other plants will have attractive seed heads and many others will have blazing red, golden, and orange autumnal colors. As the gardens become dormant, there are many approaches the gardener can use to make their garden supportive of a variety of ecological functions.
“Leave the Leaves” is a movement that is gaining popularity across the country. Instead of raking up and removing leaves from the garden, they can be left and used as a natural mulch like you would find in nature. Depending on your aesthetic preferences in the garden, there are different approaches. Some gardeners find that leaving entire leaves can create too much of a “natural” look. Depending on the type of leaves, and their density, leaving entire leaves might create too dense of a layer, so much so, that the leaves might not entirely decompose. The following spring, this could mean the leaves are too thick, making it difficult for emerging perennials to break through. If this is the case, then a good compromise is to mow over the large leaves, which will cut them into smaller sizes. Then, you can rake them back into the beds still creating natural organic mulch. These leaves will decompose and enrich the soil and the leaf layer will support a host of overwintering organisms.
Historically, mulching has been done to help suppress weeds, conserve moisture and for other aesthetic reasons. There may be places in the garden where some types of organic mulch like leaves, leaf compost, or wood chips are necessary, especially in areas with significant weeds. Putting mulch down repeatedly year after year can start to create a thick, and somewhat impenetrable layer of mulch that suppresses ground dwelling organisms and can result in water cascading off the soil versus being absorbed by the garden. Consider deciding where you need mulch for moisture sequestration, weed suppression, and amending the soil. Also, think about where you want to mulch for aesthetic reasons and decide if other areas can go unmulched or can be limited to occasional mulching.
Applying too much mulch or mulching repeatedly year after year can have many detrimental effects in the garden. It can start to destroy the ecosystem between the leaves and the soil. This naturally decomposing layer is “home” to bees, especially ground nesting bees, beetles, caterpillars, and other insects. Many birds count on this layer for foraging. Over mulching on newly emerging perennials, including native spring ephemerals, can create a suffocating impact. If the mulch layer is too thick and too heavy, the emerging perennials might not be able to penetrate this layer and will die.
There are a myriad of perennials, shrubs, and trees that can be planted as fruit and seed sources for wintering birds. Many of these plants can also be grown for their ornamental attributes. The winterberry holly, Ilex verticillata is covered with plump red and orange berries from the middle of September and sometimes all the way to the following March. ‘Winter Red’ and ‘Maryland Beauty’ have bright red fruits, while ‘Winter Gold’ has soft orange berries. These plants can also be an attractive food source for the American Robin and the Grey Catbird. The chokeberry is another native shrub with persistent red fruits. Aronia arbutifolia ‘Brilliantissima’ is an upright and multi-stemmed shrub and has an abundance of berries for most of the winter. Aronia melanocarpa, the black chokeberry is more diminutive in stature and has large purple-black fruits. Chickadees, nuthatches, and many species of woodpeckers feed on pinecones. There are many native species of pines including the iconic white pine, Pinus strobus of the northeastern states, as well as the Virginia pine, Pinus virginiana, found throughout coastal plain regions of the East.
Traditionally, gardeners at the end of the season have embarked on what is commonly called a “fall cleanup.” This includes cutting back any perennials that don’t have winter interest to create a tidy garden heading into the winter months. Many members of the daisy family like Echinacea, Rudbeckia, Helianthus, Silphium, and others have dried seed heads that are a food source for seed-eating birds like a host of native sparrow, finches, and the American Goldfinch to name a few. Plus, these heads do provide some ornament to the garden.
Many other perennials and grasses should be left to provide overwintering habitat for a host of native insects and pollinators, including plants with hollow stems such many of the ornamental grasses; wild quinine, Parthenium integrifolium; thread-leaf bluestar, Amsonia hubrichtii and many of the species of baptisia. These plants have chambers in their hollow stems which make a perfect spot for hibernating insects.
There may be a need to do some selective cutting back of perennials over the course of the winter. However, if there is also an intentional approach to leaving some perennials intact over the course of the winter as overwintering habitat, this would be ideal for striking a balance between aesthetic preferences and ecological function in the garden.
It is important that we view the ornamental garden as a part of the ecosystem. There are many approaches the home garden can use to ensure their garden contributes to the broader, surrounding natural systems. Simply leaving perennials and shrubs or limiting the use of mulch while “leaving the leaves” in the fall can help provide vital habitat and food sources for wildlife.