By Chris Erickson
Winter is an overlooked season in gardening. Too often the winter garden, unless it is full of evergreens, is written off altogether and the plants are cut back to the ground in fall. This is a shame. Proper management can enhance both the health and the aesthetics of plants during the long winter months. Nowhere are the opportunities for better utilizing planted areas in winter greater than on urban rooftops.
I will look at three reasons why urban roofs deserve extra consideration for proper winter management, whether the roof is composed of an intensive green roof or containers. Then, I will look at some basic strategies for getting the most out of a winter garden.
Why is winter management important on urban rooftops?
The first consideration is practical and concerns the health of your plants. Generally speaking, the more plant material you can refrain from cutting back over the winter, the healthier your plants will be in spring. The leftover plant material creates a layer over the ground, protecting it from harsh winds and cold dry air. This creates a more gentle environment in the soil for dormant plants. Even a simple stand of old flower stalks is effective at trapping leaves and can soon build up a valuable buffering layer over the soil surface. I see the results of this every spring. Where last year’s stems and leaves have been left there is always more new green growth than in areas with barer ground. Rooftops are exposed habitats, subject to strong winds and composed of thin soils, and providing extra protection through winter on them is especially beneficial.
A second reason considers the habitat value of rooftops. Many insects use old plant material to overwinter. If this old plant material gets tossed, so do next year’s insects. Hollow plant stems, specifically, are a common winter home. Birds will also continue to scavenge left behind seeds as food gets short. One small garden or collection of balcony planters may seem insignificant as habitat but in an urban environment like New York City where natural areas are so limited, every space becomes significant. If you value seeing butterflies and bees in the summer then consider the value of your garden as a winter home for them.
Finally, the same way wildlife habitat is at a premium in the city, so are the natural textures and colors of plants. Though roofs get physically used less in winter they are still valuable places to retreat from the commotion of the city. On top of this, many roofs and balconies are visible from numerous windows. The harsh lines of the urban environment make the organic shapes and patterns of any planted area precious. It is often easy to overlook the value of what is there until it has been cut back to nothing but barren ground.
What goes into maintaining winter interest in the garden?
Ask someone to picture a beautiful garden and many people will picture deep, rich colors. Probably the most challenging part of managing a garden in winter is that colors are much more muted. Besides the obvious evergreens which maintain their rich foliage, there are some wonderful if brief color displays from plants that flower or produce berries in the winter months. Winterberry (Ilex verticillata) with its bright red berries and witch hazel (Hamamelis virginiana), with subtle yellow and orange flowers, are two standbyes. Similarly, some plants hold on to their fall foliage well. Little bluestem (Schizachyrium scoparium) is an example, it turns a gorgeous reddish hue in fall and holds the color well into the winter. It is also a tough plant that can thrive on a rooftop. Utilizing pops of these colors can go a long way. But in general the best advice is to lean into what you do have to work with. Winter is a time to play with and closely observe form and texture.
The two elements I suggest starting with are branches and seedheads. Winter provides an opportunity to appreciate the branching structure of shrubs and trees in a way that is not possible when they are covered with green leaves. One of my favorite trees for a nice branching shape is alternate leaf dogwood (Cornus alternifolia). It typically grows in a measured, layered fashion. Another group of plants we have had success with on rooftops are the crape myrtles, which have a stately vase-like form. One of the images below show the whimsical shape of a staghorn sumac (Rhus typhina) tree that is still holding on to its summer fruit. Other woody plants feature bark with special colors or textures. Examples that we have seen grow well on rooftops include ninebark and river birch, which both have peeling bark, and red twig dogwoods which have brilliant red stems.
If the branching of trees is more about lines and silhouettes, seedheads are about shapes and patterns. Much of the time the shapes and forms of seedheads will mirror the flowers that came earlier in the year and a dormant garden of seedheads creates a special ghostly symmetry with the loud, colorful displays of summer. Once the colorful flowers have receded structure becomes much more visually important. The most striking thing about flowering purple coneflower (Echinacea purpurea) and short-toothed mountain mint (Pycnanthemum muticum – see our blog article on this wonderful plant for urban roofs) paired together is their color contrast, but as seedheads the verticality of spiky coneflowers stand out against the flat platforms of the mountain mints. Maintaining your seedheads for winter can take some planning ahead and discipline. Coneflowers are an example of a plant that goes to seed while other plants are blooming. If you really like a tidy garden you may be tempted to remove all these drying stems. Holding off can pay dividends over the winter months.
I think there is one overlooked thing that is key in making a winter garden look great: volume. A full garden looks like a healthy one, both in summer and winter. Some plants retain volume better than others. A plant we love at Highview Creations is swamp milkweed (Asclepias incarnata). Unfortunately by early winter it usually loses all the leaves from its stems and it’s seed pods become droopy and soggy. A patch of solid swamp milkweed, though beautiful in summer bloom with butterflies swarming all about it, will look bare in winter. At the Manhattan penthouse meadow pictured above, we combined swamp milkweed with robust grasses like panicums that hold their upright structure well, as well as perennials like goldenrods or coneflowers that have more durable stems, foliage, and seedheads. There is a fine line between full and messy, especially on roofs where strong winds can quickly topple dried-out stalks. Walking this line can require some strategic partial cutbacks in the fall. I look to remove weight, clean up long scraggly stems, and create more defined shapes and boundaries.
Winter is a unique time, not to mention a hefty chunk of the year. We at Highview Creations love designing and maintaining gardens that add beauty to our urban landscape in all four seasons. If you are interested in turning your outdoor space into a year round garden we’d love to chat.