The plant palette for green walls is much wider than for most green roofs. The term “green walls” encompasses all forms of vegetated wall surfaces -- green façades, living walls and retaining living walls. Living walls can be indoors or out, and made of soil, fabric or synthetics. They are nearly always irrigated, so drought tolerance is not particularly a constraint. In choosing plants for a green wall project, these parameters need to be outlined to give shape to the plant list. For indoor walls, tropical plants need to be used. Even with auxiliary lighting, the light levels indoors almost always necessitate plants that do not expect winter, which generally means tropical species.
Selecting Plants for Green Walls
By Marguerite Wells
The plant palette for green walls is much wider than for most green roofs. The term “green walls” encompasses all forms of vegetated wall surfaces — green façades, living walls and retaining living walls. Living walls can be indoors or out, and made of soil, fabric or synthetics. They are nearly always irrigated, so drought tolerance is not particularly a constraint. In choosing plants for a green wall project, these parameters need to be outlined to give shape to the plant list. For indoor walls, tropical plants need to be used. Even with auxiliary lighting, the light levels indoors almost always necessitate plants that do not expect winter, which generally means tropical species.
Outdoor walls can grow a wide range of plants, and the direction a wall faces is critical in determining the plant list. For example, a north-facing wall will sometimes never receive a single ray of direct sun. A south-facing wall may never have a moment of shade. East and west have different temperatures. Surrounding buildings affect these parameters as well. Also, winter care is a challenging part of wall maintenance. In the north, fabric-based, hydroponic walls are difficult to maintain perennial plants because the roots are exposed to the coldest outdoor temperatures, without the usual buffering effect that soil provides plants in winter. Even in soil-based walls, drying in winter is a problem — so look for plants that can withstand substantial drying in winter. Winter aesthetic interest is also a factor — a wall looking green and beautiful in summer is not too hard to achieve, but keeping things looking even halfway decent in winter, and managing customer expectations regarding this, is a challenge in colder climates for exterior green walls.
So, with all those parameters in play, how can I recommend any particular species for living walls? One category is known as Mondo Grass. Evergreen, tough and grass-like, I see this used commonly in exterior green walls. Some cultivars are very popular, and, as a result, can be hard to get and/or expensive. Others are very common and relatively cheap. They are propagated mostly by divisions, and are big plants that are not commonly found in small plug sizes because they just won’t fit. There are several genera of plants that use the common name Mondo Grass, including Dwarf and Black Mondo Grass, which are in the genus [ital>Ophiopogon<ital], and the many cultivars of the genus [ital>Liriope<ital], which come in mostly green foliage, including variegated green leaves, and many shades of blue and purple flowers. Some like more sun than others; some of the bigger varieties can be aggressive groundcovers; and some smaller selections are slower growers.
Another common denizen of green walls are ferns. There are different species to use indoors or out (for interior and exterior living walls), sun versus shade and evergreen versus deciduous. Some ferns stay small, others get quite big. A common North American native fern that is evergreen, tough, winter hardy and easy to grow is called Christmas Fern ([ital>Polystichum acrostichoides<ital]). A common sight in the woods east of the Mississippi, it grows to a height and spread of 2 to 3 feet, and keeps its leaves, although it lays flat, under the snow, in winter. Easily available in the nursery trade, it tolerates drought, wet feet, part sun and full shade.
Propagation of ferns can be by division of large ferns, or by sora, a fern’s version of tiny spore-like seeds. Ferns take a long time to grow from sora, so you may want to buy in small plants. Because they are coming up from very small, ferns can be bought as small or large plugs, 4-inch pots or gallons.
Ferns have only one edible phase in their life cycle — fiddleheads, which are the young fronds when they first shoot up in spring. Steamed with butter they are tender and delicious. Mature fern fronds are unpalatable even to deer (large swaths of forest can become dominated by ferns when the deer have eaten everything else). A few ferns have been used historically for medicinal purposes, but nothing salient in the modern herbal or medicinal chest.
Ferns and Mondo Grass can be used in most locations, except the tropics. They not only thrive in the Northeast, but western and southern states as well.
In selecting plants for green walls, I strongly recommend that designers look at existing walls and how they are faring, and how much maintenance is done to keep them that way. Keeping a line on availability for green wall plant replacements is also important. When plants die on a wall, the owners generally want replacement asap, since walls are showcases in most places. Not all plants are available everywhere all the time — plan for this. And, as always with green walls, it’s all about the water; keep the water flowing, and have backup water sources just in case.
Marguerite Wells is the owner of Motherplants.
Article courtesy of Green Roofs for Healthy Cities (GRHC), and was originally published in GRHC’s Living Architecture Monitor: www.livingarchitecturemonitor.com.