How to Keep Exterior Green Walls Alive in the Winter
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Due in part to their beauty and dazzling array of benefits, there is a growing demand for planting exterior green walls in cold climate areas of the northern US and across Canada. But freezing cold temperatures can take a toll on plants in these systems, so we asked three experts to share their winter survival secrets: Joep van Vilsteren of Sempergreen; award-winning green roof and wall designer Randy Sharp; and Lucas Moran of green wall manufacturer New Earth Solutions.
An exterior living wall on the 12th floor of an office building in Boston’s Seaport area. The living wall was installed in 2017, and faces harsh conditions created by its orientation towards the bay. Due to the resilience of the plants and the living wall system, it is able to keep nearly all plants alive over the winter. Photo: Sempergreen
Living Architecture Monitor: Exterior green wall plants often suffer during the winter months, particularly when temperatures drop below zero? What is going on here that make their survival difficult?
Joep van Vilsteren: Let’s start with stating that is NOT necessary for exterior living walls in cold climates to suffer in the winter and for plants to fail. If plant failure happens its likely caused by the inabilities of the system to protect the plants. Some systems will just simply not ever have the capability of preventing plant failure in the winter. Typical causes for failure on exterior living walls in the winter months are poor plant selection, the systems inability to protect or even develop good roots and lack of irrigation and root hydration during winter months.
At Sempergreen Services we typically use perennials, shrubs, ferns, and ornamental grasses on exterior walls in colder climates. Within these categories we select plants suitable for the projects’ exposure and location. Also, it is important to select plants that maintain foliage in the winter. Some perennials drop their leaves in the winter and come back in spring resulting in a very poor looking wall in the winter months.
Assuming we selected the right plants for the project, it is now very important that they are provided with the appropriate resources to actually make it through the cold and dry winter months. One of the most important resource during the winter is water. A plant, assuming it’s planted in the appropriate climate, will not fail just because of cold temperatures. This plant will fail because it is completely dehydrated while exposed to these cold temperatures. For that reason, its critical to keep the plants hydrated continuously during the winter.
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Green facade at the Chicago Zoo. A 12 x 12" (30cm) lattice of cables supports Vitis riparia. The River grape provides food, shelter and a perch for songbirds. The translucent green facade shades the hot west facing windows. Leaf drop in winter allows low angle sun and daylight inside the high performance zoo structure for the comfort of wooly occupants. Photo: Randy Sharp
Randy Sharp: From Halifax to Chicago and across the Prairies, designers and horticulturalists are monitoring hardy plants and irrigation systems to improve the performance of cold climate green walls. For the purposes of this interview, I am defining ‘cold climate’ as USDA hardiness zones 5 and 6. An equivalent metric is Canadian hardiness zone 6B (-17.8 to -20.6°C; 0 to -5°F) in Halifax and Toronto. By comparison, tough hardy climbers on green façades will survive in northern cities like Edmonton, Alberta (zone 3).
Important considerations for green walls when designing a site or a building include orientation to the sun, protection from desiccating wind, access for lift equipment, structural attachment and selection of a proven system. Living walls have extra design and logistical requirements comprising pre-growing plants in modules off-site for immediate appeal, shipping to the site in containers, equipment to lift heavy vegetated panels, and installing automatic irrigation with controls to monitor performance. An integrated design team often comprises the architect, landscape architect, building envelope consultant and structural, electrical and mechanical engineers as well the owner’s representative, the property manager, general contractor and the green wall installer.
In both Halifax and Toronto, living walls are located on east facing walls protected from Arctic winds. Walls with protective south and west orientations are also effective at cooling the building through evaporative transpiration. Plant loss or freezing damage is more likely to be a result of desiccation from wind rather than the actual temperature below freezing. Winterproofing entails programming the irrigation system to drain at a temperature near freezing with an air compressor; and recharge when temperature rise.
Lucas Moran: Plants rely on sunlight and water to grow and thrive. During the dark winter months, plant growth is hindered from fewer hours of sunlight, which is crucial for photosynthesis. The freezing temperatures reduce circulation of water within the plants, as well as reduce water availability in the environment. When the water in the environment is frozen, the plants are not able to access it as they normally do. Another problem associated with cold temperatures is the potential for plant cells to burst when rapidly freezing and thawing. When this occurs, the entire plant will unfortunately expire.
LAM: From a design perspective, what would you suggest the top three things that green wall designers do to help ensure green wall plant survival in cold climates?
Designed by Sharp & Diamond Landscape Architecture and supplied in pre-vegetated plant boxes by Gsky Plant Systems, this was possibly the first modular living wall commercially installed in North America in 2007. Ferns, herbs and groundcovers were sourced to replace a remnant strip of Coastal rainforest lost to the expansion of the Vancouver Aquarium. Vancouver lies in hardiness Zone 7B; however, the Landscape Architect consulted with a local nursery to test and select tough hardy plants. Gautheria procumbens was planted at the windswept top of the wall. The Wintergreen was complemented by Evergreen Huckleberry, Licorice and Spiny Wood Fern, Woodland Strawberry, Foamflower, Fringe Cup and Bleeding Heart. Unfortunately, the living wall was permanently removed five years later to allow for further expansion of the Aquarium. Photo: Randy Sharp
Randy Sharp: Living wall plants are much more exposed to the elements than plants growing in the ground or on flat green roofs. Typically, smaller more compact species are pre-grown as plugs or in small pots to transplant into small pockets or modules. Hydroponic living walls are even more exposed to extremes of weather conditions. Soil is removed from the plants roots and they are planted bare rooted between layers of fabric.
Choosing the right plant for the right location is an essential role of the designer and/or green wall specialist. Local knowledge of climatic conditions and hardy plants indigenous to the region is an asset. Poor plant performance can also be attributed to lack of moisture at critical times during the growing season, and especially during the winter months. Living wall plants used in colder climates often have shallow fibrous root systems that thrive in thin soils. Consider a plant layout which features a diversity of tough hardy evergreen groundcovers and ferns, deciduous shrubs with attractive foliage and winter branching as well as flowering perennials and seasonal interest. For green facades, hardy climbers include Clematis, Climbing Hydrangea, Honeysuckle, Virginia Creeper and Wisteria.
Lucas Moran: Insulate the growth media. Reducing the temperature fluctuations could be done by insulating the grow media / surfaces to mimic that of ground insulation. This would allow the cells in the root systems to remain intact throughout the winter season. This can be done with a more dense media along with a deeper pocket or planter.
It is also crucial that the designer of an outdoor wall consider the temperature zones when selecting species for the wall. Considering the fluctuation in temperature during the hottest most humid times of year in relation to the coldest days with wind chill will ensure the wall survives the range of temperatures present. In addition, it could be beneficial to use a wall type like a trellis system where the roots are planted in ground and therefore given greater insulation than a standard green wall with potted plants only affording the roots 6-8” in depth. We would always recommend 18-24” of depth for the root system to be well insulated.
Maintaining the wall will ensure someone can monitor the plant health during all times of year. This will inform replanting decisions after a full winter season, expectations from specific species as well as an understanding of the wind velocity and access to lighting. Finally, trimming, shearing and pruning the foliage aids in preparing the plants for winter.
Joep van Vilsteren: Ensure appropriate plants are chosen and collaborate with the system provider to ensure plants are suitable for the climate and will maintain healthy foliage throughout the winter.
Ensure a proper rooting substrate is chosen to allow plants to develop extensive and resilient root systems. It’s important that plants are not confined in small spaces. Pots or pockets will limit the plants growth and root development making it very susceptive to cold damages and/or failure. We see the best results with a mineral wool-based system allowing for unrestricted root development and protection in the winter.
Ensure that the irrigation equipment has the ability to water the plants year-round. ALSO, in the winter months. Ensure that the irrigation equipment and computer will manage the winter watering instead of relying on a maintenance technician to make daily visits to water the plants. An automated irrigation computer that waters year-round and has special features to prevent pipe bursting should be used.
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LAM: What steps should green wall maintenance professionals take to help exterior green walls survive the winter in cold climates?
Joep van Vilsteren: Even though it is extremely important for a maintenance professional to appropriately care for plants throughout the year, their ability to prevent any plant failure in the winter is strongly dependent on how well the above-mentioned factors (plant selection, system selection, irrigation equipment selection) are done. Some plants will simply not ever make it through the winter and some systems will just simply not ever have the capability of preventing plant failure in the winter.
It is very important for the living wall maintenance professionals to ensure that plants are thriving during the growing season and so are building a substantial and mature root system with enough resilience to be able to deal with cold winter conditions.
Throughout the winter the maintenance team should keep a very close eye on the irrigation equipment ensuring that everything is running. Ideally, the equipment gathers and communicates several data points allowing the maintenance professional to monitor the irrigation remotely preventing lots of expensive ‘checkup’ visits.
Lucas Moran: Selection of appropriate species is key in ensuring green walls survive winter seasons. Species that possess the following characteristics are ideal candidates for outdoor living walls; seasonal foliage retreat, deciduous species, evergreen species, etc. Other steps including pruning the dead foliage at the end of the fall season (which will allow more lush growth in the following season as the plants will have less obstruction from dead organic matter from the previous season), and covering the wall with burlap, fabric, or plastic covering (which will reduce wind blistering and add an additional layer of insulation for the plants).
Randy Sharp: Maintenance of living walls begins immediately when plants arrive on site. Ensure that the irrigation system is operational to establish new roots through constant moisture and regular injections of liquid fertilizer. The most common reason for living wall failure is interruption of water flow, even for just a few days. Install segregated irrigation zones by grouping plants with common moisture, pH and sunlight requirements. Remote monitoring with a tablet or laptop facilitates the remote operation of an automatic irrigation system. This includes a moisture sensor, flow meter, thermal control and compressor for use throughout the year especially in the winter. The irrigation system shuts down when temperature drops to +3oC and completely drains to avoid winter damage. The irrigation is reactivated when temperature reaches +5oC. Heat tracing can also be used to keep driplines open during freezing.
Green façade maintenance is much less intensive then living walls. Ensuring the roots of climbing plants have good access to moisture, supplemental irrigation and protection from compaction. Pruning and training climbing plants on an annual or semi-annual basis is recommended. Climbers, aggressive by nature, will race to the top of the wall and bunch up casting a shadow on the lower part of the wall. The solution is to prune or cut back the top one-third (1/3) of the vegetation in October. This will allow more light lower down to encourage better coverage. Providing access for ladders, platforms either fixed or on a horizontal track, scissor’s lift and/or a Genie boom.
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The greenscreen trellis panel system at the rear of a Whole Foods store in Chicago supports Parthenocissus quinquefolia, a native deciduous climber. Virginia creeper turns orange and brilliant red in the fall before dropping its leaves. In winter, the contorted branches are silhouetted against the blank concrete wall. The design of the building provides an amenity for persons biking, walking and rolling on the greenway along the North Chicago River. The shade of the Spring and Summer foliage also helps to cool the interior of the big box retail and mitigates the severity of the otherwise blank concrete walls in the Windy City. Photos: John Souza.
The application of living walls planted with hardy species are successful in Chicago, Pittsburgh, Boston, Toronto, Montreal and Halifax. These northern cities are located in USDA hardiness zones 5 and 6 as well as Canadian hardiness zone 6B. I encourage the industry to expand its plant pallet by testing living wall plants in exposed cold climate conditions. Successful living walls feature a lush assortment of evergreen groundcovers, compact shrubs, ferns, grasses, perennials and a filigree of deciduous woody plants. Green facades offer a cost-effective alternative to living walls. Climbers can be planted in elevated planters, or at grade where there is not enough room to plant trees.
Purchasing a green wall by an owner, pension fund or property manager requires a major commitment both in time and resources as well as budgeting for the high capital and operational costs. Green wall companies should train maintenance personnel who can also access out of town installations. Affiliation with a plant maintenance company or landscape contractor that offers a full-service maintenance contract and replacement warranty is an option.
The future is bright for green wall companies gaining more experience in selecting attractive, high performance, winter hardy and attractive plants for cold climates. Bringing nature back to the city with living walls and green facades provides the backdrop for gathering, collaborating and refreshing in outdoor settings.
Joep van Vilsteren has been with Sempergreen since 2016, starting as a Sales Manager. Joep later had the opportunity to become the director of Sempergreen Services. At Sempergreen Services, Joep leads the design, manufacture, installation and maintenance of Living Walls, and offering restoration and maintenance services for green roofs. Sempergreen Services prides themselves for approaching all Green Roof and Living Wall projects with great care and understanding of the vegetation resulting in high quality installations with great longevity.
Randy Sharp is an award-winning landscape architect and Green Roof Professional (GRP) with over 100 green roofs, living walls and vertical landscapes under his belt. He is the co-founder of Sharp & Diamond Landscape Architecture Inc., which was later rebranded as Connect Landscape Architecture. Randy consults on special assignments as SHARP Landscape Architecture. He is part of a team comprising Genevieve Noel of Mindful Architecture and Kerry Ross of Green T Design, designing a northern climate green roof with PVs and restoring the site landscape and Boreal forest for the McMurray Métis Cultural Centre in northern Alberta. Contact Randy for a list of plant species to consider for testing and potential use on cold climate green facades and living walls
Lucas Moran is the greenhouse manager and maintenance lead at New Earth Solutions. Lucas’ love for plants began to grow after propagating his fist spider plant early in his secondary schooling. His collection of plants quickly blossomed into an assortment of tropicals that many know and love, from parlour palms to pothos. After receiving his degree in bio-resource management from University of Guelph, he began in his position with New Earth Solutions. There he spends his time maintaining living walls and experimenting with new maintenance techniques. He hopes to continue on this path and eventually become a positive influence in the world of biophilic design.
