The Importance of Proactive Green Roof Maintenance
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Maintenance. You’ll hear a lot of talk about this when designing and planning for a green roof installation. How often? What does it entail? How expensive is it? Is it necessary?
Some clients, specifically those who are mainly interested in undergoing the project to meet their municipality’s stormwater retention requirements, think they can skip out on investing in a professional green roof maintenance contract, and go with a cheaper, less qualified landscaper to do the work instead. Or, even worse, they don’t sign on with anyone at all to do maintenance. These clients often end up calling back after a few years because the plants are not doing very well on their roof.
Proper maintenance is critical to the long-term survival of the plants and the likelihood they will thrive. Healthy plants contribute to more stormwater retention, more evapotranspiration, support biodiversity and look great. Green roof maintenance has nuances that a traditional, ground-level, landscaper could easily overlook without prior experience with rooftop greenery. This skillful practice is not a one-size-fits-all type of deal; maintenance regimes will look different for intensive green roofs, sedum extensive roofs, pollinator meadows, and rooftop farms. Implementing spring maintenance of a green roof is especially critical to ensure a prosperous growing season.
This roof in Amherst, Massachusetts was experiencing extreme weed pressure before we remediated it with a biodiverse plant palette and mycorrhizal fungi powder. Originally a sedum roof, the newly added herbaceous perennials, self-seeding annuals, and nurse crops have since reinvigorated the soil and alleviated weed pressure. Photo: Pete Ellis, Senior Project Manager & Estimator, Recover Green Roofs
Beyond the different types of green roofs, your spring maintenance regime will look slightly different based on your respective climate zone. For example, a green roof in a tropical climate like Singapore will have a much different plant palette than one installed in Denver, Colorado. A big goal of the roof in Denver is to retain as much moisture as possible to withstand the summer heat and possible drought. A core challenge of a green roof in Singapore is ensuring proper drainage so the plants don’t drown from excessive rainfall. In essence, a significant difference in climate will translate to differences in the kind of maintenance conducted. This is due largely to the difference in plants chosen based on the desired functions of the system. However, the main pillars of spring maintenance remain more or less the same, no matter the type of system or geographic location.
Soil/Growing Medium Health
As a green roof owner or maintainer, you’ll want to pay close attention to nutrient levels in the soil/growing medium to ensure it is healthy and able to foster optimal, long-term plant growth. Once there is no longer a concern of frost, that is a good time to conduct a low cost, lab-tested soil/growing media sample. Although there should be sufficient nutrients in the growing media used on a roof, it is proactive to have samples taken once or twice per year, even for juvenile green roofs, ones that are still in the establishment period. The first tests on a new roof set a baseline for that site’s nutrient makeup, so that every test after that point can be looked at in comparison.
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Even after the plants and their roots are well established on a particular roof, it is still a good idea to have the soil/growing media tested annually; this is being proactive rather than reactive. If you wait until plants start failing to take a sample to see what the problem is, you’ll find yourself in a situation that could’ve been solved earlier by checking in on soil health annually.
Based on the results of the testing, some soil amendments might be necessary. These could include adding natural nitrogen fixing plants, organic fertilizers with the desired NPK ratio, or even a pro-booster fertilizer that is high in either nitrogen, phosphorus, or potassium. Monitoring the nutrient levels in the growing medium after each winter is important because it needs to be healthy enough to withstand the rise in nutrient uptake plants sustain when the air starts to warm up. When perennials realize it’s spring, they begin pushing out new growth, which takes energy. If the soil doesn’t contain the proper amounts of the vital nutrients plants use, there won’t be new growth and some perennials may go dormant until they have suitable growing conditions. On rooftop farms, the amount of organic matter by volume in the growing medium is typically much higher than extensive, sedum based roofs. These extra nutrients are needed because food crops pull a significant amount from the soil each year. The management of soil health is critical to ensuring significant yields on rooftop farms, many of which have onsite composting facilities.
Irrigation
With warmer weather comes the possibility of drought in many regions. Spring is the perfect time to start planning for drought season by thinking about an irrigation system. Whether it be routine hand watering, a temporary irrigation system with timers and hoses, or a permanent system with pop-up sprinkler heads or drip lines, making sure your soil receives and retains the right amount of moisture is key to the success of the plants on your green roof.
There are differing opinions in the industry about irrigation and whether or not it’s necessary after a roof is established and has full plant coverage. This topic should be considered on a case-by-case basis. Deciding on a type of irrigation and to what extent it runs depends fully on factors such as the water needs of the species of plants on the roof, how extreme the temperature conditions are, the amount of rainfall in the region, and how established the roof is. It is very possible that water is only needed for the first year or two to establish the plants’ root mass. At Recover, we maintain quite a few sites that have no need for irrigation due to their ability to retain water and withstand drought. Fully established sedum roofs, moss covered roofs that are semi-shaded, and those with drought-tolerant plant diversity are examples of green roofs that perform well without a permanent or temporary irrigation setup in the Boston area.
It is also important to note that irrigation systems require maintenance as well. In cold climates, these systems need to be started up in the spring, or “summerized”, as well as configured to have each “zone” of the roof watered on certain days and times throughout the week. The design, installation, programming, and upkeep of these irrigation systems is something that should be done by professional green roof contractors or irrigation specialists.
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Weeding
Located in downtown Boston, this intensive green roof is one that we at Recover watch very closely for invasive weeds. We try to maximize coverage here to push out unwanted species like clover, oxalis, spurge, etc. Photo: Andrew Mihalchick
As plants emerge from dormancy, pesky weeds will also be waking up and getting ready to reseed. On our roofs around New England, some spring weeds we keep an eye out for are chickweed, dandelion, and various species of clover. Just like soil testing, it is essential to be proactive when it comes to dealing with weeds. Preventative weed management can be achieved in the spring by thoroughly scanning the roof and hand pulling the unwanted undergrowth every 2-3 weeks. By making sure to remove the root system of the weeds, the chance of them flowering and going to seed decreases and the overall weed pressure throughout the warmer seasons will be much lower.
Albeit, in a case where a client has a struggling green roof and they are comfortable with moving away from the original plant design, we may decide to let some species of weeds grow to help rebuild the soil. Clover and dandelion are good examples of weeds that we would let go for the purpose of regenerating the nutrient levels to create a healthier ecosystem. Beyond that, it is important to note that the addition of a more biodiverse plant palette has, in our experience, also proven to help build soil structure, relieve weed pressure, and promote fuller coverage. Needless to say, if the life on their roof doesn’t match the client’s expectations, sometimes it is necessary to try different methods of remediation to achieve the desired look.
Moreover, aside from weeds, maintainers should be wary of various species of tree saplings as their woody stems have the potential to penetrate the waterproof membrane or flashing, if left unchecked for too long. Invasive species, unwanted seeds, and tree saplings can all be brought to the green roof by wind or birds over the course of the dormant season, creating all the more reason to be thorough in your spring maintenance visits.
It is also wise to be mindful when bringing materials that could be carrying weed seeds onto each site. Things like ropes, brooms, clothing, boots, reused bags, and even potted plants or plugs can all introduce unwanted invasive species. Taking the time to clean tools and other frequently used items over the winter often proves to be beneficial come spring time.
Spring Maintenance Tasks
Below is a list of common maintenance tasks performed by green roof contractors and/or maintainers like Recover. Each point is followed up with a bit more detail.
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This lush sedum extensive roof is a prime example of a place where cuttings can be taken from for use on other sedum green roofs with less-than-ideal coverage. This roof, because it is very well established with drought tolerant vegetation, has no need for irrigation. Photo: Sidney Hare, Horticulture Specialist, Recover Green Roofs
Tool Preparation
Cleaning/sharpening of any tools that may be carrying weed seeds not easily seen.
Spring Cutbacks
Perennials and grasses will be cut back in late winter or early spring to encourage plants to flush out new growth. Cutbacks include deadheading spent foliage, fine pruning shrubs and trees, transplanting and thinning out plants that have become overgrown.
At Recover, we typically leave grasses and most perennials with their foliage throughout the winter for a number of reasons; The foliage provides insects and/or birds with habitat in the colder months, it protects the crown of the plant so it doesn’t rot under accumulated snowfall, and it can provide some winter interest from an aesthetic standpoint.
Drain Inspection and Clearing
For those of us who deal with snow in the winter, it is critical to keep our roof drains clear when temperatures begin to rise, ensuring water from snowmelt can flow freely off the roof and avoid puddling.
This principle applies to green roofs in any climate; it is always best practice to keep drains clear.
Debris Removal
Depending on type, as well as the location of the green roof, there might be a buildup of dead leaves and other plant debris that could inhibit new spring growth if not dealt with promptly.
For roofs with public access, it is good practice to dispose of any trash or construction rubble that may have accumulated over the dormant season.
Filling in Bare Spots
It is common to have small areas of exposed growing media after a long and cold winter. Weeds will fight to overtake these spaces if not filled in with new plants, plugs, or cuttings, which can often be harvested from healthier areas of the roof.
With a sedum based extensive roof, it is common practice to collect cuttings from fuller sections with long and leggy sedum plants, spread them throughout the bare spots, and coat them with a thin layer of compost or growing media to encourage regeneration.
For meadow based green roofs, we typically seed wildflowers in the spring to fill in areas that have gone bare or lack interest.
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This award-winning 24,000 square foot rooftop meadow, completed summer of 2021 by Recover Green Roofs, is outfitted with dozens of pop-up sprinkler heads. Irrigation is critical on this roof due to its full sun location, as well as the absence of drought tolerant plants. Each spring, Recover’s maintenance team cuts all of the spent foliage down to the base, leaving behind a layer of organic material that decomposes and acts as a nutrient rich mulch which supports new growth. Photo: Andrew Mihalchick
Irrigation Preparation
Again, not all green roofs require irrigation. However, ones that are in their infancy will almost always need some type of temporary setup to ensure the vegetation establishes a healthy root mass.
For roofs with a permanent setup, early spring is the right time to get the system ready for operation by making sure that it is distributing water effectively throughout the roof.
Soil/Growing Media Testing
It is crucial to have insight on the health of your soil before each growing season.
At Recover, we collect multiple soil samples from each of our sites to be lab-tested at least once a year, giving us a valuable understanding of what’s going on beneath the plants.
Soil/Growing Media Building
Aside from the technical results from the soil tests, some roofs may require the addition of plants that will help build the soil in various ways.
On some of Recover’s sites, we have used buckwheat and daikon radishes to build the soil and help aerate it to prevent compaction. Clover is also a great option for soil building, however, due to its ability to spread rapidly, it is not always our first choice.
A technique still being researched, mycorrhizal fungi powder can help promote root growth, nutrient uptake, and improve soil structure when added to newly planted plugs or perennials.
Conclusion
Plants have been providing this planet with life-sustaining benefits for half a billion years. The more green spaces we create, the more we can assist Mother Nature in returning these benefits to our built environment, and proper maintenance of these spaces ensures that we are maximizing their utility.
As of now, a maintenance-free green roof is practically non-existent in this industry. Thus, proactive maintenance, particularly in the spring, can help augment the performance of your green roof and make it more resilient in the face of a changing climate.
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Andrew Mihalchick is a Builder & Horticulture Technician at Recover Green Roofs, located in Boston, Mass. You can reach him here.
