Roof Garden Maintenance: How to Keep a Green Roof Green
Green roofs are gorgeously green in color, not just in name. The term refers to gardens planted atop houses, office buildings, and many other structures ... for a whole slew of environmentally friendly reasons. They form a protective layer that lowers the cost of energy needed to heat and cool the building below. Roof gardens also act as an air filter, removing and storing pollution and greenhouse gas emissions. Perhaps most important, they add a touch of natural beauty to the landscape, especially important in the midst of a jungle of urban high-rises.
There are two types. Extensive green roofs are basically a light 2" tall cover of shallow-rooted, hardy alpine plants such as sedums, which require minimal maintenance. Intensive green roofs feature a wide variety of vegetation, including trees, making them much heavier in weight and more maintenance-intensive. How do you maintain a green roof garden?
Feeding and Watering
A green roof is essentially a giant container garden, planted in a relatively shallow layer of soil to reduce weight on the roof. Due to the limited amount of nutrients available to the garden via the soil, you may need to apply a slow-release fertilizer. Once the plantings are well established, an extensive green roof may not require any water apart from natural precipitation. Intensive green roofs, on the other hand, usually must have an additional water supply (especially true on a steeply pitched roof that allows stormwater to run off more quickly). The easiest way to provide this is with a drip irrigation system, which has the added benefit of increasing evaporative cooling. Under extremely hot, dry conditions, supplement with hand watering or an overhead spray.
Weeding and Trimming
Although less susceptible to weeds than its earthbound counterpart, a green roof will require regular weeding in the early stages, tapering off as it matures. Be sure to remove weeds before they go to seed, in order to minimize spreading. The thin layer of soil will tend to encourage horizontal, rather than vertical, growth; as a result, you won't need to mow or trim the garden as frequently as a conventional one. However, a certain amount of pruning and clipping will still be required to keep the plantings healthy and good-looking.
With every green roof, a waterproofing membrane must be installed to protect the roof material and the building underneath. Test the membrane for leaks by flooding it with water prior to planting. There are a number of membrane materials on the market. Ask your landscaper to recommend one that is highly root resistant. Continue to inspect the membrane on a routine schedule in order to detect any leakage or drainage backup.
The abundant food, water, and shelter offered by a roof garden make it appealing not only to human beings but also to animal and insect pests. Inspect the garden regularly for signs of infestation such as chewed-up plants, burrows or nests, and droppings. Remove decaying vegetation that may attract ants, as well as stagnant water -- a potential mosquito breeding ground. Decrease the pace of drip irrigation to deter moisture-loving snails and slugs. Deter pigeons with anti-roosting devices. You may need to call in pest control professionals to get rid of a persistent infestation such as squirrels or rats; ask them to use no-kill traps.
If a green roof is allowed to dry out excessively, it is likely to become a fire hazard. Danger can be avoided by:
- Installing a drip irrigation system, possibly hooked up to a fire alarm.
- Planting fire-resistant succulents, which do not tend to dry out, even in hot weather
- Using a growing medium that contains a high percentage of inorganic material, like perlite or sand
- Spacing 2' stretches of gravel or pebbles every 130', as fire breaks on large public green roofs
Laura Firszt writes for networx.com.
Updated April 22, 2018.