Green roof systems are typically planted with water-conserving succulents that perform well in hot, dry conditions. Prolonged drought, however, can weaken even the most resilient plants and result in thinning or losses on the green roof. If you’re in a region of the country that is experiencing unusually low natural precipitation rates, you may need to irrigate your green roof to protect your investment. Not only does watering your roof result in healthier plants, it provides a cooling effect that reduces air-conditioning demand.
In fact, depending on the energy source for air conditioning, you may actually be saving water by using green roof irrigation to naturally cool your building. This is because virtually all electricity has substantial water cost for its production. So, when you crank up the A/C, you’re also “using” water.
Are you facing drought? Many areas that are not typically prone to drought are experiencing unusually dry weather. Drought information is available from the National Weather Service Drought Information Center or the Agriculture and Agri-Food Canada Drought Watch. Below are current maps from each of these drought monitoring services, as well as a map of the projected drought outlook for the United States.
How Much Water? How Often? From Where? The amount of irrigation water that you green roof needs will vary based on temperature, natural precipitation, the plants chosen for your green roof system, soil depth, and other site conditions such as lighting and wind. As a rule of thumb, a four inch deep green roof system needs a good inch of water every four weeks. This is shortened in excessive heat and/or humidity, next to highly reflective surfaces and if non-succulents are used in the plant mix. Every two weeks during the summer months will typically ensure healthy plants in most green roof applications. Consult your local LiveRoof grower to discuss your site conditions and a recommended watering schedule.
The best source of water for green roofs is from natural precipitation. Rainfall is nitrogen rich and doesn’t contain the salts typically found in treated potable water. Therefore, rainwater harvesting for green roof irrigation during drought is encouraged when possible. With sound engineering, water may be recycled or harvested from other sources, such as air-conditioning condensate or grey water. An expert should be involved in planning the treatment of most recycled water sources for irrigation. Local water harvesting equipment vendors and designers such as Wahaso can be found using an internet search or a directory such as HarvestH2O.com.