Irrigation

An irrigation system is highly advisable on green roofs: to extend plant species selection options, to improve plant growth rates and increase long-term vegetation success – thereby ensuring aesthetic and environmental (such as building cooling, stormwater reduction) outcomes can be achieved. Planning for irrigation on a green roof should consider the site layout and conditions (access, exposure), type of plants, climate and water supply issues (pressure, quality, etc.). Substrate properties and depth are also important as they influence water infiltration, holding capacity and drainage. In most cases, irrigation design will be heavily influenced by the nature of the water supply resource (for example, harvested vs. potable water) and a water budget should be used to guide not just irrigation but also plant selection (see here). In larger green roof projects, irrigation is best undertaken by a specialist consultant to guide system design, component selection, installation and maintenance. Table 11 provides an outline of different options for irrigation of green roofs.

An irrigation system is laid on a large green roof. Image: ASPECT Studios

An irrigation system is laid on a large green roof. Image: ASPECT Studios

Automatic systems

If an automatic irrigation system is to be installed on a green roof, consider a system that incorporates a rain sensor that shuts off the system in the event of rainfall above a certain threshold. This removes the risk that the roof loading may be compromised if the irrigation system is running during a heavy downpour. Even automatic systems require regular physical checks and operation tests.

Irrigation frequency

During the establishment phase after planting, irrigation may be frequent, for example, two to three times per week. For food crops, irrigation will also be necessary during high activity phases of the growth cycle, such as when the plant moves into flowering and fruit set. The frequency of irrigation should be matched to the drainage and water-holding capacity of the mix: frequent irrigation of a very free-draining substrate is likely to waste water.

 

The irrigation delivery method will partly determine the timing of irrigation. Watering in high daytime temperatures will transfer more heat into the building, as water heats up as it passes through the hot growing substrate and transfers some of this heat into the building when it drains onto the roof surface. For surface and sub-surface irrigation, there is little to no wetting of the foliage, which lowers the risk of fungal disease. If spray irrigation is used, it should be applied very early in the morning to enable foliage to dry off throughout the day and thus reduce the likelihood of disease.

Moisture sensors

Be aware that moisture sensors used to estimate moisture content in standard landscaping soils do not provide reliable information about the moisture content of free-draining, porous growing substrates used on green roofs.

Low irrigation

It is important to understand that, while some succulent species can survive on rainfall alone, plants that experience moisture stress will decline over time. This leads to a loss of vigour, leaf shedding, canopy reductions and ultimately plant death. Plant failure means more water run-off from the roof, less transpirational cooling and more opportunities for weed invasion. As such, irrigation is advisable. However, if designing for very low water use, select plant species that are better able to tolerate the extreme moisture stress on a green roof during a typical summer. Selection tools to guide the identification of low water use plant species can be found at the Smart Garden Watering website. Another option is to provide supplementary irrigation on a contingent basis during the hotter periods of the year.

 

Depending on the level of attention that can be provided, inclusion of indicator species with moderate requirements for water may be helpful to show when supplementary irrigation is necessary. This avoids compromising the performance of all the plants.

Irrigation methods for green roofs

 

Delivery method

Advantages

Disadvantages

Microspray

Low cost, visible, easy to install, reliable

Uneven distribution (plant interception), high water loss (wind, evaporation), foliage wetting (increased disease potential)

Surface drippers/perforated pipes

Low cost, visible, even delivery of water

Moderate water loss

Sub-surface drippers/perforated pipes

Low cost, moderate efficiency (water delivery to root zone)

Non visible (difficult for maintenance), higher potential for damage by people digging

Sub-surface capillary

High efficiency

High cost, maintenance and repair is difficult because not visible, ‘capillary rise’ of substrate needed or water will not reach plants

Wicking associated with irrigation in drainage layer

High efficiency, ease of installation

Linked to proprietary systems, ‘capillary rise’ of substrate needed, this may be unsuitable for plant establishment if the water is applied too deep for the plant roots to reach

Hose

Good for domestic application for easily accessed areas, not so good for other areas. Allows monitoring to occur at the same time

Cost (requires someone to be present on-site), low water efficiency, foliage wetting, uneven distribution