The best irrigation method is the one that most efficiently applies the water and provides the lowest lifecycle cost. In the case of green roof applications, this choice is straightforward and simple. Spray rotor systems are the least expensive to install, easiest to maintain, and apply water most efficiently. This may run counter to what is sometimes spoken about spray vs. drip or sub-irrigation, but it’s the truth and has been born out by testing at LiveRoof, LLC and research at Michigan State University.

Closeup views of drip, sub, and overhead irrigation.

Closeup views of drip and sub-irrigation emitters and layout of overhead irrigation system showing four of six emitters.

To understand why spray irrigation uses less water than drip or sub-irrigation systems on green roofs, one must realize that green roof soils are engineered to maintain their volume and last the life of the building, and therefore are different in composition and characteristics than nursery or natural field soils. Nursery and natural field soils are typically high in organic content. But, green roof soils are very low in organic content, to avoid volume loss via decomposition of the organic matter. In nursery and natural field soils, drip or sub-irrigation systems can often conserve water, because these types of soil are good at dispersing water laterally or wicking it upward by capillary action – through the absorbent and capillary qualities of the soil. But green roof soils, which are predominantly composed of inorganic aggregates, have poor capillary qualities and do not conduct water effectively in either a lateral or vertical manner. To irrigate them with drip irrigation, one must set the tubes so closely together that the cost is prohibitive. And, to irrigate them with sub-irrigation, one will use more water than spray irrigation and may risk root rotting diseases, from too much water saturation to the lower level roots.

Horizontal movement of LiveRoof substrate subjected to drip irrigation.

Representative horizontal movement of wetting front for Live Roof substrate subjected to drip irrigation after 5, 15, and 25 min.

Horizontal movement of LiveRoof substrate subjected to sub-irrigation.

Representative horizontal movement of wetting front for Live Roof substrate subjected to sub-irrigation after 10, 20, and 30 min. Orange flags show location ofsub-surface emitters.

A well-designed overhead spray irrigation system, very much like what would be used in a home or commercial landscape, provides even coverage to the surface of plants and soil at a very low installation cost. Such systems are simple and inexpensive to maintain, and apply the water quickly and efficiently. Studies by Dr. Bradley Rowe, PhD, and others at Michigan State University Department of Horticulture compare the effectiveness of drip, sub, and overhead spray irrigation methods, and the researchers found that drip and sub-irrigation systems wasted more than double the water lost in an overhead spray irrigation system.

Sedum growth by several irrigation methods.

Plant growth of Sedum album and Sedum floriferum subjected to overhead, drip, sub, and no irrigation following 12 weeks.

Images courtesy of: D.B. Rowe et al. / Ecological Engineering 64 (2014) 306– 313