Mix it for South Florida

The following is a summary of some of our observations on planting media thus far. These observations will be incorporated into green roof planting tests to determine the best design approaches.

Use a minimum of 6-8” of planting media for grasses. If the particular situation can only accommodate less than 6” of planting media, only use the toughest of the tough spreading ground covers.

Peat moss has a spongy structure that provides a high nutrient, water and air holding capacity. It is a light organic material that will retain 20% of its weight in moisture and will reduce the need for additional water. Peat moss is sterile and therefore free of weed seeds and pathogens.

Unfortunately, peat is a non renewable resource that is harvested from wetlands. Peat harvesting accelerates the decomposition process; the carbon that is stored in peat is released to the atmosphere as carbon dioxide. Thus, peat harvesting is being regulated and peat prices are increasing as supplies decrease. The University of Florida is currently researching an alternative to Canadian and Florida peat – Cowpeat. Results from seed germination and rooting showed that the alternative media formulated with cowpeat had little difference in the percentage of seed germination and the percentage of rooting compared to those formulated with either Canadian peat or Florida peat. The use of composted manure, such as cowpeat, appears to offer a viable alternative which reduces peat mining and will contribute to the well-being of Florida’s environment. Source: For Peat’s Sake, Qiansheng Li, Jianjun Chen, Juanita Popenoe and Lelan Parker; Ornamental Outlook, August 2008.

Lava rock is a sterile media with tiny holes that wick up water and release it slowly to plants. Lava rock is a good choice for weight considerations and drainage, but black lava rock can get very hot.

Red lava rock can create ph problems.

When mixed with peat, black lava rock will give the soil mix structure, hold the peat in place and provide pore space for aeration.

Sand can often be too heavy for installations on existing roofs.

This summary was compiled from our observations at the Nancy Foster Florida Keys Environmental Center green roof, discussions with Glen Acomb, professor at University of Florida and designer of the Charles R. Perry Construction Yard green roof at University of Florida and conversations with Atlas Peat and Soil, a local supplier. These observations are not meant to be all inclusive and are above and beyond base recommendations regarding layering etc. that can be researched on national green roof sites.