An alleopathic tree usually exerts negative influence on adjacent vegetation via a number of different processes including;
- Fog & dew drip
- Leaf litter
- Sap drip
- Other biological processes
Trees impacting the recent green roof include;
- Triadica sebifera, Chinese tallow - western border of green roof
- Quercus laurifolia, Laurel Oak - northwestern corner and eastern side of green roof
- Platanus occidentalis, American sycamore - southeastern and southwestern corners of green roof
According to the University of Georgia, School of Forestry Resources , there are a number of significant allelopathic trees requiring attention when planting other plants nearby. They include;
Strong Potential for Allelopathic Impacts
Prunus serotina leaf
Quercus falcata leaf
Moderate Potential for Allelopathic Impacts
Slight Potential for Allelopathic Impacts
As mentioned, a recently completed Green Roof will be adjacent Chinese tallow trees, Laurel Oaks and American Sycamores.
Quercus laurifolia, Laurel oak - although literature suggests laurel oak does not possess allelopathic qualities, care should be given to potential impacts of pollen and flower litter. The laurel oak adjacent the northwest corner of the green roof has stained the white TPO and covered the roofing material with a layer of leaf and pollen litter. Though laurel oak may not exhibit direct allelopathic influence on the green roof plants, potential for covering the plants with litter exists. Continued site inspection will be required to confirm any impacts on the green roof plantings.
Triadica sebifera, Chinese tallow - one medium height tree exists adjacent the western border of the BGC green roof. Chinese tallow has been the subject of numerous allelopathic studies and research. Interestingly, research exists to support the theory of Chinese tallow leaf litter and fog drip may actually support germination and shoot growth on adjacent plants. In fact, Chinese tallow was shown to actually improve germination and growth rates in Little Bluestem, Schizachyrium
Importantly, the American Sycamores, Platanus occidentalis located in the southeastern and southwestern corners of the green roof have the potential to exert significant negative influence over the green roof plants. As indicated in the above list, American sycamore produces strong allelopathic effects. Data exists showing the active ingredients, scopoletin and chlorogenic acid found in the sycamore leaf may interfere with the ability of stomata on certain plant's leaves to malfunction, interrupting the vital processes of photosynthesis and either stunting plant growth or killing the plant. Close observation will be required on the effects of the American sycamore on the entire green roof planting area and in particular, the southeast and southwest corner plantings. Pruning of sycamore limbs away from the green roof may be necessary.
Finally, good green roof design incorporates the effects of adjacent trees and other vegetation and allelopathic effect possibilities. Recognizing and dealing with a potential allelopathic problem is much easier and more cost-effective up front. Know the basics of adjacent tree and plant allelopathism and how your green roof design integrates into a site with pre-existing trees.
One of the related positive issues of anti-allelopathism and green roof adjacent trees is a benefit derived from leaf micro-nutrient content.