Impact of Adjacent Allelopathic Trees and Anti-Allelopathism Potential on Green Roofs

Allelopathy is the little referenced yet extremely important green roof secondary design principle of the bio-chemical influences certain plants and trees have on other plants and in this instance - on green roof plants.  The Breaking Ground Green Roof planting area is surrounded on three sides by tall trees. Some of these trees are deciduous and loose their leaves during the winter, others like the laurel oaks keep leaf cover most of the year.


An alleopathic tree usually exerts negative influence on adjacent vegetation via a number of different processes including;

• Fog & dew drip
• Leaf litter
• Volatilization 
• Sap drip
• Pollen
• Other biological processes

Trees impacting the Breaking Ground Contracting 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 
Acacia spp
Acer saccharum
Ailanthus altissima
Celtis laevigata
Celtis occidentalis
Eucalyptus camaldulensis
Eucalyptus globulus
Eucalyptus spp 
Juglans cinerea
Juglans nigra
Leucaena spp
Myrica cerifera
Picea engelmannii
Platanus occidentalis 
Populus deltoides
Prosopis juliflora
Prunus cornuta
Prunus serotina leaf 
Quercus falcata leaf 
Quercus marilandica
Quercus rubra
Quercus stellata
Robinia pseudoacacia
Sassafras albidum
Ulmus americana


Moderate Potential for Allelopathic Impacts
Abies amabilis
Abies balsamea
Abies grandis
Acer circinatum
Acer negundo
Acer platanoides
Acer pseudoplatanus
Acer saccharinum
Aesculus glabra
Aesculus hippocastanum
Aesculus octandra
Arbutus menziesii
Carya illinoensis
Carya ovate
Corylus spp
Crataegus spp
Fraxinus excelsior
Ginkgo biloba
Gleditsia triacanthos
Juniperus monosperma
Juniperus scopulorum
Kalmia spp
Picea spp
Pinus banksiana
Pinus contorta
Pinus densiflora
Pinus edulis
Pinus elliotii
Pinus monophylla
Pinus ponderosa
Pinus sylvestris
Prunus pumila
Quercus alba
Quercus borealis
Quercus douglasii
Quercus gambelii
Quercus michauxii
Quercus shumardii
Rhododendron maximum
Rhus copallina 
Sorbus sitchensis
Tsuga canadensi



Slight Potential for Allelopathic Impacts
Abies concolor
Aesculus spp
Betula pendula
Carpinus spp
Casuarina spp
Cupressus macrocarpa
Fagus spp
Fraxinus spp
Larix decidua
Picea excelso
Pinus palustris
Pinus spp
Populus spp
Pseudotsuga menziesii
Quercus petraea
Quercus robur
Quercus rubra
Salix pellita
Sambucus racemosa
Sequoia sempervirens
Taxus brevifolia
Thuja plicata
Tilia americana
Tilia cordata
Tilia planifolia
Ulmus laevis
Ulmus parvifolia
Umbellularia californica

As mentioned, the BReaking Ground Contracting Green Roof will be adjoined by 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 BGC 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.  Tomorrow's topic will explore the antithesis of allelopathic impacts and look at the potential biological and chemical benefits from adjacent trees.