Creating
healthy plant neighborhoods – Allelopathy
by Catherine Kavassalis for the Oakville Horticultural Society
Several years ago, I laid out a long garden border and edged it with
creeping phlox (phlox subulata). The
phlox thrived and spread except in one area. At first, I thought it was simply
poor stalk and transplanted proven plants – to no avail. Why were the plants
in that region growing sparsely? The answer was - allelopathy.
All plants produce a wide range of chemicals. Some are primary metabolites
like sugars and amino acids that are essential to the life of the plant. Then,
there are the more complex compounds, called secondary metabolites that confer
the unique properties of the plant – its colour and odour, for instance.
Allelochemicals are a particular group of secondary metabolites (such as
alkaloids, phenolics, flavonoids, terpenoides etc.) that act upon other
organisms. In 1937, botanist Hans Molisch wrote, The influence of one plant
on another: Allelopathy, coining the term that has established this branch
of phytochemical research. In 1996, the International Allelopathy Society
defined
allelopathy as “Any process involving secondary metabolites produced
by plants, micro-organisms, viruses, and fungi that influence the growth
and development of agricultural and biological systems (excluding animals),
including positive and negative effects.” However,
this definition is under review. Narrowly
speaking, allelopathy describes a plant’s biochemical defense system. More
broadly understood, it concerns the complex biochemical interactions among
plants, algae and microorganisms.
I suppose the poster child of allelopathy would be the Black Walnut
(Juglans nigra). Many members of the Juglandaceae family (including butternuts,
pecans and hickories) produce a relatively harmless chemical called
hydrojuglone. This allelochemical is exuded from walnut roots and leaches from
leaves and nuts. When exposed to oxygen in the soil, hydrojuglone is transformed
into juglone, a phytotoxic quinone (plant poison). A tomato plant in the
vicinity of a walnut will wilt, turn yellow (chlorosis), and die. In natural
forests, this allelochemical keeps the pine, birch, lindens, and hackberries,
among others, from competing for territory. Walnuts are not alone in their
ability to inhibit the growth of other species. Many landscape trees and shrubs
produce allelochemicals that discourage certain plant neighbors: sugar maples
produce phenolics that deter yellow birch and white spruce; sycamores produce
coumarins that suppress birch, herbs and grasses; the black cherry produces
cyanogenic glycosides that affect red maple and red pine. These are but a few of
the trees that have been studied.
Fortunately, while juglone is toxic to many species, particularly those in
the nightshade family (Solanaceae),
many plants not only tolerate juglone but like Kentucky
bluegrass (Poa pratensis) actually
thrive in its presence. Allelochemicals
often display complex and differential actions. They interact with other
organisms and compounds making it very difficult to isolate cause and effect. In
addition, although all plants create allelochemicals, some are only produced in
response to stress – fungal attack, drought, etc. For instance, when a plant
is attacked by insects, it will produce a signaling hormone, like jasmonic acid.
Jasmonic acid signals the production of other plant-defense compounds (like
salicylic acid - “aspirin to ease the pain”) and when it is released to the
air (volatilization), it signals nearby plants to create their own defense
compounds. Because allelochemicals are often intermittent in their action, they
can be missed by researchers and gardeners alike.
It’s hard to miss my gasplant, however. On warm summer days, Dictamnus
albus exudes some rather noxious, though not altogether unpleasant smelling,
chemicals - including the very flammable isoprene (gasplant is “considered by
many to be the Burning Bush of the Bible,” 2004, J.
Essential Oil Research). It releases these odiferous compounds to reduce
heat stress. Unfortunately, my little bordering phlox does not flourish in its
fumes. In addition to volatile gases that exude from its pores, Dictamnus, like other members of the Rutaceae family, releases furocoumarins and cineole from leaves and
roots - herbicides. In fact, Cinmethylin derived from plant-sourced cineole is a
commercial herbicide. (A strong word of warning, furocoumarins in Rutaceaes,
like the gasplant and garden rue [also in citrus], can produce
phytophotodermatitis – I know from painful experience – do not get their sap
on your skin on a sunny day!) Not only does my phlox have to contend with this
VOC factory, but it also must put up with cousins of the spotted knapweed –
the “wicked weed of the west” (2004, Smithsonian).
Centaurea biebersteinii,
like many other Centaureas,
can be highly competitive - releasing catechin, a phytotoxic compound, which can
inhibit both seed germination as well as root growth of neighboring plants.
Many invasive plants use such inhibitory allelochemicals. Unfortunately, I have
a penchant for several Centaurea
species to the dismay of some of their garden companions.
And that’s the trick, finding compatible neighbors. Companion planting
is as much science as it is art. Knowing that french marigolds (Tagetes
patula) release thiophenes, that kill root-knot nematodes (Meloidogyne incognita),
you should plant them next to snap dragons (Antirrhinum
majus are particularly susceptible
to root-knots) - pretty and practical. In general, members of Asteraceae/Compositae
(aster/daisy family) produce inhibitory allelochemicals. Sunflowers (helianthus
annuus in particular) are known for preventing seed germination in
farmers’ fields. Others in the aster family, like Artemisia
(wormwood), can on the one hand stunt some neighbor’s growth, but on the other
hand provide some insect protection (pyrethrum daisies are also in this Anthemideae
tribe). I guess, like in any neighborhood, it’s a bit of give and take.
Adoption
of allelopathic strategies is already helping farmers reduce pesticide use and
increase productivity. Interplanting, plant rotation, mulching with the right
plant material can also help home gardeners be more environmentally friendly and
improve garden health. Unfortunately, there are few easy to use allelopathy
charts for perennial gardeners. Companion planting guides, however, that have
been created from years of observation are a good place to learn how to design
healthier plant neighborhoods. I have included a few references for you to
further explore this area of gardening knowledge.
May toads nestle
in your flower beds and help you tend your beauties in the spring.
References and
links:
Society Links
International
Allelopathic Society (Comprehensive
Overview by Azim Mallik, president of IAS for the 2005 World Congress on
Allelopathy Allelopathy: Advances, Challenges and Opportunities)
World Congresses on Allelopathy
Fourth World
Congress "Establishing the scientific basis" August 2005, Australia
Third World Congress
"Challenge for the new millennium" August 2002, Japan (nice intro to
Allelopathy)
Second World
Congress "Allelopathy: critical analysis" August 1999, Canada
First World Congress
"Allelopathy: a science for the future" September 1996, Spain
Companion Planting Charts and Info
Bayfuss, R. Winter 1994.
Companion
Planting. Cornell Cooperative Extension ECO GARDENING, Fact Sheet #10.
Kuepper,
G. & Dodson, M., 2001. “Companion Planting: Basic Concept and Resources.”
National Sustainable Agriculture Service.
Garden Guides: Herb Companion Chart
& Vegetable
Companion Chart
Melanys Companion Planting
Chart
Stromme,
L., “Trees and turf: are they compatible?”
University of
Michigan’s
Sustainable Urban Landscape Information Sheet. (well
done)
Stuart B. Companion
Plants. Hill Department of Entomology Macdonald College McGill University.
More general and many esoteric references
An,
M., Pratley J., & Haig, T. 1998. “Allelopathy:
from concept to reality.”> Proceedings of the 9th Australian
Agronomy Conference.
Agronomy Journal - Collection
from Allelopathy Symposium
Allelopathy Research at the University of Savoie
- Intro
to Allelopathy has some nice photos of Athyrium filix-femina
affecting germination of Picea abies seeds.
Alper,
J. Dec. 2004. “Wicked Weed of the West.” Smithsonian
Magazine. (spotted knapweed"Centaurea biebersteinii>
syn. C. maculosa)
Appleton, B., Berrier R., Harris, R., Alleman, D.,
Swanson, L. January 2000. “The Walnut Tree: Allelopathic Effects and Tolerant
Plants.” Virginia Cooperative Extension.
Armstrong,
W. "Major Types Of
Chemical Compounds In Plants & Animals." An Online Textbook of
Natural History. (fun overview with some interesting factoids).
Bais, H.P.,Vepachedu,
R., Gilroy, S., Callaway, R.M. and Vivanco,
J.M. 2003. "Allelopathy and Exotic Plant Invasion: From Molecules and Genes to Species
Interactions." Science 301(
2003): 1377-1380.
Balandrin, M. F., J. A. Klocke, E. S.
Wurtele, and W. H. Bollinger. 1985. Natural
plant chemicals: Sources of industrial and medicinal materials. Science
228: 1154-60.
Chromadex Dictamnus
- burning bush phytochemicals (Botanical reference standards)
Coder, K. 1999. Allelopathy
in trees and forests: a selected bibliography Warnell School of
Forests Resources
Cornell Education Project on Allelopathy
Dover, K. et. al. 2003. Marigolds
as crop cover Department of Entomology & Nematology, University of Florida
Duke, S.O. 1990. Natural
pesticides from plants. 511-517. In: Janick & Simon (eds.), Advances
in new crops. Timber Press,
Portland
,
OR.
Duryea, M., English, R., Hermansen, L.
1999.
A comparison of landscape mulches: chemical, allelopathic, and decomposition
properties. Journal of Arboriculture.
Ferguson, J. &
Rathinasabapathi, B. July 2003. Allelopathy:
How plants suppress other plants University of Florida Extension
Service.
Fleisher, A., &
Fleisher, Z. Jan/Feb 2004. Study
of Dictamnus gymnostylis volatiles and plausible explanation of the
"Burning Bush" phenomenon. Journal of Essential Oil Research
Focassi, D. Physiology
of Plantae - Botany. This is a general repository of info and links.
Hierro, J., Callaway,
R. 2003. "Allelopathy
and exotic plant invasion." Plant and soil 256. 29-30
Internet
Dermatology Society. 1995-2000. Phytodermatitis.
Kruse, M. Strandberg,
M. Strandberg, B. March 2000. “The Ecological Effects of Allelopathic Plants – A
review.” (pdf) National Environmental Research Institute, Denmark.
Mallik, A. Prof of
Biology at Lakehead University in Ontario. Pres. of IAS. Ecological
Research in Northern Forests.
PanDagger, Z., et. al.
July 1998. Aspirin Inhibition and Acetylation of the Plant Cytochrome P450, Allene Oxide
Synthase, Resembles that of Animal Prostaglandin Endoperoxide H Synthase. J Biol Chem, Vol. 273, Issue 29, 18139-18145.
Pliny the Elder,
77+AD.
Natural
History (eds.
John Bostock, M.D., F.R.S., H.T. Riley, Esq., B.A.)
Sengbusch, P., Bergfeld, A., Bermann,
R. 2003. The
Secondary Metabolism of Plants: Secondary Defense Compounds for Botany
online - Internet Hypertextbook.
Segelken, R. Dec. 8, 2003. Researchers find plant immune system's 'take two aspirin' gene, offering hope for disease control without agricultural pesticides.
Cornell News.
Silva, E. 1997. "Plant Against
Plant." The Virginia Gardener Newsletter, Volume 8, Number 4.
Tebo,
M. 2005. "Killer
Plants." HGTV
Whiting,
D., Wilson, C. & Card, A. December 01, 2005. Organic
Fertilizer. Colorado State Extension Service.