In Australia, one of the most common trees are eucalyptus trees, also known as eucalypts or gum trees. These evergreen trees manage to drop a decent amount of their leaves, branches, twigs and gumnuts below their canopy. Since this debris doesn’t rot very easily, it can persist for a very long time.
It’s tempting for gardeners to gather these readily available eucalyptus leaves, so they can add them to their compost, or use them as a mulch over their garden beds, but that would be a very bad idea!
When growing in harsh conditions, plants and trees have to compete for resources. In order to survive, sometimes the best defence is offence. Many trees and plants have evolved a defensive mechanism known as allelopathy, where they release inhibitory chemicals to suppress the development and growth of neighbouring plants.
The term allelopathy is derived from the Greek words allelon which means “of each other”, and pathos which means “to suffer”, which when combined literally means “to suffer from each other”.
The Effects of Eucalyptus Allelopathy
Take a look beneath most eucalyptus trees to see allelopathy in action. The bare patches of soil beneath their canopies are not caused by shading, because most eucalypt canopies are quite sparse and offer very little shade. The aggressive eucalypt roots can outcompete many plants for water, but this ‘Swiss cheese’ effect of the forest floor under eucalypts also occurs when water is not scarce. The allelopathic chemicals released from roots, leaves and branches act as natural herbicides, clearing the ground beneath them.
In the plant world, allelopathic chemicals can be found in leaves, flowers, roots, fruits, or stems, as well as the surrounding soil.
These toxins affect their targets in different ways such as:
- Inhibition of shoot and root growth
- Inhibition of nutrient uptake
- Attacking naturally occurring symbiotic relationship to destroy sources of nutrients
In general, Eucalyptus species contain allelopathic chemicals in their roots, leaves and stems, including branches and twigs.
Eucalyptus camaldulensis (River Red Gum) releases volatile allelopathic chemicals, and they also leach out from the litter beneath the trees.
Eucalyptus globules (Southern Blue Gum) exerts its allelopathy from chemicals in roots, leaves and stems, but also by fog drip, a phenomenon which occurs in specialized forest communities where allelopathic compounds are released through foliage leaching.
Some plants develop their own defences against allelopathic chemicals, and can grow perfectly well under eucalyptus trees. The Australian native groundcover plant Nodding Saltbush (Einadia nutans ssp. nutans) will form a thick carpet beneath gum trees, and produces tiny red-orange berries around 3mm (1/8”) wide which are used as a bush food and also attract native lizards.
What Happens if Eucalyptus Leaves are Composted or Used as Mulch?
If eucalyptus leaves are put into compost or used as garden mulch, they will break down, albeit very slowly because of their thick, waxy skin, but they will release allelopathic compounds which are toxic to plants and soil organisms, contaminating the compost or soil.
Eucalypt leaves release large amounts of volatile organic compounds (hydrocarbons) which are both repellent and toxic to earthworms, so they’re best kept out of the compost for that reason too.
It’s important to keep in mind that these allelopathic chemicals don’t affect all plants, they’re selective to a certain degree, but they will generally reduce seed germination, and reduce the growth of many plant roots and shoots.
Don’t use gum leaves in your compost or as a mulch! Period.
How to Safely Use Eucalyptus Leaves in the Garden
Work with nature, don’t fight it! If eucalypts are dropping lots of plant debris, gather it and use it to mulch around them. It won’t affect them, as they are using their allelopathic chemical defences to keep the soil around their canopy clear of other plants. Give them what they want! The mulch will reduce evaporative moisture loss from the soil in summer, reducing water requirements, and as it very slowly breaks down will help recycle the nutrients from their fallen leaves.
Managing Allelopathic Plants and Trees
As we know from the practice of companion planting, some plants grow better together, while and some are best kept apart. If plants are allelopathic, place their leaves and mulched branches beneath them, where they can do no harm, but benefit the plant they came from, and keep them away from sensitive plants.
Use common sense when planting allelopathic plants and trees:
- Don’t plant allelopathic plants and trees uphill if they release allelopathic water soluble toxins that can wash downhill onto other plants, or if their fallen laves and other debris will be carried down by water runoff.
- Don’t plant monocultures of allelopathic plants and trees because monocultures are unnatural, and reduce biodiversity, and a mass of allelopathic plants will create a toxic environment for many other plants, further reducing biodiversity.
Not Convinced? Here’s the Details…
Science is objective, and evidence based. Eucalypts are allelopathic, as are many other trees. They’re considered amongst the most allelopathic trees, rated as ‘strongly allelopathic’, alongside a list of other trees, as detailed below:
Strongly Allelopathic Trees and their Pathway of Effect
- Acacia spp., wattle tree – (root, leaf, stem)
- Acer saccharum, sugar maple – (roots)
- Ailanthus altissima, tree of heaven – (root, leaf, stem)
- Celtis laevigata, sugarberry, Southern hackberry – (root, leaf, stem)
- Celtis occidentalis, common hackberry – (root, leaf, stem)
- Eucalyptus camaldulensis, river red gum – (volatile, litter)
- Eucalyptus globulus, Southern blue gum – (fog drip, root, leaf, stem)
- Eucalyptus spp., gum trees -(root, leaf, stem)
- Juglans cinerea, butternut, white walnut – (root, leaf, stem)
- Juglans nigra, black walnut – (root, leaf, stem)
- Leucaena spp. (root, leaf, stem)
- Myrica cerifera, southern wax myrtle, southern bayberry, candleberry, bayberry tree, tallow shrub – (root, leaf, stem)
- Picea engelmannii, Engelmann spruce, white spruce, mountain spruce, or silver spruce – (root, leaf, stem)
- Platanus occidentalis, sycamore, American sycamore, eastern sycamore, American plane tree, western plane, occidental plane, buttonwood, buttonball tree – (root, leaf, stem, litter)
- Populus deltoides, eastern cottonwood, necklace poplar – (root, leaf, stem)
- Prosopis juliflora, mesquite – (root, leaf, stem)
- Prunus cornuta, Himalayan bird cherry – (root, leaf, stem)
- Prunus serotina, wild black cherry, rum cherry, mountain black cherry – (leaf)
- Quercus falcata, Southern red oak, Spanish oak, bottomland red oak, three-lobed red oak – (leaf)
- Quercus marilandica, blackjack oak – (root, leaf, stem)
- Quercus rubra, red oak, northern red oak – (root, leaf, stem)
- Quercus stellata, post oak,iron oak – (root, leaf, stem)
- Robinia pseudoacacia, black locust – (bark, roots)
- Sassafras albidum, sassafras – (root, leaf, stem)
- Ulmus americana, American elm – (root, leaf, stem)
Eucalyptus Allelopathy Research Abstracts
Here are some of the abstracts (summaries) in part, or whole, from the cited research, detailing the findings, explaining how and why eucalyptus trees are allelopathic:
Allelopathic effects of Eucalyptus on native and introduced tree species
“In this study, we conducted two field trials and a greenhouse trial to assess the influence of soil allelopathy, allelochemical volatilization, and foliage litter decomposition on seed germination and seedling growth of three native (Acmena acuminatissima, Cryptocarya concinna and Pterospermum lanceaefolium) and one introduced (Albizia lebbeck) tree species in a Eucalyptus urophylla and Pinus elliottii plantation.”… “In the allelopathy experiment, the root length of the three native species was significantly inhibited in the E. urophylla plantation compared with that in the P. elliottii plantation. In the volatilization experiments, the seedling survival rate of the three native species was greater in the E. urophylla plantation, but significant differences were found for A. acuminatissima and C. concinna. Root length and dry weight of P. lanceaefolium increased significantly in the E. urophylla plantation, in the foliage litter decomposition experiments.”… “We concluded that allelopathy in the E. urophylla plantation was selective, which inhibited the growth of the native tree species but had no significant influence on the introduced A. lebbeck species. Allelopathy from volatilization and foliage litter decomposition contributed little to the inhibitory effects.”
Allelopathic effects of volatile organic compounds from Eucalyptus grandis rhizosphere soil on Eisenia fetida assessed using avoidance bioassays, enzyme activity, and comet assays
“Allelopathy has been identified as an underlying mechanism of detrimental environmental impacts within commercial plantations. Eucalyptus spp. are known to generate huge amounts of volatile organic compounds (VOCs) that can function as phytotoxins and thus inhibit other plants. In the present study, biochemical markers, including activities of acetylcholinesterase (AChE) and oxidative stress enzymes, such as superoxide dismutase (SOD) and glutathione S-transferase (GST), were assayed to assess changes in Eisenia fetida at the physiological level induced by different doses of VOCs as part of an acute toxicity test over 7 and 14-day exposures. In addition, the toxicities of VOCs were investigated using a soil avoidance test and comet assay. The results revealed that E. fetida exhibited significant avoidance behavior towards the highest concentrations of undecane, decane, 2,4-dimethyl heptane, and 2,2,4,6,6-pentametyl heptane.”…
Editor’s note – Eisenia fetida are earthworms, this is a study of how allelochemicals in certain Eucalyptus species affect them.
Allelopathic Influence of Eucalyptus (Eucalyptus camadulensis L.) on Germination and Seedling Growth of Crops
“Experiments were conducted to evaluate allelopathic influence of eucalyptus on the germination and seedling growth of cotton, sunflower, sorghum, mungbean and mothbean under the laboratory conditions. One kg of fresh leaves of eucalyptus was soaked in ten liter of water for 72 hours. This extract was used in seeds soaking prior to sowing in the pure sand culture. The same extract was applied to the germinated seeds in comparison with simple water application to the control treatments. The results revealed that application of aqueous extract of fresh leaves to seeds of all crops reduced the germination and suppressed the growth of roots and shoots of seedlings of crops belonging to different families.”
Allelopathic effects of four eucalyptus species on redgram ( Cajanus cajan L.)
“Investigations to identify the allelopathic compounds in the leachates of bark, fresh leaves and leaf litter of Eucalyptus tereticornis , E. camaldulensis , E. polycarpa and E. microtheca using paper and as chromatography showed the presence of coumaric, gallic, gentisic, hydroxybenzoic, syringic and vanillic acids and catechol. The influence of identified phenolics as well as leachates on the germination, seedling length, dry matter production, vigour index and nitrogenase activity of redgram (CO.5) was studied. Germination was inhibited by each individual compounds tested while vigour index was significantly affected by catechol, ferulic, gallic and syringic acids, compared to control. Bioassay with leachates revealed significant reduction in germination over control in all the cased, 7 days after sowing. Dry matter production was affected by E. camaldulensis and E. microtheca. Meanwhile, vigour index was affected by E. camaldulensis , E. polycarpa and E. microtheca . Seedling length was affected in all the cases except E. camaldulensis , 37 days after sowing. Simultaneously, reduction in vigour index and nitrogenase activity was also noted in all the cases, compared to control.”
Editor’s note – this study identifies the various allelopathic chemicals in four eucalyptus species.
Potential allelopathic effect of Eucalyptus grandis across a range of plantation ages
“Allelopathy of the eucalypt has been considered as an important mechanism for the biodiversity reduction in the eucalypt plantation. To understand the allelopathic potential of the eucalypt (Eucalyptus grandis) roots and rhizosphere soil along a chronosequence (2, 4, 6, 8, 10 years), the germination and growth characteristics of three plant species (Raphanus sativus, Phaseolus aureus, and Lolium perenne) growing nearby or beneath the eucalypt plantations were measured. The results showed that aqueous extract of E. grandis root suppressed the germination and early seedling growth of the target plants. The younger E. grandis exhibited a comparatively stronger allelopathic potential. The highest dose root extracts from 4 years old E. grandis showed the strongest inhibitory effects on the germination rates of the target species, the inhibitory rates were about 48, 51.2, and 56.56% for R. sativus, P. aureus, and L. perenne, respectively. However, present biotests of rhizosphere soils from 6, 8, and 10-year-old plantations exhibited a remarkable stimulative effect on L. perenne, which indicated that the soil might neutralize or dilute allelopathic agents with the increase of plantation age. In addition, according to GC–MS analysis, more allelopathic potential compounds were found in the rhizosphere soil and roots of younger E. grandis plantation. Moreover, more allelochemicals were obtained from soil than from roots. The allelopathic compounds in roots and rhizosphere soil may play important roles in allelopathy of E. grandis plantation. More attention should be paid to the younger E. grandis plantations for the relative higher allelopathic effects.”
Review: Agroforestry Species Have Negative Allelopathic Effect on Food and Fodder Crops
“The aim of the review was to assess the effects of different allelochemicals contained in different tree species used for food and fodder. Information was gathered from different papers using secondary data and an essay was prepared covering different tree species used in Agroforestry. Many tree species were found to have negative allelopathic effects on food crops such as maize, covo and wheat. Eucalyptus species were found to contain allelochemicals which have negative effects on growth of food crops.”
Editor’s note – this review of previous studies identifies the various paths of action of allelopathic chemicals in eucalyptus species, how they affect food crops. It lists the following mechanisms:
- Allelopathy chemicals in Eucalyptus camaldulensis could effect on germination and early seedling growth of Vicia villosa, Onobrychis sativa, Festuca arundinacea and Trifolium rigidom (Saberi et al., 2013).
- Trifolium rigidom and Onobrychis sativa are more sensitive to allelopathic chemicals from Eucalyptus camaldulensis than all other species. These allelopathic chemicals decreases germination because they inhibit hormones, especially gibberellins. Allelopathy from Eucalyptus camaldulensis causers changes in enzyme activity which restrict the conversion of nutritive compounds during germination of seeds (Lu and Yanar, 2004; Saberi et al., 2013).
- Allelochemicals in Eucalyptus camaldulensis causes delay or stimulate in using nutritive matter that can cause lack of production of respirable vesicles and finally result in lack of adenosine triphosphate (ATP) in seeds exposed to allelochemical compounds (Saberi et al., 2013; Farajo et al., 2012).
- Allelochemical compounds restrict metabolic energy by disorder in respiration rate consequently decreased the early growth of seedling (Lu and Yanar, 2004; Rice, 1984).
- Growth of radicle and shoots of food crops such as Zea mays and Glycine max is decreased when these crops are grown near or within Eucalyptus camaldulensis and other Eucalyptus species because these food crops are exposed to allelochemical compounds (Bogatek et al., 2005).
- Allelochemicals in Eucalyptus grandis, Eucalyptus tereticornis and Eucalyptus camaldulensis inhibit cell division and elongation of cells. They also decrease stimulate effect of gibberellins and acetic acid in plants (Lu and Yanar, 2004; Bogatek et al., 2005; Saberi et al., 2013).
- Allelopathic compounds could disorder in vital activities of food and fodder crops by other mechanisms such as restriction of nutritive absorption, disorder in respiration, oxidative phosphorylation and photosynthesis ( Saberi et al., 2013).
- Foliar and leaf litter leachates of Eucalyptus species are more toxic than bark leachates to some food crops. (Ferguson et al., 2013).
- Allelopathic chemicals in Eucalyptus tereticornis inhibit growth and yield in cowpeas if grown together or near each other (Rizvi et al., 1999).
- Eucalyptus globulus produces volatile emanations that inhibit root growth of Cucumis species seedlings and the growth of hypocotyls but not the roots of Eucalyptus globulus seedlings (Bhatt and Chauhan, 2000).
- Leaf leachates from Eucalyptus globulus inhibits seed germination of Glaucium flavus (Singh and Bawa, 1982).
Plenty of Australian native plants have evolved to be able to grow under or alongside eucalyptus trees, so it’s not the end of the world! Similarly, there are many other trees whose leaves can be used for mulch, and a walk through a public park in autumn with a large bag will yield a plentiful supply of leaves that are safe to compost. Just remember to avoid some of the other trees on the allelopathy list above.
When lots of autumn leaves from deciduous trees need to be composted, it’s best to pile them on their own in a compost bay or a frame made of posts and chicken wire, and leave them there for a year. Any harmful compounds can leach out that way, and the leaves will partially break down, making them much easier to compost.
- Chaojun Chu, P.E. Mortimer, Hecong Wang, Yongfan Wang, Xubing Liu, Shixiao Yu, Allelopathic effects of Eucalyptus on native and introduced tree species, Forest Ecology and Management, Volume 323, 2014, Pages 79-84, ISSN 0378-1127, https://doi.org/10.1016/j.foreco.2014.03.004.
- Potential Allelopathy In Different Tree Species by Dr. Kim D. Coder, Daniel B. Warnell School of Forest Resources, University of Georgia 4/99
- Zhiqun T, Jian Z, Junli Y, Chunzi W, Danju Z. Allelopathic effects of volatile organic compounds from Eucalyptus grandis rhizosphere soil on Eisenia fetida assessed using avoidance bioassays, enzyme activity, and comet assays. Chemosphere. 2017 Apr;173:307-317. doi: 10.1016/j.chemosphere.2017.01.004. Epub 2017 Jan 11. PMID: 28113065.
- Muhammad Ayyaz Khan, Ishtiaq Taj, Ijaz Ahmad and Muhammad Safdar Baloch, 1999. Allelopathic Influence of Eucalyptus (Eucalyptus camadulensis L.) on Germination and Seedling Growth of Crops. Pakistan Journal of Biological Sciences, 2: 737-738.
- Sasikumar, K., Vijayalakshmi, C., & Parthiban, K. (2006). Allelopathic effects of four eucalyptus species on redgram ( Cajanus cajan L.). Journal of Tropical Agriculture, 39, 134-138.
- Zhang, Dj., Zhang, J., Yang, Wq. et al. Potential allelopathic effect of Eucalyptus grandis across a range of plantation ages. Ecol Res 25, 13–23 (2010). https://doi.org/10.1007/s11284-009-0627-0
- Kugedera, A. T & Kokerai, L. K, Department of Agriculture Management, Faculty of Agriculture, Zimbabwe Open University
Department of Livestock, Wildlife and Fisheries, Great Zimbabwe University, Masvingo, Zimbabwe, Published on International Journal of Agriculture & Agribusiness, ISSN: 2391-3991, Volume 2, Issue 2, page 78 – 83, Publication Date: March 11, 2019