Elisabeth C. Miller Library

Gardening Answers Knowledgebase

Search Results for: Allelopathy | Search the catalog for: Allelopathy

Plant Answer Line Question

Keywords: Juglans, Compost, Allelopathy

Will black walnut leaves cause compost to be allelopathic? Should they be kept out of compost? Or is this folklore? The specific compost is made with chicken manure (fresh), grass clippings and walnut leaves. Are there plants that tolerate the toxin in black walnut?


It seems that the main source of toxicity is the roots of the walnut tree, rather than the leaves or shells. However, there are still those who believe that there is enough juglone in the leaves that they should be fully composted before use in the garden. Below are excerpts from information published online in various university extension websites, by various authors, and now unavailable:

"This toxic affect on surrounding plants appears to be related to root contact, as walnut hulls and leaves used as mulch have not shown toxic effects on plant growth. [Warning- Frank Robinson disagrees.] Because Walnut roots do not occupy the surface layers in most soil, many shallow rooted plants growing under walnut trees don't come in contact with the roots and are not affected by them." [Michigan State University]

"You've probably always heard that you should never add black walnut sawdust [or wood chips] to the compost pile because the juglone will kill everything that grows in the compost. Abraham says that's not necessarily true; that juglone is not found in walnut saw dust or wood chips. Nor do dead walnut trees exude juglone. Juglone is harmless to humans so you can go right ahead and safely eat fruit and vegetables grown near walnuts."[Katy Abraham]

"Robinson doesn't agree on the use of walnut residue in composting. He has this to say about black walnut saw dust, husks and leaves affecting plants. 'Tomatoes growing in clean soil in pots were severely stunted when leaves and nuts fell into the pots while we were on vacation. I know what juglone can do. I have seen a 15-year-old rhododendron killed a few weeks after its owner mulched it with black-walnut husks, and roses injured by an application of compost containing black-walnut sawdust.'" [Robinson]

"The juglone toxin occurs in the leaves, bark, and wood of the walnut but these contain lower concentrations than the roots. Juglone is poorly soluble in water and does not move very far in the soil. Walnut leaves can be composted because the toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks. In soil, breakdown may take up to two months. Black walnut leaves may be composted separately, and the finished compost tested for toxicity by planting tomato seedlings in it. Sawdust mulch, fresh sawdust or chips from street trees prunings are not suggested for plants sensitive to juglone, such as blueberry. However, composting of bark for a minimum of six months provides a safe mulch even for plants sensitive to juglone." [Ohio State University]

"To be on the safe side, composted material containing juglone should be allowed to breakdown over a period of time before use. This composted material can be used with plants that are not susceptible to juglone damage. If it is important to use it for general composting purposes, testing it first with a few tomato plants for a few weeks should reveal its level of toxicity." [Abraham]

This may also be of interest: The Walnut Tree: Allelopathic Effects and Tolerant Plants from the Virginia Cooperative Extension.

Frank Robinson's article "Under the Black Walnut Tree," Horticulture magazine, October 1986, pp. 30-33 concludes that many plants are indeed able to tolerate juglone's toxicity. Some of the juglone-tolerant plants listed in the article and in other sources are included on Morton Arboretum website.

Date 2018-09-12
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Prunus, Allelopathy

I have a laurel hedge that I am taking down. Can I use the chips as mulch or will the mulch kill things as I've heard that laurel is poisonous?


Is your laurel an English laurel? If so, it is the plant Prunus laurocerasus which does have toxic properties (cyanogenic glycoside and amygdalin, according to the information here from North Carolina State University Extension) but I think that the toxicity mainly affects people and other creatures if they eat its leaves, twigs or seeds. My guess is that the wood chips should be safe to use, but what you could do is use the mulch on a path or other area where you do not want plants to grow and it will not touch anything you will eat.

University of Georgia School of Forest Resources has a list of potentially allelopathic tree species and Prunus laurocerasus is not among them.

Date 2018-10-25
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Juglans, Allelopathy

I would like a plant list of plants that are resistant to the juglone toxin.


Juglone, a toxin produced by walnuts (Juglans spp.), can be a problem for many plants. Luckily, some don't mind it.

Here is a list of juglone-tolerant and juglone-sensitive plants, from University of Wisconsin Urban Horticulture.

Virginia Cooperative Extension also has a list.

Ontario's Ministry of Agriculture has similar information.

Date 2018-12-19
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Wood chips, Mulching, Allelopathy

We've taken down some big cedars and chipped the branches. I've heard that cedar mulch can damage plants. What is your take on this? I already put it around some choice pines and some viburnums, but I could move it if need be.


Washington State University Extension horticulturist Linda Chalker-Scott has written about this very issue, and her scientific research on the subject says that cedar (both Thuja and true Cedrus) wood chips are not allelopathic (toxic) to plant tissue. Here is the article.
This author has further information on the general benefits of wood chips as mulch. Here is a newer Washington State University factsheet on the subject.

Date 2019-01-05
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Eucalyptus, Allelopathy

I have a young Eucalyptus pulverulenta tree. I'm concerned that it might be emitting toxic chemicals into the surrounding soil that will harm other plants. Is this indeed a cause for concern?


My instinct is that your Eucalpytus tree will probably not be toxic to most other plants. Eucalyptus is often seen growing in the midst of garden beds here in Seattle, to no ill effect. I have a tree growing in a perennial bed, and have noticed no problems. However, the leaves do contain essential oils (phenolics and terpenoids). This does make the tree more flammable, if that is a concern. Eucalyptus leaf litter (to a greater degree than exudates from the tree's roots) can inhibit specific food crops, like wheat. This is what is meant in this University of Florida Extension article, which refers to "selective activity of tree allelochemicals on crops and other plants."

To show how selective this chemical property can be, see the following documentation of a University of California, Davis experiment using composted Eucalyptus as mulch.
"Mulch products made from Eucalyptus are an asset to maintenance of fine landscapes. Composted Eucalyptus makes an excellent seed cover and will aid in germination and establishment of seedlings such as California Poppy. Fresh Eucalyptus is an excellent mulch for woody landscape plant materials and palms; the main effects of its use are weed control and water conservation."

Unless you are growing your tree in the middle of a field of wheat or among other grassy plants, your landscape is probably in no danger of inhibited growth from the Eucalyptus.

You may find the following veterinary perspective (from Cornell University) on the toxicity of Eucalyptus of peripheral interest.

Date 2018-10-25
Link to this record only (permalink)

Didn't find an answer to your question? Ask us directly!

Browse keywords

Search Again: