Cook's Landscape Rejuvenation Consulting & Designs

BLACK WALNUT TOXICITY

In 1998, we were approached by a client who had a large mature Black Walnut tree removed from his front yard. Since the trees canopy occupied most of the yard, without the tree it appeared very desolate. The tree was removed because it appeared to affect other plants growing in the same general area. We were asked to design an attractive landscape in the area where the walnut tree had stood.

Although we were aware that Black Walnut trees release a toxicity into the surrounding soil, we did not have a comprehensive understanding of the problems this toxicity can cause. Our research has shown that it is a controversial subject and not everyone agrees. The following is the results of the information gathered from reliable sources*.

*What we discovered is that research regarding toxic effects on surrounding plants has been conducted on edible garden plants. Less has been done on ornamental plants, shrubs and trees.

Question #1

To what extent beyond the tree, does this toxicity ooze?

Answer

The toxic zone from a mature tree extends to the drip line of the branches, but may extend some distance beyond. However, the largest concentration of Juglone will occur within the dripline.

Question # 2

If the trunk of the tree has been removed, will that end the problem of juglone toxicity?

Answer

This answer requires two parts. The general answer is "no". If the roots have not been removed, they may continue to release toxicity into the soil for some years. There is a debate as to the length of time that the the soil remains after the complete removal of the trunk and roots. Some researchers state one year. Other sources indicate the problem may persist for several years. It is known that the roots must fully decompose before all danger of toxicity will disappear.

Question #3

Is this toxicity confined to the roots of the Black Walnut?

Answer

The largest concentration of Juglone is produced by the roots. However, some sources indicate that the leaves, hulls, nuts and bark from the tree contribute to the toxicity of the soil. Other sources indicate that is debatable but may depend on the natural resistance of some plants to the toxicity.

Three sources indicate that leaves should not be added to the compost, but should be sent with household garbage to the landfill. Another source states that leaves can be composted because toxins, when exposed to air, break down rapidly. Another source suggests composting them separately and then testing for toxicity by planting tomato seedlings in this compost. The seedlings will quickly die of the toxin ratio is high.

All sources tend to agree that the husks, nuts and bark should be removed and not allowed to decompose in the soil. Black Walnut sawdust or bark chips should not be applied to the garden or flowerbeds.

For a list of plants which are tolerant of the Juglone toxin and sources of information, click here.

SOME INTERESTING FACTS

1. The husks of the Black Walnut were once in great demand. They were made into high quality coal for use in the filters of gas masks during World War 1.

2. In the 1930's the walnut husks were ground into a type of "meal" which was then used as an insecticide.

3. Because the tree requires rich well drained loam slightly alkaline for good growth, early pioneers in search of land for a homestead would look for black walnut trees. This was considered fertile land.

4. Ancient Greeks referred to the Walnut Tree as the "Royal Tree." The Romans dedicated this tree to Jupiter, who was the most powerful of the gods, the name of this tree being a contraction of two Latin words 'Jovis' and 'Glans' meaning Jupiter's Acorn.

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- The Canadian Rose Society

Organizations

- Wilmot Horticultural Society

Local Interest

- Castle Kilbride

Other Garden Related Sites

- Garden Humour featuring David Hobson, Writer, Speaker, Gardener, Humorist.