Elisabeth C. Miller Library

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Search Results for: Juglans | Search the catalog for: Juglans

Plant Answer Line Question

Keywords: Zelkova, Xylosma, Thujopsis, Taxodium, Tamarix, Poa, Pennisetum, Casuarina, Baccharis, Festuca, Juglans, Alnus, Palms, Allergies

What could cause sinus allergy symptoms every December?


According to Thomas Ogren's book, Allergy Free Gardening , the genera Alnus, Baccharis, Casuarina, Festuca, Pennisetum, Juglans, Poa, Tamarix, Taxodium, Thujopsis, Xylosma, Zelkova, and palm trees all produce pollen during December.

(Source: Ogren, T.L., Allergy-Free Gardening: The revolutionary guide to healthy landscaping , 2000, pp.262-265)

Also check out Allergy Free Gardening website. There are a number of articles on low-allergy gardening listed.

Date 2019-01-26
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Plant Answer Line Question

Keywords: Grafting, Woody plant propagation, Juglans

Is it possible to graft a walnut scion onto a maple tree?


The book, Plant Propagation edited by Alan Toogood (American Horticultural Society/DK Publishing 1999) says that Juglans regia and Juglans nigra, grown for their edible nuts, are usually whip-and-tongue grafted. You would "use a slightly narrower scion than the stock so the thinner scion bark will align with the stock's cambium more easily."

I was not able to find any information on grafting a walnut scion onto a maple, but here is an article (pdf) on propagating Eastern black walnut trees by William Reid, which has detailed information.

This publication from the University of Georgia College of Agricultural & Environmental Sciences, entitled "Propagating deciduous fruit plants common to Georgia" (1999) indicates that whip grafting or ring budding will work best for walnuts.

Date 2018-10-25
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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
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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
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Plant Answer Line Question

Keywords: Juglans, Edible plants, Poisonous plants

Is it safe to eat pickles made from unripe walnuts (including hulls)? Are some types of unripe walnuts safe to eat and others not safe? I am not sure what kind of walnut is in my garden.

I am also a bit worried by articles I found online which say that juglone from walnuts can cause cell damage. Maybe I should skip this culinary adventure?


Pickled walnuts (from English walnut, Juglans regia) are a traditional British delicacy. The Royal Horticultural Society even mentions them. Alys Fowler's book, The Thrifty Forager (2011), says the walnuts for pickling must be picked in early summer before they harden. Traditionally, walnuts for pickling were harvested June 15, St. John's feast day. The famous 16th century herbalist John Gerard said, "the green and tender Nuts boyled in Sugar eaten as a Suckad, are a most pleasant and delectable meat, comfort the stomacke, and expell poison."

All walnut species have edible properties, though black walnut may be more bitter than English walnut. Edible East Bay published an article by Kristen Rasmussen in summer 2015 on pickling green walnuts from a native Californian species of walnut, Juglans californica.

Anyone who is sensitive to walnuts probably should avoid the pickled ones, too. Like many plants, walnuts have both edible uses and toxic properties. If you do not consume large quantities of walnuts (pickled or otherwise), I do not think there should be dire medical consequences. Toxic Plants of North America, 2nd ed., 2013 (Burrows and Tyrl) has a section on walnut (Juglans). The main toxicity concern discussed is that to horses, and in their case, it is mainly due to the use of walnut wood shavings in horse stalls.

A word about finding random articles on the internet: Context matters, and the citation you found about cell damage is in the context of using juglone (administered in a medical research facility) to kill cancer cells. It is not the context of everyday consumption of walnuts. Reliable sources are hard to find via the internet, and I would view with skepticism any site that is primarily commercial and does not cite trustworthy sources.

I could not find any references to the effects of pickling on the chemical composition of walnuts. Certainly, pickling (like any form of food preparation) will have some effects on nutrients. But since pickled walnuts are likely an occasional snack and not a staple upon which one's diet is founded, there is no cause for concern.

Date 2019-05-15
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Garden Tip

Keywords: Juglans, Gleditsia, Prunus, Pruning, Cornus, Birch

A common question gardeners have is when to prune. "When the shears are sharp!" is the often-heard answer. In reality there are a few timing guidelines that do matter.

First of all, certain trees are known to "bleed" when pruned while the sap is rising in late winter and early spring. Maples, dogwoods, birch, elm, walnut and honey locust are the most common.
Bleeding usually won't hurt the tree, but the pruning cuts are slower to heal which may leave susceptible trees vulnerable to infection. These trees should be pruned right after leaves fall off in autumn.

Cherry trees are at risk from the destructive cherry bark tortrix. The tortrix is attracted to fresh pruning cuts, so cherry trees should not be pruned between May and August when the tortrix is active.

Spring flowering shrubs should be pruned immediately after flowering so that the new growth has time to form next year's flower buds. Summer flowering shrubs may be pruned in winter because flowers are formed on this season's growth.

Pruning resources online:

Date: 2007-05-17
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