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)

Season Winter
Date 2007-04-02
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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 walnut trees from the University of Missouri Center for Agroforestry (2001), which has excellent detailed information and illustrations.

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

Season All Season
Date 2007-04-03
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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 a Cornell University Extension document on the subject:

"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- Robertson 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. [Mich]

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. Abrahams 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. [Abraham]

Robertson 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]

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."

Here are a few more links:

The Walnut Tree: Allelopathic Effects and Tolerant Plants from the Virginia Cooperative Extension.

Black Walnut Toxicity to Plants, Humans and Horses from Ohio State University Extension.

Season All Season
Date 2008-03-27
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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 plants, from University of Wisconsin Urban Horticulture.

Virginia Cooperative Extension also has a list.

Ontario's Ministry of Agriculture has similar information.

Season All Season
Date 2008-06-04
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