They'll take a really long time to break down so I think they'd be a better mulch than compost.

Although concentrations are higher in black walnuts, English walnuts also emit a substance called juglone that has growth suppressing properties termed allelopathy. All parts of the tree emit this substance to varying degrees, including the shells. I'd consider them a better material for pathways or other non-plant areas than using either as a regular mulch or adding to the compost.

Gardengal--considering how slowly the shells break down, would enough juglone be released for it to make a difference? I wasn't thinking about juglone when I wrote my response, because I thought it was only an issue with the black walnut, and also because I thought the breakdown of the shells would be so slow it wouldn't really affect anything.

I dunno - never used it myself. Commercially packaged walnut shell mulches are available, so I'd assume the effects are not that severe or far reaching. You do see references to avoiding including walnut shells or other walnut materials in compost, but I've got no walnuts around to test out these concepts :-)

From just a personal perspective, I prefer mulches that breakdown a bit faster and return OM and nutrients to the soil. I do use nut shells - hazelnuts, specifically - as pathway materials throughout my garden as they hold up well to weather and provide a dry, unmuddy walkway and provide a great source of entertainment from foraging squirrels. And they make a great crackling sound when you walk on them.

The Purdue University horticulture web site says "English walnuts do not have the high levels of juglone (the chemical that causes nearby plants to suffer) like the black walnut, unless the English walnut is grafted to a black walnut root stock."

Hmmmm....good thoughts everyone. Maybe I'll just put them on the path between my raised beds. Thanks!

For whatever it is worth, we have some black walnut trees that do not seem to be suppressing nearby plant growth much if at all.