Black walnut trees produce juglone, which inhibits growth of other plants.

When I googled "magnolia juglone" & scanned the results, it looks like saucer magnolia is the only one that has appreciable amounts.

You might check it more thoroughly before you use magnolia leaves around juglone-sensitive plants (not all plants are susceptible to juglone).

Thanks very much. I found a site that lists daylilies as resistant to juglone, in reference to black walnut trees. Since that is my main activity lately, I put them all on a bed as winter mulch. I didn't shred them, so I suppose they will blow around a bit.

Here is a link that might be useful: Green Tips --Black Walnut Toxicity

Sylvia...juglone is produced only by plants within the Juglandacea family. Some plants, like the Saucer Magnolia, are sensitive to juglone, while others are not (thank goodness). But there are many, many different kinds of allelopathic chemicals besides juglone.

Southern Magnolias, for example, have long been suspected of having allelopathic properties and recent studies have proven this. More research needs to be done before we know the extent of kinds of plants that may be affected by Magnolia, however.

3:30 ALLELOPATHY IN SOUTHERN MAGNOLIA (MAGNOLIA GRANDIFLORA), Karl Knights*, Lorena Castillo, Rochelle Tiedemann, Ruth Williams, and Mark Dauis, North Georgia College & State University, Dahlonega, GA, 30597. We investigated possible allelopathic effects of Magnolia grandiflora on rapid radishes, Raphanus satiuus, a small herbaceous annual with a short life cycle. Raphanus sativus seeds were germinated and grown in growth chambers (24°C, 24 h light) in small pots containing potting soil mixed with 0% (control), 2.5%, or 5% ground dried M. grandiflora leaves. Plants were watered each 2 d, and harvested after 25 d. Shoots were separated from roots, dried in a drying oven for 4 d at 65°C, then weighed. Shoot biomass of plants in the control group was significantly higher compared to plants exposed to ground M. grandiflora leaves (one-way ANOVA: F2 32 = 4.79, P = 0.015), but we found no significant `difference in shoot biomass of plants grown in the 2.5% and 5% potting soil-ground M. grandiflora leaf mixture (Tukey-Kramer post-hoc test). A related congener, M. uirginia, has recently been shown to be allelopathic, but our data provide the first evidence for allelopathy in southern magnolia. Magnolia grandiflora leaves are slow to decompose and accumulate to a depth of several centimeters in a large area around each tree. Our results suggest that studies on the alleopathic effects of southern magnolia on population densities of local herbaceous floras are warranted.

Aha. Well, the leaves are out there now, and I'll leave them on for tonight's freeze. I didn't shred them, so they'll be relatively easy to remove to my path where I'd prefer nothing grow anyway. I have plenty of old straw to take their place. :)

I don't quite have words to express how dopey I felt this morning as I replaced the magnolia leaves in one of the same bags I got them in. :D I put them out for my garbage man. :( I put the others inside a little fenced enclosure around a pool pump, where I'd considered placing plastic and gravel. They do look better than gravel.

You could still compost them, just don't use them for mulch.