Q: I subscribe to the heavy mulching philosophy of gardening, for water retention, weed reduction, and feeding the soil. I’ve heard that some plants (cedar, black walnut, etc.) contain natural herbicidal agents, and may be useful/harmful for different applications.
A: Cedar has tanins but when used as a mulch should not cause any trouble with established plants.
Walnut on the other hand is one of the allelopathic trees. Allelopathy is chemical competition between plants with one plant claiming the space and causing growth problems with neighboring plants. Juglone, a chemical in walnuts, inhibits the growth of many other plants that try to grow nearby. Some plants are resistant to Juglone and will grow fine alongside walnuts. All parts of the walnut tree contain Juglone. Wood, leaves or branches from walnuts should not be used as mulch or compost.
No wood should be incorporated into the soil as an amendment.