I hope to make this theme - "Tree Walk Wednesday" - a regular post. I was inspired by Take a Tree Walk by Jane Kirkland and Tree Tuesday at Spacing Wire. Jane Kirkland defines a tree walk as "an adventure" that might begin in your backyard, schoolyard, or park. The goal is to take a walk and observe the trees along the way. The book is designed for children and provides guidance on how to take field notes about the trees one sees. On the second page of the book, Jane provides an example of a tree walk adventure; she notices a large oak during a neighborhood bike ride. For my first tree walk, I did not have to go on a new adventure. I chose a tree I see quite often - a large walnut in the Le Conte neighborhood. The reason I chose the tree is twofold: (1) I am in interested in edible street trees and (2) my landlady told me that squirrels bury the nuts on our lot and during the spring they search for the buried nuts which explains the many messy holes and exposed feeder roots in our planting beds.
The black walnut (genus: Juglans) produces an allelopathic compound called juglone which inhibits the growth of vegetation sensitive to the toxin. A list of insensitive and sensitive plant species is available through the Virginia Cooperative Extension. Other walnut species produce juglone, but to a lesser degree. A sparse bed of vegetation grows beneath the walnut tree I observed, but the planting area might have been "weeded" by the adjacent resident. I cannot positively identify the species of walnut I photographed, but I assume it is the Northern California black walnut. The leaves on this tree (the lowest branch is too high to reach and no leaves are on the ground) appear similar to those in my Pacific Tree Finder. Also, the species is a street tree in the Crescent Park neighborhood of Palo Alto. The UC Davis California Backyard Orchard project bulletin describe walnuts as "lovely shade trees."
There are two varieties of the native California walnut (Juglans californica): the Northern California black walnut (J. californica hindsii) and the Southern California black walnut (J. californica var. californica). The UC Davis orchard project notes that the most popularly grown varieties are English. The walnut entry on Wikipedia notes that the Northern California black walnut is used as a rootstock for English varieties (J. regia). According to Stuart and Sawyer in Trees and Shrubs of California, species in the northern range are hybrids of California black walnut and eastern black walnut (J. nigra).
If you can gather the walnuts before the squirrels do, you can prepare recipes published by the Hammon Company. For more botanical information on the Northern California black walnut and the English walnut visit the UC Davis orchard project website.