Home & Garden Columns

Who Put the Walnuts in Walnut Creek?

By Ron Sullivan, Special to the Planet
Tuesday December 12, 2006

A tree student learns a set of categories: the 50-mph tree, the 30-mph tree, the stop-and-think tree. The distinctions here concern how fast-moving and far away you can be and still be able to identify a tree—how distinctive it is from a distance.  

A subset of 50-mph trees is the drive-by orchard. Citrus orchards are pretty obvious from miles away, if they’re in bloom and you’re downwind. Pear orchards are easy when they’re out of leaf, because their youngest twigs are so determinedly vertical.  

Walnut orchards aren’t hard if you can see the trunks. Even without their distinctive compound leaves, they’re marked by the funny-looking trunks, dark at the bottom and pale starting from a foot or three off the ground. The height of the change varies, but it’s usually uniform within each orchard, which gives it the odd effect of an orderly regiment of trees in knickers, or cavalry boots. 

What’s going on here is a marriage of convenience. The walnuts we find in the market most of the time are English (or Persian) walnuts, from trees of the species Juglans regia, with their familiar pale and relatively easy-to-open shells. That tree prospers here, but for one thing: it doesn’t like poorly drained clay soils.  

But we have our own native walnut, the California black walnut, Juglans californica. I myself like black walnuts better than English walnuts; blacks have a haunting perfume, a heady port-wine note in their flavor that I treasure. The problem with black walnuts as a commercial nut is that they’re really hard to crack. There are jokes about strewing them on the driveway and backing your car over them a few times.  

The native walnut, no surprise, likes the native soils just fine. So walnut ranchers (Don’t you love living here, where we have dairy ranches and walnut ranches and, up the hill, a Fish Ranch?) graft English walnut scions onto black walnut trunks, and everybody grows up to be a happy tree in kneesocks.  

The walnuts that Walnut Creek is named for were the native California species. Europeans found them growing around the sites of indigenous villages. Donald Culross Peattie cites “the oldest records” in limiting the oldest stands to “the valley of Walnut Creek, in Contra Costa County, the banks of the Sacramento River, particularly at Walnut Grove, and Wooden Valley east of Napa.”  

There’s a magnificent senior black walnut living next to a well-kept big Victorian in Rockridge. Walnut trees turn up in random backyards where squirrels have planted them; we have a ten-foot whip in the skinny space along our driveway, where the squirrels also bequeathed us a few native live oaks. The problem with a gift walnut is that they don’t play well with others, at least some others.  

Black walnut roots exude juglone, which inhibits growth in some plants—tomatoes (which you wouldn’t grow in a tree’s shade anyway) and their kin; azaleas and rhodedendrons; mountain laurel (and presumably its California cousin, Kalmiopsis); blackberries and blueberries.  

But hot or long composting breaks down the juglone compound into its nontoxic components, and there are so many plants that don’t mind juglone that it’s barely a limitation. Japanese maples supposedly get along just fine with black walnuts, and so do plants as diverse as daffodils, hibiscus, honeysuckle, and heuchera.  

There are lots of wild or feral black walnuts along the roads and streamways up in the Delta and north, and over the hills in Contra Costa. The only time you see bare spaces under them is when they’re along a cleared road, or when they’re rootstock that has overgrown the English walnut top—I see that often at the edge of an old orchard. Otherwise, natives and invaders elbow black walnuts like any other tree. 

Sometimes black walnuts are the roadside row of a working orchard. I’ve heard that tourists and passers-by can be obnoxious about helping themselves to orchard fruit; maybe the black walnuts are bait. (Professional nut rustlers typically strike after the nuts are picked and ready to ship.) You can tell them from the English walnuts by their bark—dark and furrowed all the way up the trunk—and their narrower and pointer leaflets.  

I’d thought that these and the black walnuts I see in the wild were insurance against the species’ extinction, but maybe not. Apparently most of the black walnuts we have are hybrids between our Californians—the two varieties, J. C. californica and J. C. hindsii used to be considered separate species—and Eastern black walnuts, brought here for more rootstock. The wild genepool is a resource against diseases like the butternut blight that’s whacking the southeastern Juglans cinerea. Hybrids are fine, if we keep them leashed.  

If you want to see orchard walnuts used as street trees, go to Vacaville, where there are memorials of the groves that were built over, not far from the Adobe exit from I-80. 


Photograph by Ron Sullivan  

Foreground, black walnut: dark bark, narrow leaflets. Behind, English walnuts grafted onto black walnut stock.