I mentioned in the last post that we’re planting baby walnut trees with the hope of grafting soon. For the uninitiated, most commercial walnut trees in California are grafted trees; the rootstock is a different variety than the walnut producing tree grafted to it. There are many reasons for this, not the least of which is that walnut varieties with the best tasting, best producing walnuts don’t necessarily have the most disease-resistant roots. Grafting is a way to get the best of both worlds.
How do you pick which variety to graft? Institutions like UC Davis have been doing field experiments “crossing” different walnut varieties to come up with new better-producing varieties, like the Fordes which we are now planting. The following is my Dad’s account of his work in the evolving world of commercial walnut production. Thanks, Dad, for guest blogging!
For the past 10 years or so we have hosted field trials for new walnut varieties bred by UC Davis. Even though most of these new crosses will eventually be abandoned for one reason or another, I find it to be an interesting enterprise, satisfying my need for new challenges.
Two years ago we set out our most ambitious field trial to date, 12 experimental varieties on 4 acres. Some were older crosses that have proved promising enough as individual trees to merit planting in a larger block (30 to 60 trees) and some new crosses not yet field tested (3 to 4 trees).
As is common with the slow process of evaluating tree varieties, now after two years we have already decided to abandon a couple of varieties and replace them with seven new promising crosses .
The trees are still small enough to be readily grafted over to a new variety. To do this the trunk is severed at 4 or 5 feet and scions of the new variety are inserted under the bark in a process quite logically called bark grafting. The process itself is referred to in the trade as topworking trees.
Topworking Experimental Walnut Varieties
This is obviously a traumatic event for the young tree so the lower limbs (nurse limbs) are left, not to be removed for a couple of years until the growth above the graft is sufficient to nourish the tree and its roots.
Our foreman Jose Luis has developed a great deal of skill in grafting walnuts. Nonetheless topworking is a slow process and Jose Luis spent most of the morning on this project.
Secured by 
Thankyou for this! My son is doing agricultural development work in a mountainous region of Nepal. There are many ‘wild’ walnut trees growing there that are well adapted to the conditions but they do not produce very good walnuts, so he would like to try some grafting. In his remote part of the world, he doesn’t have high speed internet so asked me to research grafting techniques.
I have read that the scion should be cut when the tree is dormant and then refrigerated in a plastic bag until the grafting is done in Spring (at leaf bud). Do you follow that timing? Or….?
Thankyou again.
Bob
Scion wood is cut while it is dormant, and it is stored and refrigerated in a moist (not wet) media, commonly wood chips. Grafting in walnuts is done in the spring when the buds are beginning to push. We use a piece of scionwood that contains two buds. It is important to match the cambium of the scionwood to the cambium of the rootstock. Wrap the graft union with a layer or two of grafting tape and waterproof the tape and the end of the scionwood with tree seal.
California walnut varieties are cold sensitive and may not be adapted for Nepal. Our trees have withstood temperature dips as low as 10 degrees F.
I have a situation at our school garden in Sacramento. We have a 7 year old walnut tree planted by us when it was just a stick. It has been producing for 2 years now and the tree is huge. Anyway we just noticed that all the walnuts are on one branch and only that one branch that resembles an English ? Walnut’s leaves. The rest of the tree has black walnut leaves. What went wrong and is there anything we can do about it? We also like the shade the tree gives ( the students call the tree Wally). The joint where the producing branch grows out of is off to the side.
The tree you planted was an English walnut variety grafted to a black walnut rootstock, which is not an unusual graft. However, you apparently let a sucker/shoot grow from below the graft (a black walnut sucker/shoot), and since only one branch displays English characteristics it sounds as if that black walnut sucker now dominates the tree. If shade is what you want you can live with it as it is. If you want English walnuts you should begin pruning back the black walnut portion of the tree with goal of eventually removing it. Without seeing the tree I would hesitate to suggest you remove all the black walnut growth in one year, it might be too big a shock. If your pruning leaves the English trunk unshaded you should consider painting the exposed surface with interior white latex paint to protect it from sunburn.
My neighborhood has TONS of black walnut trees, frankly the black walnut is a weed here, growing every and anywhere. I’ve observed in the almost 2 years I’ve been here, a “volunteer” black walnut just starting out, my height a year later. What we have a problem with is, this used to be small family farms and a lot of walnut production. The area became a “bedroom” community for the greater San Jose area, and the new inhabitants don’t know diddly about walnut trees. The result is all the fine, producing, trees, being overtaken by their black walnut rootstock. I see, and harvest from, many that are about half’n'half, English and black. I’m tempted to start cutting off black walnut branches in a campaign to keep more English walnuts growing in my area. I collect the nuts for my own use, out of ditches and beside the road, I guess I’m just a slightly larger squirrel. I plan to learn about the grafting of walnuts, and may offer a service to my neighbors of grafting their black walnut trees with english walnut scions and instructing them in their care and maintenance.