Most walnut growers assume that if they plant a healthy tree in good soil, the nuts will just appear by autumn. When it’s just planted, most walnut growers think a healthy tree in good soil should mean nuts come fall. It sounds like a reasonable assumption, until you consider what is happening biologically in spring, up in the canopy.
Unless you time things just right, walnuts don’t pollinate themselves well at all. You’ve got open blooms and no pollen from a neighbor’s, and then where are you? Frustrated, with empty hulls. This chart shows it visually: you need two trees to work together to get a decent crop.
Why Walnuts Need Two Trees to Grow Nuts
But here’s the thing: Walnut trees does not provide all they can produce by themselves. All species described as self-fertile require some sort of nearby cross-pollinator anyway. And many, such as English (commercially grown) walnuts, are fussy about whom they get to play with. Both an early-blooming and a late-bloomer must be present; each deposits pollen first and then receives it in return from the other. That’s what dichogamy means.
The flowers’ timing doesn’t match up, which is just how nature designed things to promote genetic diversity. But you’re the one who has to close the gap on purpose by planting. The trick: You have to match Type I trees with Type II trees (see infographic above).
Why? Because on Type I species, catkins gets released before the females’ pistils are primed to accept pollen. On Type II trees, it’s just the opposite: The pistils open first and wait for some wind to waft pollen over from a nearby tree. If all your trees are of one type and planted alongside each other, all those trees will drop pollen simultaneously, then close up shop long before any partners show up. No dice. Put them together, however, and there’s a window of overlap, when that pollen is in the air right as the stigmas go sticky and ready.
In the orchard, timing is also everything. Depending on your winter chill amounts and local climate, bloom time typically occurs from April to May. One year, the seasons might be off by a hair, and some of those looser pairings may completely miss each other. Why are the exact pairings so important? Certain combinations has aligned windows that work together reliably each season. Pairing two kinds of things isn’t enough, you want partners whose biological clocks line up under all sorts of weather conditions. Chandler works beautifully as a partner for Franquette, for instance, since their windows lines up reliably each season.
Placement also matters. Bees aren’t necessary; walnuts are pollinated by wind. And it’s no good tossing a pollinator tree out into the far back corner of yard and hoping for the best. Pollen moves well only within a given radius (which I’ll describe below), and more trees means greater likelihood of enough grains landing on the right one. Ideally, keep your partner trees within about sixty feet of each other. In a small home orchard where space is limited, try to achieve a spacing between them of maybe thirty or even twenty feet. Usually closer is better then farther.
There are other factors in caring for your trees besides what you choose to plant and the spacing between them. The physical environment surrounding the trees matter. The canopy should be kept relatively open so that wind can reach the inner branches to fertilize them with pollen. This also allows air to circulate around them. Water them while they’re blooming. This keeps the pistils moist longer, which gives them more time to trap passing pollen grains. No sprays during those precious two weeks when everything’s tender and the plant is trying to reproduce. Spraying residues harms reproductive structures.
The heavy walnut is no coincidence. Biology, wind, and soil work together in this well-planned team-up. If you have the right varieties and allow the breeze to blow, it will pay out in a large September crop of green husks ready to go. You get heavier nuts, plumper kernels, and a rewarding handful for your labor. It would of been a great harvest if everything goes right.