Black walnut is near and dear to my heart, and I love to see folks have success in planting it. Here are the three biggest pieces of advice I have to folks planting black walnut.
1) Plant black walnut only in rich, well aerated bottom land soil. If you have hilly ground plant black walnut in the bottoms and oaks up the slopes. Otherwise only the walnuts in the bottoms will do well while those in the shallower soil of the slopes and tops will struggle mightily.
2) If you can only use tree tubes a certain percentage of your trees, don’t waste tubes on the “runty” or smallest seedlings. Use the tubes on the seedlings with the thickest trunks and best root systems. These trees have the best chance of becoming your champion trees, so it makes sense to invest in protecting them.
3) Weed control, weed control, weed control – and that includes grass. Black walnut does not compete well with weeds, especially grass. Later in life the walnuts will shade out the grass and they exude a chemical from their roots called juglone that inhibits the growth of weeds and other trees. But in the early years grass will dramatically stunt the growth of the walnuts. By weed control I mean killing the grass, not just mowing it. Mowing helps reduce competition for light, but does nothing to reduce competition in the root zone for nutrients and moisture – in fact it invigorates the grass and makes it a fiercer competitor for these resources.
Years ago an elderly forester from Ontario Canada gave a presentation I will never forget. He planted an acre of black walnut seedlings. On one half of the acre he mowed the grass weekly. On the other half he sprayed herbicide around the seedlings to kill the grass. After 6 years the trees in the mowed area were less than 8ft tall, while those in the sprayed area were well over 20ft tall. He couldn’t publish the study because it was not laid out in a “randomized block” design so the results were not “statistically significant,” but he whacked the screen with his pointer and said, “but a blind man with a cane can see the difference between these two treatments!” Perhaps not the most politically correct way to make his point, but obviously effective; I still remember his presentation vividly after nearly 25 years, and there aren’t too many technical presentations I can say that about!