Walnut tree seedlings can be seen growing in weeded areas where the walnuts have fallen or been scattered by squirrels and then left to freeze and thaw over the winter months. They are the easiest way of growing your own walnut trees, since the tree is quite a vigorous grower once the germination process has started. If you happen to find a walnut tree seedling, there are just a few things to do to keep it growing strong for years to come.
Dig up the seedling if it is one you have found growing outside, maybe in a wooded area near other walnut trees. Mark a circle around the trunk at least 6 inches out, and then dig straight down for about 8 inches and then dig in at a 45-degree angle for another 6-8 inches, depending on the size of the seedling. Dig under the roots as much as possible, so if your seedling is only 6 inches tall, the roots will probably only be about 6 inches down.
Prepare a new hole for the seedling where it will get a good amount of sun, and will not be crowded out by other trees. Plan on giving it a 50-foot area from other plantings. Walnut trees do not like to compete for the sunlight. Make the hole at least 6 inches wider and deeper than the root ball you just dug up, or purchased. It should be well-draining soil without too much rock.
Add some well-rotted compost to the hole and work it in the the soil at the bottom. Set your tree in place and hold it straight up and down while you continue to fill in the hole around it. Tamp the soil down hard with the heel of your shoe so that there are no air pockets that might dry out the roots.
Water the tree well for the first week and then at least once a week for the first growing season if you are working in the summer. If you transplant the seedling in the later fall, only water the tree for the first couple of weeks before the cold weather comes. Walnut trees need about 25 inches of water a year or about an inch every two weeks.
Mulch around the tree with a good six inch layer thick and out about 12 inches from the trunk of the seedling. Pull the mulch back from the trunk so air can circulate around the bark. As the tree gets bigger you can extend the mulch line farther.