Black walnut trees (Juglans nigra) grow quickly, shooting up multiple feet per year, and can be desirable shade trees. This fast growth rate can also be a problem if they're growing in undesirable areas. Additionally, black walnut trees emit a toxic sap that makes it impossible for some types of plants to grow near them, including garden vegetables like the tomato. You have several options at your disposal should you need to kill a black walnut tree.
Remove the black walnut tree manually by uprooting it. This method is ideal for small seedlings and saplings. Grasp the tree around its base with gloved hands and pull upward. Optionally, you can use a spade to loosen the soil first to make uprooting easier.
Spray the black walnut tree with a glyphosate-based herbicide or a commercially prepared mixture of triclopyr and picloram. This strategy is best for large collections of seedlings or saplings for which manual removal would be too labor intensive. Mist the glyphosate spray on all exposed foliage. The trees will die within 14 days.
Cut down the black walnut tree. This option works for one to two year old black walnut saplings that are 4 to 10 feet in height and for which spraying would be unfeasible. Use a standard ax and chop down the tree to a height of 6 to 12 inches.
Coat the remaining cut stump with a stump killer solution, available from most garden stores and nurseries. This kills the black walnut's underground roots and prevents the stump from regrowing.
Inject the black walnut tree with glyphosate, a systemic herbicide. This is best for killing mature black walnut trees. Use a ruler and a marker to mark a ring of dots around the black walnut tree at a height of 4 feet from the ground. Each dot should be 1.5 inches apart.
Use a standard tree injection kit--available from some nurseries and garden stores--to inject glyphosate into the tree at each of the marks you made in Step 5. Inject 1ml to 2ml at each injection site, which is typically one or two squeezes of the injection kit's gun. The tree will die within several weeks and can then be chopped down by a professional tree removal company.