Walnut trees are planted for several good reasons such as their appearance, shade and of course, their fruit. But there are many factors to consider when you incorporate a walnut tree into your landscape; first they can be considered a very messy plant dropping their fruit all over the yard which is too large and too hard to mow over. With the fruit lying around you can expect to see many squirrels running around digging and planting the nuts everywhere in your yard, and they will grow. And lastly, walnut trees produce a substance known as juglone that will have a bad effect on many garden plants such as tomatoes, potatoes, blackberries and blueberries, azalea, mums, peonies, rhubarb, apples, hydrangeas, asparagus, lilies and rhododendron.
These plants and many others can be injured or killed in just one to two months after you plant them. Your tomatoes, etc. can be affected from a distance of 50 to 60 feet, even as far as 80 feet from the walnut tree.
The roots of the walnut tree are the most powerful killer so the closer you plant affected plants, the more likely they will die. Walnut leaves are also toxic however it is safe to put the leaves into a compost pile as long as the pile is "at work" for at least 4 weeks. The juglone in the leaves breaks down quickly; even the bark breaks down within 6 months.
Not all plants die around walnut trees. Japanese Maples, Redbud and Hemlock do well close to walnut trees as do Clematis, Euonymus, Forsythia, Rose of Sharon (hibiscus), Honeysuckle, Roses, Arborvitae and Viburnum. Annuals include marigolds, begonias, morning glories, pansies and zinnias. Vegetables include squash, melons, beans, carrots and corn and fruit trees such as cherries, plums, pear and peaches. The list of perennials is just too long to include.