Walnut trees grown for their fruit are self-fertile. They are hardy, and positively like cold winters. According to variety, they may come into production only after 5 to 6 years. Remember that walnut leaves are toxic to other plants and trees, and so the tree should be positioned with care. Don’t put walnut leaves into your compost. Do not plant fruit trees close to walnut trees. Grafted walnut trees are a special item, much more expensive, but they need less room.