Growing 50 to 75 feet tall and nearly as wide at maturity, black walnut (Juglans nigra) is a shade tree that provides tasty fruits (filled with nutmeat) and a deep-colored lumber. A spacious landscape with a deep, fertile soil that is moist supports the best growth of this tree. Excessively dry soils stunts root and branch growth rates as well as causes premature leaf and fruit drop. Walnut trees are grown across U.S. Department of Agriculture hardiness zones 4 through 9a.
Black walnut trees naturally grow across their native range of the Eastern United States in near-neutral pH soils that are well-drained and moist. According to the U.S. Forest Service, walnut will grow best in fertile loams comprised of silt or clay. Growth rate is reduce when soils are too dry and gravelly or when soils are too wet. Moreover, trees grew more over time when planted in topsoil that is over 30 inches deep and never too dry or overly wet.
Walnut trees develop a tap root after seeds germinate and saplings initially establish. While established, larger trees develop a spreading root matrix that extends out at least two times wider than the branch canopy, young trees establish their roots better if competition from grasses and other weeds is eradicated within 6 feet of the tree trunk, according to the U.S. Forest Service. Large trees develop some roots that plunge as much as 10 feet into the ground to supply water during periods of drought.
Melvin J. Baughman and Carl Vogt of the University of Minnesota Extension Service write that walnut trees need at least 25 inches of water annually. Over 35 inches of rainfall with irrigation each year is optimal. Avoid planting walnut trees on southern or western slopes as the conditions increase water needs from the increased heat and light and likelihood of drier soils. Adding a 2- to 3-inch layer of an organic mulch over the root system, extending well past the branch tips helps conserve soil moisture, diminish weed competition and provides trace nutrients to tree roots.
Black walnut trees appreciate a deep, fertile soil. Organic matter compost and mulches provides non-stop seepage of minerals into the soil for tree roots to absorb. A study conducted by Felix Ponder of the U.S. Forest Service's Northern Research Station in Columbia, Missouri, revealed black walnut trees grew larger when provided a slow-release granular fertilizer. Both the height of the several walnut trees and their survival rate after the nine-year experiment were greater with the use of a 16-6-8 formulation.
Baughman and Vogt comment that growth of walnut trees is faster when they are not densely planted, or in close proximity to other trees that are competing for the same soil and water resources. Interestingly, the slower the growth rate, the more dark-colored heartwood is produced--beneficial to people wanting to cut down trees for making furniture, paneling or other items. The U.S. Forest Service mentions that walnut trees' growth period extends for only 110 to 135 days, regardless of elevation or latitude. This explains why the foliage of walnut yellows and drops earlier in the season than other American native hardwood tree species.