The black walnut, Juglans nigra, is a deciduous tree valued for its rich, dark hardwood and edible nuts. Propagating this species from seed presents a hurdle because the nuts possess a tough shell and are naturally programmed for dormancy. To successfully grow a black walnut tree from a seed, the grower must simulate the natural conditions of winter to break this dormancy. This process requires careful attention, beginning immediately after the nuts fall from the tree.
Harvesting and Preparing the Seed
Black walnut fruits are typically collected in the fall, usually around September or October, once they naturally drop from the trees. It is helpful to gather more nuts than needed, as a portion will not be viable for germination. The large, round fruit consists of a fibrous, fleshy green or yellowish-green husk surrounding the hard, corrugated nut shell.
The outer husk must be completely removed before the seed can be prepared for planting or stratification. This husk contains juglone, which inhibits germination and encourages mold if left intact. Hull removal is commonly achieved by rolling the nuts underfoot on a hard surface or by using a dedicated wire brush.
Wear gloves during this process, as the husk contains a potent dark brown pigment that will severely stain skin and clothing. Once the husk is off, perform a viability test by placing the shelled nuts in water. Nuts that sink are viable, while those that float should be discarded because they are likely dried out or damaged.
The Mandatory Step of Stratification
Cold, moist stratification is a biological requirement for black walnut seeds to germinate, breaking the deep dormancy induced by the hard shell and natural inhibitors. Dormancy prevents the seed from sprouting during a warm fall period and dying in the winter cold. The seed must experience a prolonged period of cold and moisture to signal that spring has arrived.
The chilling requirement for Juglans nigra is typically a minimum of 90 to 120 days at temperatures just above freezing, specifically between 34° and 41°F. This can be accomplished through two methods: natural or artificial stratification. Natural stratification involves planting the prepared nuts directly into the ground in the fall, allowing the winter climate to perform the chilling naturally.
For artificial stratification, nuts are placed in a sealable bag or container mixed with a damp medium, such as sand, peat moss, or vermiculite. The medium should be moist like a wrung-out sponge, not soaking wet, to ensure adequate moisture without promoting mold growth. This container is then stored in a refrigerator for the required three to four months, where the consistent low temperature satisfies the dormancy requirement.
Direct Planting Techniques
After stratification is complete, typically in early spring after the last hard frost, the chilled nuts are ready to be planted immediately. Site selection is important, as black walnuts perform best in full sun. The ideal soil is deep, fertile, and well-drained, often a loam or sandy loam, with a near-neutral pH range of 6.0 to 7.2.
The physical act of planting involves burying the nut at a shallow depth, typically 1 to 2 inches below the soil surface. This shallow depth ensures the emerging seedling can reach sunlight efficiently. While the exact orientation is not strictly necessary, planting the nut on its side or with the flat, pointed end facing down is common practice.
Protection from rodents, particularly squirrels, is essential when planting black walnuts, as they will dig up and consume the nuts. Protection can be provided by covering the planted area with hardware cloth or fine-mesh wire screen secured with stakes. Alternatively, a small cage or a large, flat stone placed over the planting spot can deter animals until the sprout emerges.
Early Seedling Care and Protection
Once the nut successfully germinates and the seedling emerges, attention shifts to providing optimal conditions for initial growth and survival. Consistent soil moisture is important for young seedlings, especially during their first year, but the area should never become waterlogged, which can damage the developing root system. Deep watering once a week during dry periods is more beneficial than frequent, shallow sprinklings.
Controlling competing vegetation is also necessary, as black walnut seedlings do not tolerate shade or competition well. Maintaining a weed-free zone in a three- to five-foot radius around the young tree for the first three to five years significantly improves growth rate and survival. Protection from browsing animals like deer and rabbits is also warranted, often requiring the use of tree tubes or protective fencing around the young saplings.
On good sites, black walnut seedlings can exhibit strong initial growth, sometimes reaching three feet in height during the first year. The tree develops a strong taproot early on, which is why deep, well-drained soil is so beneficial. After one to two growing seasons, when the root system is well-established and the seedling shows vigorous growth, it can be safely transplanted to its permanent location if necessary.