Walnut seeds won’t germinate immediately when planted because they are in a dormant state, and you must break their dormancy before the seed can germinate. Both scarification and stratification are required to break a walnut seed’s dormant state, for germination to occur.
The black walnut’s dormancy is caused by the thick, hard seed coat. Breaking or weakening the seed coat is referred to as scarification, and is the first step necessary to break the seed’s dormancy. A metal file or coarse sandpaper can be used, but is difficult and time-consuming, unless you have a power sander.
Treatment with boiling water also works. Place seeds in water of 170 to 210 degrees F. Make sure it’s not boiling! After the water cools, continue to soak seeds for 12 to 24 hours. The process is slow because you need to use 10-20 times the volume of hot water as seed.
Whatever scarification method you use, you must be careful not to damage the embryo inside. Once scarified, seeds will not store well and should be planted as soon as possible after treatment.
If scarification is done naturally after planting the seed coat may be broken by microbial action, exposure to alternate freezing and thawing, or fire. Depending on nature to scarify your seeds may require leaving them in the ground longer than one year.
The second step in breaking the black walnut seed’s dormancy is stratification. This requires being exposed to cool temperatures and moist conditions for several months. Winter weather in the Northern USA provides the necessary conditions to break dormancy naturally.
You can also break the seed’s dormancy by stratification in a refrigerator. Using a coffee can, plastic jar, cottage cheese container, or a plastic bag, place the seed in a moist 50:50 mixture of sand and peat moss. Punch holes in the lid of the container to provide air.
Let’s take the process from the start: Collect your walnuts immediately after they fall to the ground – before the squirrels get them. Remove the husks, then place the nuts in water. Nuts that float are not viable and should be discarded. The viable nuts will sink to the bottom.
Scarify your nuts, and plant 1 to 2″ deep in the fall or stratify the nuts in a refrigerator at 34 to 41°F for 90 to 120 days and plant in the spring. Use the natural soil or Grow-Boxes that are open to the soil beneath the box, because walnuts produce a long taproot.
Prepare the soil with Pre-Plant and Weekly Feed, then after the seedlings emerge feed 3 times each year. Walnut seedlings grow fast, and it’s recommended they be transplanted into the orchard within 2 years after germination, unless you have tree-planting equipment.