The seeds, or nuts, of chestnut trees (Castanea spp.), which are hardy in U.S. Department of Agriculture plant hardiness zones 4 through 8, don't produce seedlings identical to the mother tree. Also, the American chestnut tree (Castanea dentata) is highly susceptible to chestnut blight. Its nuts tend to be smaller than the nuts of other chestnuts, too; American chestnut's nuts are about 1/2 to 1 inch in diameter while non-native varieties' nuts often measure 3/4 to 1 1/2 inches across. The Chinese chestnut (Castanea mollissima) is considered the most blight-resistant species.
Harvest nuts directly from chestnut trees or purchase them from a seed seller because store-bought nuts may have been treated with hot water, which can adversely affect their germination. Plant at least two types of chestnuts for the best pollination.
Chestnuts' nuts usually ripen in late summer to mid-autumn. Shortly after their outer coverings, or burs, crack open, gather the nuts you wish to use as seeds, wearing heavy gloves to protect your hands from the spiky burs. Nuts that fall to the ground generally remain viable for no longer than one week.
After removing the burs, soak the nuts in water overnight, and discard all nuts that float rather than sink. Drain the remaining nuts, and allow the surfaces of their shells to dry before you stash them in a plastic bag filled with damp peat moss. Disperse the nuts throughout the peat moss, and perforate the sides of the bag with a toothpick before you place the bag in a refrigerator. Leave it there until early to mid-autumn if you plan to sow the nuts outdoors, and until late winter if you plan to sow them indoors.
When sowing the nuts outdoors in autumn, choose a plot of well-drained, loamy soil with an acidic pH of 4.5 to 6.5 and exposure to full sun. Ensure good drainage by making a planting row about 3 inches deep for nuts 1 inch in diameter and filling the bottom of that furrow with a 1-inch depth of damp compost or peat moss. Sow the nuts 4 to 6 inches apart, laying them on their sides and burying them under an additional 1/2 inch of damp compost or peat moss topped with 1/2 inch of moist soil.
Afterward, place hardware cloth over the planting site, bending the hardware cloth about 1 foot into the ground on each side of the planting row to exclude rodents. Pile at least 6 inches of mulch on top of the hardware cloth to protect the nuts from freezing temperatures during winter, and remove the mulch in mid-spring.
Chestnut nuts kept in a refrigerator should begin to form roots in or by early February. After they do so, remove them from their plastic bag, and pot them in individual tree pots about 4 inches in diameter and 12 inches tall. If you don’t have access to tree pots, use 2-quart or 2-liter milk or juice cartons with their tops removed and drainage holes punched in their bases. Alternatively, plant about 6 rooted nuts evenly spaced in a single 1-gallon pot.
Fill each pot or carton to within 1 inch of its rim with a mixture that is 1 part peat moss, 1 part vermiculite and 1 part perlite, adding about 1/2 tablespoon of garden lime for each 1 gallon of the mixture. Plant the nuts with their roots down, and cover the nuts with 1/2 inch of the mixture. Afterward, enclose the containers in transparent plastic bags to keep the mixture damp until the little trees sprout in about one month.
When the new trees sprout, strip off the plastic bags, and set the pots on a sunny windowsill or under the center of a grow light. Keep their soil moist. Once they have two sets of leaves, feed them once every two weeks with a one-half-strength dose of 30-10-10, water-soluble plant food, dissolving 1/4 teaspoon of the fertilizer crystals in each 1 gallon of water. When the little trees are 1 month old, raise the fertilizer amount to 1/2 teaspoon of crystals per 1 gallon of water.
After the danger of frost has passed, move the seedlings outdoors, starting them in shade and gradually getting them accustomed to more and more sun exposure. Once they have adapted to sunny conditions, transplant them into the ground, setting them at least 20 to 25 feet apart. Remove the underground nuts before you transplant the seedlings, or else a rodent may dig up the plants to get at the nuts. Keep the trees' soil well-watered while they become established, providing them with at least 1 inch of water per week. Chestnut seedlings usually won't produce nuts until they are at least 5 to 8 years old.