Occasionally called nettle tree, the frequent hackberry (Celtis occidentalis) is a deciduous tree valued for its umbrella-shaped canopy and attractive fall foliage. It grows in U.S. Department of Agriculture plant hardiness zones 2 to 9, where it will rise in urban, rural and subtropical landscapes with equal vigor. Common hackberry trees propagate best out of seeds, which you gather in fall once they ripen. The seeds sprout quickly if kept moist, warm conditions after their initial treatments.
Collect hackberry fruit in late summer or early fall, after they ripen to a sound, purplish-red shade. Choose berries with unblemished flesh and steer clear of those with insect holes or other signs of damage.
Put the hackberry fruits in a bucket. Gently crush the flesh having a soft mallet to generate the seeds easier to extract. Cover the seeds with water and soak them overnight, stirring the water sometimes.
Scoop out and discard the fruit flesh and rotten seeds which float to the surface of the water. Drain off the remaining water. Choose through the seeds at the bottom of the bucket and discard any with clear signs of damage.
Rub each hackberry seed with a fine rasp to tighten the seed coat. Apply very light pressure when scarifying the seed, and rub only until a pale spot develops. Don’t make a hole in the seed coat.
Fill 3-inch starter pots using a mix of 3 parts coarse sand and one part seed compost. Drizzle water onto the sand mix until it feels quite moist at the top half.
Poke a 1/2-inch-deep hole at the center of the mud mixture and place a single hackberry seed in each pot. Cover the seed loosely using the mud and compost mix. Mist the mix to settle it.
Wrap the starter pots with plastic wrap. Place the pots in the fridge or outside from a north-facing wall. Chill them for three months to cold-stratify the seeds. Remoisten the mud mixture whenever it dries out in the top 1/4 inch.
Move the pots to a cold frame or indoors near a large window. Remove the plastic wrap. Establish the pots to a germination mat and cover them with a plastic propagation dome or a sheet of plastic wrap. Adjust the temperature on the germination mat to between 75 and 80 degrees Fahrenheit.
Water that the hackberry seeds whenever the mud mixture feels mostly dry only under the surface. Avoid over-watering them to prevent mold and fungi problems.
Watch for sprouting in about three months. Maintain the hackberry seedlings on the germination mat to get another 2 weeks to encourage growth. Transplant them to 6-inch pots full of standard potting soil as soon as they hit 2 inches tall.
Move the hackberry seedlings to a lightly shaded area of the garden in spring after the last frost. Grow them under dappled shade with regular watering to get their initial summer. Transplant them into a garden bed at fall, only after the first rainfall.