Aesculus is comprised of approximately 20 species of trees and woody shrubs, with six native to North America and the rest native to Europe and Asia. All aesculus species have white flowers in spring and produce conkers, or spiny seed husks encasing a large seed. In Europe, aesculus species are referred to as horse chestnuts. In the United States, they are referred to as buckeyes. Horse chestnuts are aesculus hippocastanum and are not as hardy as buckeyes. Horse chestnuts are hardy in Zones 3 to 8 and form rounded 50- to 75-foot specimen trees.
Plant the Seeds
Gather twice as many horse chestnut conkers than you think you will need. Each conker contains one seed so you will need to gather 8 conkers if you want 4 trees, because not all the seeds that you start will germinate. Only gather conkers that have fallen from trees. Conkers fall in September and October, depending on where you live.
Remove the seed from the conker by pulling one half of the spiny shell (conker) off (conkers open when they hit the ground) and popping the seed out. Seeds are very large and easy to handle, however, when first removed they may be slippery.
Store the seeds overwinter by placing them in a plastic storage bag packed with sphagnum or peat moss. Keep the moss moist by misting periodically with water. Store in a cool room (approximately 40 degrees F) or in a refrigerator. Do not seal the bag; rather, allow it to remain slightly open or poke several holes in the plastic to allow some air flow. You will need to start the seeds in mid spring after all chances of frost have passed.
Sterilize as many 1-inch pots as you have seeds in a 1:10 bleach/water solution. Rinse thoroughly in warm water and allow them to air dry.
Make your own seed starting mix for horse chestnuts by mixing compost and potting soil together to make a 50/50 compost/soil mix. Moisten the compost/soil mix with water and fill the 1-inch pots to the top.
Push one seed into a pot filled with compost/soil mix until it is just covered. Continue with the other seeds, planting one seed per pot. Seeds will germinate in 4 to 10 days.
Place the pots outdoors in a spot that receives morning sun and afternoon shade. Bring the pots indoors if nighttime temperatures drop below freezing.
Keep the seeds moist but don't allow the soil to become soggy. Allow the top 1/2 inch of soil to dry out before watering again.
Transplant seedlings into a sheltered area of your garden, such as a spot where they have protection from the prevailing wind and receive afternoon shade. They should be at least 2 to 3 inches high to transplant successfully. Plant them at the same depth they were growing in the pots.
Give your seedlings 1 to 2 inches of water a week. Allow the top 1 inch of soil to dry slightly before watering. Do not allow the soil around the seedlings to become soggy as the seedlings will succumb to root rot if left in standing water.
Apply 1/2 to 1 inch of mulch around your seedling. Pull the mulch an inch away from the horse chestnut's trunk. Spread the mulch so it extends 3 to 4 inches from the trunk. Mulch keeps the soil evenly moist and helps control weeds.
Transplanting the Saplings
Choose a spot in full sun or part shade (horse chestnuts need at 6 to 8 hours of direct sunlight) with well drained soil. Horse chestnuts prefer slightly moist soil to excessively dry soil so they are the perfect trees for clay and clay loam soils.
Transplant your horse chestnut saplings to their final location when they reach 1 to 3 feet high (usually at the end of their first summer). Time the transplanting so it is 4 to 6 weeks before your average first frost date (this gives the sapling time to settle in) or, if you will be transplanting in spring, you can move your sapling as soon as the soil can be worked.
Dig a hole just large enough to accommodate the root ball. Plant your horse chestnut sapling at the same depth it was growing in.
Give your horse chestnut sapling 1 to 2 inches of water per week during its first year.
Use 2 to 3 inches of mulch around the base of the sapling. Remember to pull the mulch 1 to 2 inches away from the base of the tree. Spread the mulch so it reaches the drip line (the imaginary line around trees that marks where the tips of the branches reach).