Clay is a difficult medium in which to grow trees because it's wet and heavy. Clay soil is made up of tiny particles that cling to moisture. It's so thick that there are few air pockets, which plants need for root growth and spread. In addition, clay soil is usually alkaline, which means that trees that prefer acidic soil will die if planted in clay. The drainage is poor and there is limited oxygen to feed the root systems. To grow trees in clay soil, it is essential to choose a tree that does well in alkaline soils. You must also take care to properly plant the tree.
Plant trees in clay soil in the spring to give them adequate time to become established before winter sets in.
Select a planting site that gets at least six hours of sunlight daily. Most trees also grow well in light shade. Make sure to read the specific sunlight needs of the tree you purchased.
Run a rototiller over the planting area to loosen the clay soil. Dig deep with the machine because clay soil gets so compacted. Loosen it at least 12 inches deep. Make sure the tilled area is at least twice as wide as the tree's root mass.
Dig a hole that is twice as wide and twice as deep as the root ball. Scrape the sharp edge of the shovel along the sides of the hole to loosen it further.
Remove the tree from it's nursery container. Use your hands to loosen the root mass so it is not a tangled mess. Put the tree in the center of the hole and spread out the roots gently.
Fill the hole back up with a combination of the local clay soil, potting soil from the nursery container and compost. The fill should be comprised 2/3 with clay and 1/3 of compost and soil together. This will improve drainage and give the tree important nutrients.
Water the tree with a garden hose. Let it run slowly so the water soaks deep into the root ball. The water will also eliminate air pockets.
Spread 2 to 4 inches of mulch around the base of the tree. Keep it 1 inch away from the trunk. The mulch will help with moisture retention and keep weeds away.