Clay soils, also called as "heavy" soils, create problems when attempting to plant shrubs. Because they tend to compact when wet and are low in organic matter, clay soils can literally "drown" shrubs, by reducing the amount of oxygen available to shrub roots, unless planted correctly. For a long time the traditional method of planting shrubs in clay soil involved digging a hole larger than needed, adding gravel to the bottom and using topsoil as a backfill. A much more successful method of planting shrubs in clay soils is high planting, where part of the root ball is above ground.
Measure an area 2 feet larger in diameter than the root ball of the shrub. Turn the soil over using the spading fork and shovel until the ground is loosened. Add 3 inches of composting material and dig it into the soil with the spading fork. Incorporate a slow-release, balanced fertiliser according to the directions on the manufacturer's packaging.
Remove the shrub from its container and gently loosen any matted or thick areas of roots without breaking them. Measure the root ball. Dig a hole twice as wide in diameter as the root ball of the shrub, and to the depth of the root ball height.
Place the shrub in the hole and backfill with the amended soil removed from the hole. Backfill the bottom of the hole until top of the root ball is sitting higher than the adjacent ground. Add soil until the root ball is positioned 1 inch for every 8 inches of root ball height above the surrounding soil. For example, a root ball with a height of 16 inches would be raised 2 inches above the ground level. Mound the soil up over the roots and the root ball. Cover the entire tilled area with mulch and water thoroughly.
It is easier to till clay soil with a spading fork than with a shovel or spade. Clay soils are usually alkaline. Adding acidic organic matter such as pine needles will help the soil. High planting tends to dry roots out faster than other methods, so water shrubs well until they are fully established.