Clay soils, with their dense textures and abilities to retain water, can be particularly challenging to plant trees like the flowering crab apple (Malus spp.). By using container-grown trees with an intact rootball, the crab apple can be successfully planted into clay ground by positioning it higher in a gently sloping mound of soil. A wide but shallow planting hole that is mounded with unamended backfill and heavily mulched creates a conducive setting for a crab apple to grow and prosper.
Locate a spot in the landscape where the clay soil does not pool or remain waterlooged after a typical rainfall event. Likewise, avoid a hilltop or exposed site where the clay soil dries quickly or becomes hard from lack of moisture, exposure to drying winds or baking afternoon sun.
Measure the depth of the crab apple tree's rootball with a tape measure. Multiply that number by 0.6. This product reveals how deeply the hole should be dug in the heavy clay.
Measure the width of the tree's rootball. Multiply the number by 5. This product tells you the breadth of the berm of soil to be made from the natural soil grade to the top of the rootball once placed in the shallow hole.
Dig into the heavy clay soil with the shovel; utilize the potato fork to more easily break up the soil if compacted or dry. Till the soil to a depth of 10 to 12 inches. Break up any clumps of clay soil into as small of particles as possible.
Measure the hole on occasion to ensure it is the proper depth and and width for the crab apple tree's rootball, keeping in mind the "shallowed" depth determined in Step 1. If the tree's rootball is small, resting it atop the tilled soil or in a very shallow basin of 4 to 6 inches may suffice.
Remove the crab apple from its container and place it in the shallow hole. If the tree is B&B (balled and burlapped), do not remove the burlap, but set it fully wrapped in the hole (after any wire or nylon cords are removed).
Scratch the rootball with your hand or shovel to gently jar loose and cut some thin roots in the rootball. Slicing the rootball in three to four places encourages new root production and results in these roots growing out into the soil as compared to remaining tightly in the rootball.
Place soil back around the rootball, only lightly tamping it as the shallow hole is filled. Pull back the burlap on B&B rootballs so that the soil covers the top flaps. Do not allow any part of the burlap to remain exposed to the air.
Bring the supply of extra soil over to the planting hole once all the original soil from the shallow hole has be placed around the tree's rootball. A wheelbarrow is a labor-saving way to move and collect the extra soil, whether purchased or gathered from another area of the property.
Mound the extra soil up and around the rootball to a level even with its top. Tamp it lightly.
Add more extra soil, as needed, to create a gentle sloping mound from the top of the rootball to the natural level of the ground.
Water the rootball and backfill soil around the newly planted crab apple. Create a small berm moat around the tree if desired so water pools atop the tree and slowly soaks down.
Add a top-dressing mulch, such as compost, bark or pine straw, atop the mound once you finish watering. The mulch, at a depth of 3 to 5 inches, helps retain moisture, shades the soil, prevent weeds and provides nutrients and humus to the clay. Keep the mulch 3 inches away from the base of the trunk, however.