Spread 1 to 2 inches of compost or peat moss over the soil underneath the trees, and work it into the top 3 inches of the soil. Take care not to damage any tree roots when working. Ivy grows best in rich, moist soil.
Dig small holes with a trowel and plant the ivy in the holes 4 to 6 inches from the base of the tree. Buy small ivy plants that don't require large planting holes to avoid damaging tree roots. Space ivy plants 12 to 18 inches apart.
Water the ivy thoroughly after planting, and weekly as needed to keep the soil evenly moist. The ivy will naturally cling to the tree as it grows with no help from you.