Chances are that the azalea you purchase would have been grown in a container, either in a soil-less potting medium or a mixture of composted bark and sand. While these are fine mediums for plants grown in a container, making them easier to care for, potting mediums are very different in structure from native soil. Water will not flow freely from one to the other; often the native soil around the newly planted azalea will be moist while the root ball remains dry. Because of this, the day before planting, water the container-bound azalea so that it is well hydrated but not too soggy.
If you have good drainage, plant the azalea at the same level in the soil as it was in the container. The planting should be the same depth as the container and 3 to 5 times as wide, with sloping sides.
Most of the azalea's roots will grow in the top 12 inches of the soil, extending outward rather than downward. The goal in making a wide hole is to loosen the soil in the area of maximum root growth. Do not dig the hole excessively deep, otherwise the root ball will sink and settle too low.
If you have clay soil, plant the azalea slightly higher, 1 to 2 inches, than grade. Sometimes azaleas are planted 6 inches higher with great success. To do this, add coarse sand and leaf compost to the back-fill and grade on a slight slope.
When removing the azalea from the container, take care to protect the tiny hair-like feeder roots along the main roots which can be torn if the plant is pulled straight out of the container. Instead, for a small container, with one hand cup the top of the container around the stem. Tip the container upside down, cradling the plant with the cupped hand, and slide out of the container. If the root ball sticks to the container, tap it against a firm surface to dislodge the plant from the pot. For a large container, two people may be needed to hold and tip the container or to lay it on its side and gently slide the plant out.
If the azalea is pot-bound (meaning that the roots have wrapped around the root ball), it's necessary to loosen the roots before planting, otherwise they will grow in a way that compromises the health of the plant. There are several ways to loosen the root system. If the roots are slightly pot-bound, tease them out gently by hand. If the roots are tightly wound around the root ball and hard to loosen, make three or four slits vertically down the side of the root ball about a half-inch deep. Also make a crisscross slit across the base of the plant (the bottom).
One of the newer techniques for handling a pot-bound shrub is to plant the root ball first, without loosening the roots, and then slice off approximately one half to three quarter of an inch all around the root ball with a sharp spade. Sheering off the outer layers of roots after the shrub is in the ground gives the root ball more structure support so that it does not get damaged or collapse from the force of the spade.
Once the hole is dug and before planting, amend the soil if needed. If gardening in heavy clay or sand, incorporate organic matter such as leaf compost. In an ideal world you would amend as large an area as possible around the plantings. If just the planting hole is amended, the roots may stay in the rich amended area and may not move out into the surrounding soil. The wider the planting hole the better. The general rule is to add one-quarter to one-third the quantity of organic matter to the hole; you can eyeball this amount.
When planting a shrub, it is helpful to add mycorrhizal fungi or soil inoculants to encourage active soil microorganisms (read the package directions). Fertilizer can be incorporated into the back-fill at planting time, though at the Garden we often wait until the second season. Do not fertilize heavily and do not use a high nitrogen fertilizer, which will promote top growth rather than encourage the roots to grow. At the Garden we often use an organic fertilizer specifically for Ericaceous plants, Holly-tone® 4-3-4.
Place the root ball into the hole and slowly fill the hole with the back-fill. Either gently firm the soil down as you go so that there are no air pockets around the roots, or water the hole when it is halfway filled with soil, let the water subside, and finish filling the hole.
With the excess soil, build a temporary berm (a slightly raised edge) about 3 to 4 inches tall around the perimeter of the root ball. This will act as a reservoir, concentrating the water in the root ball area. Keep this temporary berm in place for 6 to 8 weeks.
It is imperative to mulch your azalea. The maximum depth is generally around two inches—mulch out, not up. Taper off the mulch so that it doesn’t touch the plant, which can potentially cause pest and disease problems.
Check your newly planted azalea every other day for about 2 to 3 weeks to make sure the root ball doesn’t dry out. Water as necessary and then once a week through the first season, until the azalea gets established. Your azalea will need an inch of water (including rain water) each week or a half-gallon per square foot.