Azalea 'Mevrouw Gerard' in bloom
image by Leonid Dzhepko: Commons.wikimedia.org
Widely grown as indoor flowering plants, azaleas, like many houseplants, will require repotting or replanting every two to three years. Replanting will release root-bound plants, give new roots room to spread, and allow you to remove old soil that may be deficient in nutrients and in which fertilizer salts may have built up. Replanting should be done when the plant is not in bloom but otherwise can be completed at any time of year.
Select a new pot that is at least 1 to 2 inches larger in diameter than the previous pot and the azalea root ball. Choosing a pot that is more than 7 inches larger in diameter is probably a waste of soil material and expense, as swamping the plant tends to raise the risk of waterlogged soil and rot in houseplants.
Fill the new pot 1/3 of the way up the sides with a good-quality commercial potting soil mix and set to one side. Slide the azalea carefully out of its old pot, and gently loosen and release the old soil from around the root ball. Untangle and free any roots that are encircling the root ball so that they can grow outward into the new soil.
Set the azalea down into the new pot, ensuring that the top of the root ball soil rests at least 1/2 inch below the lip of the pot. Fill in around the roots with more of the fresh potting mix, poking down the soil down gently with your fingers to fill in around the roots and minimize air pockets. Fill with soil until the surrounding soil is level with the top of the root ball but not any higher than that on the trunk.
Water your azalea in well after replanting. Pour in tepid water slowly, and allow it to percolate down through the potting mix and roots. Once the first watering has dissipated into the soil, add more water to settle the soil and top off with more potting mix to fill in any holes, as needed. Place your plant back in its sunny location and check back on it every three to five days to ensure that the soil is lightly and evenly moist at all times. The first few weeks after replanting are particularly critical, as the roots are recovering from the stress of being disturbed and the plant is battling slight shock. Hold off on fertilizing for the first month or so after planting to further reduce stress on the plant and allow all the plant's energy to be focused on new root development.