Azalea are one of the most popular varieties of bonsai trees as they can be trained in most bonsai styles. The plants have yearly blossoms available in a wide variety of colors. Regularly transplanting your bonsai and pruning the roots will help keep it in good health and small stature for many years.
Azaleas have a large collection of small hairlike roots, rather than a few larger tap roots. This means they take well to extensive root trimming. This helps keep the plant small, a perfect trait for a bonsai.
Transplant your azalea bonsai every 2 years. The small bonsai pot will fill with hairlike roots in this time frame and the soil nutrients will be used up.
What Time of Year?
Transplantation is best done after your azalea has finished flowering. This timing will minimize the stress on the plant. Remove all the flowers once they fade to save your plant from trying to make seed pods. Developing seed pods requires a great deal of energy, which is hard to come by in a small pot. Trying will overstress your bonsai.
If you transplant before flowering, remove the flower buds for that year to keep from over stressing your bonsai while it reestablishes itself. With the decreased root mass it will take time before your bonsai can absorb the nutrients necessary to flower again.
Step 1: Remove from the Pot
Remove the plant from the pot. This will likely remove all the soil at the same time as the roots will have filled every nook in the small pot.
Step 2: Observe the Roots
Untangle the roots and separate out the soil. You can rinse things loose or use a small rake. Any black sections indicate you have been over watering and your bonsai's roots have taken to rotting.
Step 3: Trim the Roots
Remove all black roots first and remember to watch your water levels better in the years to come. Azaleas like water, but prefer moist soil to wet roots. Don't saturate the soil.
Once the black roots are gone, continue cutting until you have approximately halved the over all root mass relative to its original amount. Use a sharp blade rather than tearing whenever possible.
Step 4: Repot Your Bonsai
Replace the roots into the pot with the azalea trunk in the proper location in the pot to suit the style of bonsai. Surround the roots with fresh soil, a lime free mix with a bit of acidity. Now that you have decreased the root mass there should be room for more soil. Water the soil well and keep a good eye on the water levels for at least a month. Removal of the roots will shock your bonsai and it will take some time to reestablish itself. Fertilize on schedule.