One of my azaleas had black leaf spots a month or so ago. I moved in and treated the situation, and think it’s under control. But… it’s in a big cheap pot. The disposable kind from a box store. I suspect the tree was harvested from the wild and stuck in there a year or two ago. With the arrival of spring I’d like to repot it to get a look at the roots and put it into a better growing medium. Is the coast clear? Here’s the answer:
First be sure you’ve done due diligence in ensuring you’ve kicked black leaf spotting. (Note this is different than white leaf spotting which looks a bit like baby powder covering the leaves and requires different treatment.) Clear the area around the azalea of fallen leaves to prevent reinfection from fungi coming in from dead leaves on the ground. Consider the weather and if it’s been wet and humid, cut back on watering accordingly (for places that are usually humid like where I live in Florida, this has meant careful vigilance including periodically moving it to a covered patio out of the rain). Remember to clean your tools after use on all your trees or before and after use on the infected tree to not spread the fungi to other neighboring trees. If these measures have been taken and black spots continue to appear it may be necessary to apply fungal spray. In spring this is ideally done after budding but before leaves emerge.
You’ve kicked the black spot when new leaves emerge that stay healthy and black spot free. Clearly, your tree’s health has improved when it looks greener, the leaves are firmer, and this healthy new growth replaces the black spot leaves. So, if possible wait to ensure no further signs of disease or infection appear on the leaves for several weeks.
Note that I said your tree’s health has improved. If you had a minor case of black spot, treated it and got rid of the fungus it may be a good time to repot but if it was a severe case of black spotted leaves repotting may be a bad idea. For me the case was relatively mild. I’ve also been systematically removing black spotted leaves as the new foliage has begun to replace it.
Part of the answer outside of the objective healthy of the plant is to ask if the tree really needs to be repotted. Bonsai are repotted when the plant is root-bound, or the soil is inhibiting the growth of the tree. When the roots have no further room for growth or are circling the pot on the inside the plant is said to be root bound. It’s fairly easy to check if a plant is root-bound by lifting it from the container it’s in and examining the outside of the soil mass before touching it. Root-bound plants have roots running all around them, and it’s unhealthy for the plant to stay in that situation too long. One way to test for unhealthy soil is if it’s too compact – if digging in your soil with your fingertip you find the soil hard and compact, the roots of your plant probably do too and can’t grow. Both are cause for repotting.
Generally while repotting the roots will be trimmed, much as the braches of the bonsai are trimmed. This removes what may be a substantial part of the root system or root ball depending on the tree in question. For this reason, if you’re debating repotting and trimming roots, when asking if it is safe to repot you are also asking if it’s safe to knowingly inflict further stress on the tree and if it will survive and flourish with that change.
If the azalea infected with black spot disease was not flourishing to begin with in it’s current position and situation before it was inflicted with black spot, it might not be the best time to repot.
Consider also that each species of plant has it’s own optimal repotting sweet spot. Likewise, within a species each plant itself has it’s own time (much as humans have their own optimal time for, say, loosing their first tooth or suffering the awkwardness of the teenage years). Frequently repotting is done in the spring because typically the production of flowers and foliage that happens in spring expends much of the energy the tree keeps in its root system. Trimming when the plant is at that sweet spot means much of the energy has left the roots and thus trimming those roots is less damaging to the plant as a whole.
In conclusion, consider first if you’ve fully done due diligence in taking measures to remove and keep black spot at bay. After treatment, wait and watch the plant for steady signs of growth. Next consider the health of the plant – how healthy do you feel the plant was before it became sick? See if the tree is root bound or the soil is too compact or if you could wait another year to repot. Consider the plant and situation itself; if the tree was severely root bound you may want to go ahead. Give the tree more time in its current pot for it’s health to improve if the black spotting was extreme. If poor soil is the cause of the tree’s floundering, you may want to repot. A good time to re-pot overall is in early spring, after budding but before blooming. Last, if you do repot be sure to use fresh clean soil to avoid any residual fungal growth re-infecting your newly potted azalea. Remember to also use a medium to strong acidic soil for any azalea repotting, and consider a low-nitrogen fertilizer specifically for rhododendrons and other low-pH loving plants.