Phlox Paniculata and Mondarda Didyama (Bee Balm) plants are stunning additions to any yard. They are, however, susceptible to powdery mildew–and especially during the Spring season. The fungus covers the leaves, which causes them to drop off and can lead to severely stunting the plant’s growth. While there are both chemical and natural spray remedies to apply to treat this fungus, it’s a good rule of thumb to find ways in preventing it from happening at all. The best answer to this is thinning the plant as it grows.
Phlox and Mondarda are naturally very dense. Over-crowding of the stems cuts down the ability for air to properly circulate around and through the foliage. Couple that with a hot, humid summer and you have the perfect environment for this fungus to thrive. Thinning the shoots gives them more room for airflow, thus eliminating the main culprit behind this issue.
Thinning Versus Pruning
Understand that thinning is different than pruning. Thinning is where you are more selective about which stems are cut or removed. You can thin your plants the traditional way by utilizing pruners. In the spring, look closely at the new shoots and see if they are beginning to get crowded. If they are, simply take your pruners and cut some of the more woody stems all the way to the ground–not just partially down. Be sure to not cut too many of the thicker stems, as that is part of their stability. Your goal should be to trim roughly one-third of the shoots, but space that out through the whole plant so you don’t affect the aesthetic appeal.
Dig and Divide Method
Another way of thinning your plants is called the dig and divide method. This is done by digging up the plant (gently, of course) and manually separating the shoots. Once you have them apart, replant a selection of shoots back into the ground, remembering to give adequate space between them. This is also a great opportunity to take some of the shoots and find new homes for them. You can plant the rest of the shoots in another location in your garden or even give them to friends, neighbors, or family to plant in their own yards.
You want to examine your plants at least twice a year for over-crowding and signs of powdery mildew. Spring and fall are the most recommended times to do this. No matter if your plants are used as ground-cover or more majestic centerpieces, this preventative measure will ensure years of enjoyment.