Why are my Phlox plants dying and what can be done to save them?
Question:
I have four creeping Phlox plants that were planted in the spring of 2023 and grew well. They looked good this spring until about 7-10 days ago when all four almost died overnight. The middle of the plants appear dead. There is some very little life at the very edges. These plants are in full sun, on a slope. They do not get too much water. To the best of our knowledge, pesticide has not been used on the property. The other perennials in the garden have not been affected so far. Any ideas on what happened or how I can revive them?
Answer:
There are several things that might cause such a sudden and dramatic decline, and you’ve ruled out several of them–insufficient sunlight, excessive watering and/or insufficient drainage (causing root rot, which does often appear as a sudden collapse of a plant), and inadvertent contact with an herbicide (which would definitely present similarly). Other common ailments for creeping phlox are spider mites and powdery mildew, but your photo doesn’t look quite right for either of those. You could check for nearby holes to indicate vole damage to the root system, as well as monitoring pet activity nearby–frequent dog urination in the same place can cause dieback in plants. It is also possible that you are looking at something like anthracnose, but you would need to submit a sample to the UMaine Plant Disease Diagnostic Lab to get confirmation on that.
I would try gently pulling at the phlox to see if it comes away from the soil easily. If so, this particular specimen is done for and you can pull it out. If it’s still attached, you may still have healthy crown and root tissue–I would trim out the dead material to just above the soil and see if you can coax the plant into flushing out new growth. Phlox can sometimes recover from above ground die-back if there is no severe underlying cause.
I’m sorry not to have a more definitive answer for you, but I wish you happy gardening, nonetheless.