I have a lot of young lavender plants that I planted this spring. I read up on lavender and followed all the recommendations. I have clay soil so I planted them on raised individual mounds. The mounds are made up of lean soil mixed with sand, starter grit, purchased top soil, wood ashes, etc.

Most of the plants are starting to turn grayish brown on the bottom. The tops are still green and look okay. Should I be worried? What could be causing this?