Experts and BHG readers answer.
Dahlia Leaves Turn Brown
It's hard to say what the problem is here without knowing more about the plants. Do they look healthy otherwise? Is it just the lower leaves? Does your soil drain well? My first instinct is to say that they could be rotting if you've had tons of rain and your soil isn't draining well. Mulching them might help, but no bulb can withstand sitting in wet, heavy clay soil.