A fungus completely destroyed my lovely bed of ajuga. Is it safe to plant another groundcover in the same spot?
Yes, but first amend the soil to improve drainage, and select another groundcover that is not subject to the crown rot that killed your ajuga, also known as bugleweed. If you want to fill the bed with ajuga again, I recommend you wait a couple of seasons before replanting. Amend the soil first, and then plant healthy disease-free transplants. Thin out the ajuga bed every few years to avoid future problems with crown rot.