Mazus is a fast, low-growing spreader that carpets bare ground in bright green foliage and covers itself in small purple-blue flowers for a few weeks in late spring, with scattered rebloom through summer. It roots as it spreads, filling gaps between stepping stones and along path edges, and shrugs off light foot traffic better than most flowering ground covers.
Its speed is the tradeoff for its usefulness — left unchecked it will move into neighboring beds and lawn, so it's best planted where a spreading mat is wanted rather than next to anything delicate.
Root rot
Plants wilt, yellow, and stunt even in moist soil; lower stems and roots turn brown, soft, and mushy and the plant may collapse. Most common in heavy, wet, or poorly drained ground.
Improve drainage and stop overwatering, and never plant too deep. Lift and destroy badly affected plants along with the soil around the roots. Grow in raised or free-draining beds to prevent it returning.
Crown Rot
Lower leaves yellow and wilt; the base of the rosette turns brown and soft, sometimes with white fungal threads at the soil line. Plants collapse in wet conditions.
Remove and destroy affected plants. Improve drainage and avoid overhead watering. Do not mulch directly against the crown. Space plants for airflow and avoid replanting in the same wet spot.