'Herrenhausen' is a compact, spreading selection of ornamental oregano grown for looks rather than the kitchen — dark green leaves flushed purple when young set off dense clusters of small, funnel-shaped pink-purple flowers with purple-red bracts that open over a long summer season. It's a mainstay of gravel gardens and dry sunny borders, drawing bees and butterflies for months.
Like culinary oregano it wants lean, gritty, well-drained soil and struggles in rich, wet ground — the poorer the soil, the tighter and more floriferous the habit. Once established it's genuinely drought tolerant and needs little beyond an occasional deep watering in extended dry spells.
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.