Bog sage is the salvia to reach for in a damp corner of the border where most other salvias would sulk. Slender stems carry clear sky-blue flowers on airy, branching spikes from late summer clear through the first hard frost, and the color reads true at a distance in a way few blues in the garden do.
Unlike its drought-tolerant relatives, this species wants moist, rich soil and struggles through dry spells with wilted foliage. It spreads by shallow rhizomes and can naturalize into thick colonies in mild, wet climates — give it room or contain it. In zones colder than 7b, treat as a tender perennial and either mulch heavily or lift the rhizomes for winter.
Powdery mildew
White or grey powdery coating on leaves — usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
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.