Rosemary is a Mediterranean evergreen shrub, 2 to 4 feet, with needle leaves and pale blue flowers in late winter and spring, and the flavour and fragrance are unmatched for roasting meats and potatoes and for infused oils. It is reliably hardy only to zone 7, and borderline in zone 6, where it survives most winters against a south-facing wall in sharp drainage but a wet cold snap kills it outright. 'Arp' and 'Hill Hardy' are the toughest named forms.
It wants full sun, eight hours or more, and sandy, alkaline soil that drains fast; the worse the drainage the faster it dies, and overwatering kills more rosemary than cold does. Plant 24 to 36 inches apart in spring and water only until it is established. In cold zones grow it in a terracotta pot and overwinter it indoors on the sunniest windowsill, watering sparingly, or accept replacing it every few years.
Propagate from stem cuttings in summer, which is faster and more reliable than seed. Prune lightly after flowering, taking no more than a third of the growth, because rosemary does not regrow from old wood. The early flowers are a lifeline for bees in late winter, and deer and rabbits never touch it. Botanically it is now Salvia rosmarinus, a true sage.
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.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Spider Mites
Tetranychus urticae →Fine stippling or bronzing on leaves, faint webbing on undersides, foliage drying out during hot, dry weather.
Spray foliage with a strong jet of water to dislodge mites. Apply insecticidal soap or horticultural oil to leaf undersides; repeat every 5–7 days as needed.