Rosemary

Identification and origin

Salvia rosmarinus, long known as Rosmarinus officinalis, belongs to the mint family, Lamiaceae, alongside mint and sage. It is native to the lands around the Mediterranean: Portugal, Spain, southern France, Italy and its large islands, the Balkans, Greece, Türkiye, Cyprus and North Africa from Morocco to Egypt. There it grows on dry, rocky ground, often near the sea: its name means “dew of the sea.”

It is a woody, upright, rounded shrub that usually reaches 2–6 ft tall and as wide where it spends the winter outdoors. Its square stems turn woody with age. The stalkless leaves, narrow as needles and less than 1 in long, are leathery: dark green above, covered in white hairs beneath, with rolled-under edges. Crushed, they smell of pine and camphor.

The small two-lipped flowers, pale blue, lavender, pink or white depending on the variety, are clustered along the branches. Bloom runs from spring into summer, and bees and butterflies visit it readily.

Common cultivars include ‘Arp’, with light blue flowers, considered the most cold-hardy; ‘Madeline Hill’, also hardier than the species; ‘Tuscan Blue’, vigorous and strongly upright, with deep blue-violet flowers; ‘Miss Jessopp’s Upright’, a vertical form; ‘Barbeque’, with straight, stiff stems; ‘Majorca Pink’, with pinkish-purple flowers; ‘Albus’, with white flowers; and creeping forms such as ‘Prostratus’, which stay under 6–12 in tall.

Light

Rosemary wants full sun: at least six hours of direct light a day. In shade it becomes leggy and flowers poorly. In the garden, choose the sunniest, best-ventilated spot you have.

Indoors, where it often spends the winter in cold regions, set it against an unobstructed south-facing window or add light with a grow lamp.

Watering

Excess water is the leading cause of rosemary decline. Water only when the mix is dry a couple of knuckles deep; then water thoroughly, let the excess run out of the drainage holes and never leave water standing in the saucer.

Once well established in the ground, it tolerates drought and needs no watering; only young plantings need water during dry spells in their first year. In containers, check more often in hot weather. In winter, cold, soggy mix rots the roots: space waterings well apart.

Browning branch tips can signal root rot from too much water, but also a mix left dry for too long: check the moisture and the roots before acting. Rosemary copes poorly with air that is both warm and humid.

Potting mix

It needs light, sandy or gravelly soil that drains perfectly, with a near-neutral pH between 6.5 and 7. It tolerates most soils except heavy, poorly drained clay: there, grow it on a mound, in a raised bed or in a container.

In a container, use a peat-free, soil-based mix with roughly a quarter horticultural grit added. The pot should be at least 12 in wide and deep, with generous drainage holes. Too much fertilizer reduces flowering and fragrance. In the ground it needs no feeding; in a container, a general-purpose granular fertilizer in late spring is enough, starting in the second year.

Temperature

Rosemary is grown in the ground in USDA zones 8a to 10b and withstands about 18 °F in well-drained soil. ‘Arp’ and ‘Madeline Hill’ survive down to zone 7a. Containers are more vulnerable than plants in the ground: group them against a sheltered wall and cover them with frost fleece during hard freezes.

Where winters are too harsh, bring the pot indoors two to three weeks before the first expected frost. The ideal spot is then cool and very bright, around 50–61 °F, such as an unheated sunroom. Our homes are usually too warm and dry for it.

Repotting

Plant or repot in spring, as the soil begins to warm. Do not set it too deep: the roots should be only lightly covered. Choose a clay pot, which dries faster, filled with the same free-draining mix.

In cold regions there are two options for summer: sink the whole pot into the garden, or plant the root ball in the ground, then dig it up and pot it again two to three weeks before the first frost.

Pruning

Trim lightly every year after flowering to keep the plant dense. Do not cut into bare old wood: it regrows poorly. An old, straggly plant is easier to replace than to rejuvenate; it often lives seven or eight years.

Harvesting doubles as pruning: snip sprigs year-round, but never remove more than a fifth of the foliage at once. The soft new growth of late spring and summer has the best flavor. For drying, harvest before flowering and hang bunches upside down in a dry, dark, airy place.

Propagation

Cuttings are the most reliable method. From late spring to early summer, take 4–6 in tips from non-flowering shoots, strip the lower leaves and, if you like, dip the base in rooting hormone. Insert them in perlite or a sterile, airy mix, ideally with bottom heat of 70–75 °F. Roots form in ten to twenty days; then pot each cutting up individually.

Layering a low branch, in spring or fall, also works. Seed is possible but disappointing: germination is slow and erratic, taking two to three weeks at about 61 °F, and it takes several years to reach a harvestable plant.

Common problems

• Decline, brown tips, a plant that suddenly dries out: root rot from overwatering or poorly drained soil, especially in winter.
• White powdery coating on leaves: powdery mildew, favored by stagnant air and humidity; improve air movement and space plants apart.
• Gray mold: botrytis.
• Indoor pests: aphids, mealybugs, scale insects, whiteflies, thrips and spider mites.
• Outdoors: rosemary beetle chews the leaves; leafhoppers leave pale mottling on the foliage.
• Damaged young shoots in spring: frost injury.

A single symptom can have several causes: consider soil, light, temperature and air movement together before treating.

Toxicity

Rosemary is not considered toxic to dogs, cats or horses, and it is an edible culinary herb for people. If an animal eats a large amount or reacts unusually, ask a veterinarian for advice.