How to Grow and Care for Rosemary

Rosemary is two completely different plants depending on where you garden. In zone 8 and warmer it is a woody evergreen shrub that wants to be left alone. In zone 6 it is an annual you keep buying, or a pot you fight to keep alive from November to March.

Knowing which situation you are in is most of the work.

Start with the name, because it changed

Rosemary was reclassified from Rosmarinus officinalis to Salvia rosmarinus — it sits inside the sage genus. Both names refer to the same plant, and older books, seed packets and the USDA PLANTS Database still use Rosmarinus. Nothing about growing it changed.

Where it is actually hardy

NC State Extension gives the species as hardy in zones 8a to 10b, reaching 4 to 5 feet tall and 3 to 4 feet wide.

That figure is the single most useful thing on this page, because most rosemary sold in colder regions is the plain species and will not survive the winter in the ground.

Some cultivars do better. ‘Arp’, ‘Madalene Hill’ (sold as ‘Hill Hardy’), ‘Athens Blue Spire’ and ‘Miss Jessop’s Upright’ are the recognised cold-hardy group, reliable to about zone 7a and sometimes through zone 6 in a protected site with sharp drainage. ‘Arp’ was found in Arp, Texas in 1972 and grows upright to 3 or 4 feet with greyer foliage than most.

If you are in zone 7 or colder and want rosemary in the ground, buy one of those by name. Buying an unlabelled rosemary and hoping is how most people end up replacing it every spring.

Sun and soil

Full sun — six or more hours of direct sun a day. Less than that and growth goes soft, sparse and disease-prone.

Drainage matters more than fertility. Rosemary is drought tolerant with, in NC State’s phrasing, a low tolerance for wet environments. It grows well in poor, rocky, shallow soil and badly in anything that stays damp. If you have heavy clay, plant it on a slope, in a raised bed, or in a pot — not in an amended hole in clay, which behaves like a bucket.

Do not enrich the soil heavily. Rich ground produces lush growth with less aromatic oil and less cold hardiness.

Watering, and the mistake almost everyone makes

Overwatering is the common cause of rosemary decline. That is NC State’s assessment, and it holds for plants in the ground and in pots.

Water a new plant while it establishes, then leave it alone. An established rosemary outdoors in most of the US needs no supplementary water except in prolonged drought. In a pot, let the top couple of inches of mix dry before watering, and never leave the pot standing in a saucer of water.

The confusing part is that a rosemary dying of root rot looks like a rosemary dying of thirst — grey, dry-looking, dropping needles. The instinct is to water more, which finishes it. Check the soil before you reach for the can.

Overwintering indoors, which is genuinely hard

Be realistic about this. Wisconsin Horticulture is blunt that rosemary is often a difficult plant to overwinter indoors, and the reasons are specific.

What it needs is an unusual combination:

• Cool. Around 50–60°F. A warm living room is actively wrong.
• Very bright. At least six hours of direct sun — a south-facing window — or supplemental grow lights if you cannot supply that.
• Humid, but with moving air. Standing the pot on a tray of pebbles and water raises humidity around the plant; good air circulation stops the mildew that humidity otherwise invites.
• Careful watering. Low light plus a pot plus enthusiasm is the classic route to root rot, which Wisconsin notes is especially deadly in pots overwintered indoors.

Clay pots suit it better than plastic, because they dry out faster. Expect a plant overwintered indoors to bloom later than one left outside — late spring into summer rather than early spring.

If you cannot offer a cool, bright room, growing rosemary as an annual and buying a fresh plant each spring is a legitimate choice rather than a failure.

Pruning and harvesting

Harvest by cutting stem tips, which doubles as pruning. Take no more than about a third of the plant at once, and avoid cutting back into old bare wood — rosemary is reluctant to regrow from leafless stems, and a hard cut into the woody base often does not recover.

Prune after flowering to keep the plant dense. Plants left unpruned for years go leggy and open at the centre, and at that point starting again from a cutting is easier than renovating.

Propagation

Stem cuttings are the reliable route, and they also let you keep a known cold-hardy cultivar going. Layering and division work too. Seed is possible but slow to germinate, and seedlings vary — you will not reliably get the parent’s hardiness.

What goes wrong

The pests are the usual sap-feeders: aphids, mealybugs, whiteflies and spider mites, all more likely on a stressed plant indoors than a healthy one outside.

The diseases are the ones you would expect from the watering advice — powdery mildew, root rot and botrytis. Every one of them is a symptom of too much moisture, too little air movement, or both. There is no spray that substitutes for drainage and spacing.

The short version

Full sun, sharp drainage, and less water than feels right. In zone 8 and warmer, plant it and forget it. In zone 7, buy ‘Arp’ or ‘Hill Hardy’ by name. In zone 6 and colder, grow it in a clay pot, give it a cool bright room in winter, and accept that some years you will lose it.