How to Grow Rosemary
Rosemary is a woody evergreen shrub hardy outdoors in USDA zones 8 to 10, and that number decides how you grow it. Everywhere, it wants full sun and fast-draining soil: wet feet kill more rosemary than cold does. In zone 7, and in sheltered zone 6 gardens, the cold-hardy cultivars ‘Arp’ and ‘Madeline Hill’ — usually sold as Hill’s Hardy — are the ones that survive. In zone 5 and colder, rosemary is a container plant that spends winter indoors. Wherever you are, buy a plant or root a cutting rather than starting from seed: seed can take up to three years to reach harvest, and cuttings root in 10 to 20 days.
How it works
Rosemary was reclassified a few years ago and is now Salvia rosmarinus rather than Rosmarinus officinalis, so nurseries and reference books currently use both names for the same plant. Where it is hardy it grows into a genuine shrub, 2 to 6 feet tall and as wide, blooming from January to April. Cultivars vary enough to matter at planting time: ‘Tuscan Blue’ reaches 6 feet, ‘Arp’ stays near 3 to 4, and prostrate types like ‘Huntington Carpet’ stay under a foot tall while spreading 4 to 6 feet sideways.
Seed is the wrong route. Viability is low, germination is slow — 14 to 21 days at 60°F when it works at all — and named cultivars do not come true, so seed cannot give you the cold-hardy plant you specifically need in a cold garden. Cuttings solve all three problems at once.
The failure mode that kills more rosemary than winter does is water. Rosemary struggles in wet or poorly drained clay, and overwatering causes root rot, which shows up first as browning tips. That symptom reads as thirst and gets treated with more water, which is precisely the wrong response. The same trap operates indoors, where dry household air browns the tips of a plant whose roots are already too wet. Push a finger 2 inches into the soil before you decide: damp at that depth means rot, not thirst.
What to do
Planting. Give rosemary full sun and at least six hours of direct light a day, in light, well-drained soil that is sandy or gravelly, at pH 6.5 to 7.0. Space plants to the size they will reach: 18 to 24 inches for compact types like ‘Arp’, four feet or more for ‘Tuscan Blue’ and the prostrate spreaders. In heavy soil, plant high and top-dress with pea gravel or sandy mulch instead of organic mulch, which holds moisture. If your winters put you at the edge of hardiness, choose the cultivar before you choose the spot: ‘Arp’ and ‘Madeline Hill’ are reliable in zone 7 and can hold on in a protected zone 6 site — possible, not something to plan around.
Taking cuttings. Take 4- to 6-inch tip cuttings in late spring or early summer. Strip the lower leaves, dip the cut base in rooting hormone, and set it in a sterile, well-aerated soilless mix with bottom heat at 70 to 75°F. Roots form in 10 to 20 days, and the rooted cutting goes out in full sun.
Watering. Let the soil dry between waterings and err toward too little. Rich soil pushes fast growth and thins the essential oils that carry the flavor; heavy watering rots the roots. Container plants need closer attention than in-ground ones simply because pots dry faster, but the rule does not change — water when the soil is dry 2 inches down, then water deeply and let it dry again.
Harvesting and pruning. Cut tender tips and foliage as you need them through the season, and take no more than 20% of the plant’s growth at a time. Regular cutting is also the pruning: it keeps the habit tight and compact instead of open and leggy. Stop hard cutting 4 to 6 weeks before the first hard freeze, because winter hardiness stays reduced until pruning wounds have healed. For the general technique, see how to prune vegetables.
Bringing it indoors. Start with a plant that has been in its container all season; one dug out of the ground in autumn rarely survives the winter indoors. Make the move 2 to 3 weeks before the first expected frost. Indoors, rosemary wants a cool, bright spot and soil kept on the dry side. Raise the humidity around it with a saucer of water and pebbles under the pot, keeping the pot itself above the waterline, and mist it when the air is very dry. Watch for thrips, aphids, whiteflies, mealybugs, and spider mites, all of which find indoor rosemary readily. Arbor tracks the first-frost date for your zone and puts the move-indoors task on your list ahead of it rather than on the day it arrives.
Zone and season notes
In zones 8 through 10 rosemary is a permanent planting and behaves like a landscape shrub, flowering in late winter and early spring when little else does. Powdery mildew and root rot are the problems to watch. Mildew is an airflow problem, so give a mature plant room; root rot comes from drainage, and no amount of space fixes it.
Cultivar choice starts doing real work in zone 7, and by zone 6 it is the whole strategy. ‘Arp’ and ‘Madeline Hill’ can come through a zone 6 winter in a protected site: good drainage and shelter from wind, not a warmer average temperature. Apply winter mulch after the first hard freeze, not before, because mulch laid down early keeps the soil warm and can reduce hardiness. Where rosemary is not reliably hardy, NC State’s herb-garden research goes further — cut the plant back to within a couple of inches of the ground after the first hard frost, cover the stub with soil, and mulch 4 to 5 inches deep over it.
Zone 5 and colder is container territory: a pot with open drainage holes and the same gritty mix, and a houseguest for part of every year. Overwintering indoors is genuinely difficult, and the reason is that the two failure modes pull in opposite directions: the plant dries out fast in heated air, and it rots if you answer that by watering more. Keep it cool, keep it bright, water sparingly, and expect to lose some plants while you calibrate.
One rule holds in every zone. Excessively wet soil and standing water decrease winter hardiness on their own, independent of how cold it gets. A rosemary that died over winter in zone 7 usually died of drainage, not temperature.