Rosemary care: keeping a potted plant alive through winter

Light

Rosemary is a full-sun shrub, and Missouri Botanical Garden is specific about winter: the best indoor spot is a sunny south window, and if the plant cannot get at least six hours of sun a day, add grow lights. Penn State Extension puts rosemary on its short list of herbs that do best in direct sunlight. UF/IFAS gives the same six-hour minimum outdoors.

Even a south window is dim compared with a patio. The University of Illinois Extension measures about 1,500 foot-candles right at a south window against 10,000 or more outside, so indoor rosemary grows slowly and that is fine. Turn the pot regularly so it does not lean. In spring, Missouri Botanical Garden suggests taking containers outside in mid-spring after the last frost date; give the plant a few days in light shade first so needles grown indoors do not scorch.

Water

Missouri Botanical Garden sums up the target well: let the mix dry out on top, but not dry throughout. Check by pushing a finger an inch or two into the pot. Dry at that depth means water thoroughly until it drains from the bottom; damp means wait. Empty the saucer every time.

Overwatering is the classic killer. NC State and Wisconsin Extension both call it the most common cause of decline and death, and UF/IFAS adds that rosemary suffers when watered too frequently. A cool room makes this worse, because the mix holds water for longer when evaporation slows.

The trap is that indoor rosemary also dries out quickly. Illinois Extension describes how brown tips and dieback appear in heated rooms, and warns that the answer is not more water but more humidity and a cooler spot. A completely dry root ball is fatal too, and the needles often stay green for a while after the roots have died, so you only notice when it is too late. Lift the pot every few days: a light pot needs water even if the surface looks damp.

Soil and pH

Drainage first. The RHS recommends a soil-based potting mix with horticultural grit added at up to 25 percent by volume, in a pot at least 12 inches wide and deep with plenty of drainage holes. Missouri Botanical Garden prefers clay pots. NC State notes rosemary tolerates most soil types except clay.

Published pH preferences disagree. NC State and Missouri Botanical Garden lean slightly acidic, while the University of Maryland Extension gives 6.5 to 7.0. Any ordinary potting mix falls close to that range, so focus on drainage rather than chasing a number.

Humidity

Here the sources seem to contradict each other, and both are right. Missouri Botanical Garden wants an overwintering room that is sunny, humid, cool and well ventilated, and Illinois Extension recommends a pebble tray and frequent misting. NC State meanwhile says rosemary has a low tolerance for wet, humid environments. The difference is moving air: humidity around the needles prevents drying, but still, stagnant humidity brings powdery mildew, which Missouri Botanical Garden links to poor air circulation. Use the pebble tray and keep a little air flowing.

Temperature

Cool is better than warm in winter. Illinois Extension asks for a cool, even cold, sunny location indoors, and Missouri Botanical Garden suggests a cool room. A bright unheated sunroom or an enclosed porch that stays above freezing is ideal. Penn State Extension notes that most herbs survive the mid to low 40s, and rosemary handles that comfortably.

Outdoors, the RHS says well-established plants in the ground survive about 18°F (minus 8°C), but potted roots are far more exposed. Bring containers indoors before the first fall frost. Wisconsin Extension offers a trick for summer: sink the pot, rim below soil level, into a garden bed so the roots are never disturbed when you lift it in fall.

Fertilizer

Feed lightly. Wisconsin Extension warns that excess fertilizer reduces flowering and fragrance, and the RHS suggests just a sprinkling of general granular feed in late spring or early summer for a plant that has been in the same container for more than a year. A diluted balanced liquid fertilizer used at the long end of the University of Illinois Extension houseplant guideline, once every one to three months during the growing season, is a reasonable alternative. Do not feed in winter.

Work out the dose for your watering can with the fertilizer dilution calculator →

Repotting

Rosemary can stay in the same pot for a long time. Wisconsin Extension notes that a slightly pot-bound plant flowers better, so repot only when roots fill the container, push through the drainage holes, or the pot dries out almost as soon as it is watered. Spring, before it moves outdoors, is the moment. Go up one size, use the same gritty mix, and keep the crown at its original level.

Knobby bumps on old stems are not a disease. The University of Maryland Extension explains these are adventitious roots and normal on older plants.

• Roots grow out of the drainage holes
• The pot dries out within a day in summer
• The plant has been in the same mix for several years

Propagation

Cuttings are the best method, and Wisconsin Extension describes them precisely: take 4 to 6 inch tip cuttings from a flower-free shoot in late spring or early summer, remove the lower leaves, and root them in perlite, using rooting hormone if you have it. The RHS also recommends layering, pegging a low stem into soil until it roots. UF/IFAS considers rosemary difficult from cuttings, so take several to be safe.

Seed is a poor option. Wisconsin Extension notes germination is irregular and slow, even at an ideal 60°F, and that seedlings can take up to three years to give a real harvest.

Methods: Tip cuttings from nonflowering shoots, Layering low stems, Seed (slow and variable) · Best season: late spring to early summer.

Common problems

Indoor rosemary attracts sap feeders. Missouri Botanical Garden lists aphids, mealybugs, whiteflies and spider mites as the pests to watch for on indoor plants, less so outside. Powdery mildew shows as a white dusty coating on the needles in stuffy rooms. Botrytis and root rot are the main diseases. Harvest gently while the plant is under stress: Wisconsin Extension suggests removing no more than 20 percent of the growth at a time.

Not sure which one it is? Try the plant problem diagnosis tool →

Is the rosemary toxic to cats and dogs?

The ASPCA lists rosemary as non-toxic to dogs, cats and horses, which makes it one of the safer herbs to keep on a kitchen sill with pets around. A dog or cat that chews a lot of any plant can still get an upset stomach, and you may not want pets nibbling the plant you cook with, but there is no poison concern. Insecticides are another matter: if you treat pests, keep pets away until the spray has dried and follow the label.

Check any other plant against the ASPCA lists →