How to Care for Rosemary: Watering, Feeding and Airflow

Water only when the soil has dried right down

Rosemary evolved on thin, stony hillsides with long dry summers. Its roots take oxygen from air pockets in the soil, and compost that stays wet fills those pockets and suffocates them. In practice this is the single biggest cause of losses: far more rosemary is drowned than is ever dried out.

The rule for a container is to push a finger in to the second knuckle, and water only when it is dry at that depth. Then water hard, until it pours from the drainage holes, and leave it alone until it dries again. In open ground an established plant needs no watering at all in a normal season.

1. Water in the morning Watering in the evening leaves the compost cold and wet overnight, which is when root rot starts. Water early so the surface dries through the day.
2. Never leave a full saucer A saucer of standing water keeps the bottom inch of compost saturated permanently, and that is where the main roots are. Tip it out within the hour every time.
3. Water less, not more, in winter Growth stops below about 45F (7C) and the plant barely drinks. Once a month is often enough for a plant under cover. Watering on a summer rhythm through winter kills more plants than the cold does.
4. Weight tells you more than the surface The surface of a gritty mix dries within a day while the middle is still wet. Lift the pot instead. Once you know what dry feels like, you will not need to guess.

A plant that has been overwatered goes gray-green and dull, and the needles drop from the inside of the bush outwards. Cutting back the watering at that stage often saves it; by the time whole branches are brown it usually does not.

Use a gritty mix and a pot that drains

You cannot fix bad drainage with careful watering. A standard all-purpose potting mix holds too much water for rosemary, so cut it by a third to a half with coarse grit, perlite or sharp sand. The mix should feel loose and fall apart in your hand rather than form a ball.

In heavy clay ground, plant on a raised mound six to eight inches (15 to 20 cm) high rather than in a hole. A planting hole in clay behaves like a bucket: water collects in it after every downpour and the plant sits in it for days.

Do not use a saucer-less decorative pot with no drainage hole, however good it looks. Drill it, or drop a plain pot inside it and lift that out to water.

Feed hardly at all

Rosemary makes its aromatic oils under mild stress. Rich soil and regular feeding give you a soft, fast, sappy plant with long gaps between the leaves, weak scent and much worse cold tolerance. A plant in open ground never needs feeding once established.

A container plant is the exception, because watering flushes nutrients out of a limited volume of compost. One light feed in early spring is enough for the year. Use a balanced or low-nitrogen feed at half the strength on the label, and stop entirely by mid summer so the growth hardens before cold weather.

1. One spring feed for pots As new growth starts in spring, scratch a small handful of slow-release granules into the surface, or give one half-strength liquid feed. That is the whole year's feeding for most container plants.
2. Avoid high-nitrogen feeds Lawn feed or a high-nitrogen tomato-stage feed pushes soft green growth that flops, smells of very little, and blackens in the first hard frost.
3. Refresh the top instead of feeding Scraping off the top two inches (5 cm) of old compost and replacing it with fresh gritty mix does more for a long-term pot plant than any liquid feed.

Give it sun and moving air

Six or more hours of direct sun is the target. In less light the stems stretch, the needles space out, the scent fades and the plant becomes far more prone to mildew. A rosemary in part shade will survive for years without ever being any good.

Airflow matters almost as much as light. A dense bush against a wall or crammed between other plants traps humid air inside, and that is where powdery mildew and gray mold start. Leave at least a foot (30 cm) of clear air around the plant, and thin the middle of the bush if you cannot see through it.

Record the date you repot, the mix you used and how often you ended up watering that summer. Rosemary is slow to show problems, so a note from last year is often the only way to tell whether a change actually helped.

Rosemary in a container needs a plan

A pot dries faster, freezes harder and runs out of root room in a way open ground never does. It is still the best option in wet or cold gardens, because you can move the plant, but it needs a size that holds enough compost to buffer both drought and frost.

Start a young plant in a pot about two inches (5 cm) wider than its rootball and step up gradually. Potting a small plant straight into a huge container surrounds the roots with wet compost they cannot use, and that is a slow way to kill it.

Repot every two or three years even if you keep the same pot size. The old mix collapses, loses its air pockets and starts behaving like the heavy soil rosemary cannot tolerate.