Rosemary

Light and exposure

Rosemary demands full sun and warmth. Give it the warmest, most sheltered spot in the garden, ideally against a wall that catches the midday sun, radiates heat and shields it from the prevailing cold winds. The less sun it gets, the less it flowers and the less fragrant it is.

This warm exposure has a double benefit in a cool climate: it encourages growth in summer and gives the plant the best chance of surviving winter, keeping it dry and gaining a few degrees at the foot of the wall. In a pot, place it in full sun and plan to be able to move it under shelter in winter.

Watering

Rosemary, like other garrigue plants, dreads excess water. Once established in the ground, it makes do with rainfall and only needs watering during marked drought. A rosemary that yellows and declines is far more often suffering from waterlogged soil than from thirst.

In a pot, let the top of the substrate dry out between waterings and water moderately. In winter, cut back sharply: a waterlogged pot that freezes is the most common cause of rosemary loss under a rainy winter.

Soil and repotting

Drainage is essential. Rosemary wants light soil, stony or sandy, rather chalky and poor, where water never stagnates. Heavy clay soil, wet all winter, is fatal to it.

On heavy ground, plant it on a mound, work in gravel or coarse sand, or grow it in a large pot with a free-draining substrate and a thick drainage layer at the bottom. Avoid nitrogen-rich feeds, which produce soft, water-filled growth that is more frost-sensitive and less fragrant.

Temperature and humidity

This is the tricky point away from a Mediterranean climate. Rosemary is only semi-hardy: it tolerates moderate frosts, around minus ten to minus twelve degrees for the common varieties, but suffers or even dies in prolonged hard cold, especially combined with damp soil. It is therefore not as reliable a winter perennial as thyme or chives. The variety 'Arp', selected in the United States for cold resistance, takes considerably more, around minus fifteen to minus eighteen degrees in soil that stays dry through winter.

Two strategies work. In the ground, plant it at the foot of a wall that catches the midday sun, in very well-drained soil, and protect the crown with a dry mulch and the foliage with a winter fleece when cold snaps are forecast. Safer still, grow it in a pot that you bring into a cool, bright, frost-free space, a veranda or a cold greenhouse, during hard cold spells. Upright varieties are a little hardier than trailing ones.

Pruning and upkeep

Prune rosemary after flowering to keep it compact and to stop it becoming bare and too woody. Cut back the current year’s growth by a third, without cutting into the old bare wood, which regrows poorly or not at all.

Light, regular pruning extends the shrub’s life and maintains a nice shape. Avoid pruning at the start of winter: the young shoots produced by a late cut would be soft and would freeze. Save pruning for spring and early summer.

Pests and diseases

Well placed in the dry, rosemary is tough. Its main enemy under a wet winter remains root rot caused by soil that is too waterlogged, rather than a pest. A plant that turns brown and dries out has most often had its roots in water or has frozen.

Among pests, the rosemary beetle, a small beetle whose larvae devour the foliage, can appear. Powdery mildew sometimes coats the young shoots in damp, still weather. Good airflow, well-drained soil and a sunny exposure are usually enough to keep the shrub healthy without treatment.

Propagation

Rosemary is mainly propagated by cuttings, the most reliable method. In summer, take ten-centimetre cuttings of semi-woody stems, remove the lower leaves and plant them in a sandy mix kept just moist, out of direct sun. Rooting takes a few weeks.

Layering works just as well: lay a low stem on the ground, cover it with soil leaving the tip exposed, and it roots on its own. Sowing is possible but slow, unpredictable and not true to variety, so cuttings are clearly preferred, giving a true-to-type plant that is already robust.

Month-by-month care calendar

Green Hub tips

• Rosemary is not fully hardy outside its Mediterranean range: protect the crown in the ground during hard cold, or grow it in a pot to bring in frost-free.
• Cold plus damp soil equals a dead plant. Drainage and near-zero watering in winter matter as much as frost protection.
• Plant at the foot of a wall that catches the midday sun: the radiated heat and shelter from the wind give it its best chance of surviving winter.
• Propagate by cuttings rather than sowing: cuttings give a true-to-type, already robust plant, while sowing is slow and unpredictable.