Rosemary bonsai

Placement

Salvia rosmarinus, long known as Rosmarinus officinalis, is an evergreen, semi-woody shrub that is sensitive to cold. It demands full sun: allow at least six hours of direct sun a day in spring, summer and fall. In a dark corner it weakens, and poor light encourages root rot.

In the ground, an established rosemary can take cold of around 18 °F, but a bonsai lacks that toughness: in a small pot, the root ball and roots freeze quickly. Bring the tree in or shelter it before the first frost. In a mild climate, group the pots against a sheltered wall, out of the wind, and keep them out of the rain: cold, soggy mix rots the roots, even in a gentle winter.

In a harsher climate, the tree spends winter indoors. Give it the brightest spot, with plenty of direct morning sun but no harsh afternoon sun, or about fourteen hours of light under a full-spectrum grow lamp. It likes nights a little cooler than days and plenty of air around the foliage. Indoor air is often dry: set the pot on a humidity tray or use a humidifier.

Soil mix

Rosemary cannot stand wet roots. It needs a very free-draining mix, for example half organic components, such as screened pine or fir bark, and half mineral components, such as pumice, perlite or other grit. Add a little limestone or dolomite to buffer the acidity of the mix, and avoid peat. Sift the components to remove dust; ordinary potting soil is too dense, stays wet and suffocates the roots.

Watering

Too much water is the leading cause of rosemary decline. Let the mix dry out almost completely between waterings: push a wooden chopstick into the pot and water only when it comes out completely dry. Then water generously, until water runs from the holes, and empty the saucer. Water preferably in the morning, so the roots do not sit in water overnight. Do not water on fixed days: a tree in full summer sun drinks far more than one wintering indoors. Rosemary tolerates a day or two of dryness, but a bonsai whose root ball has dried out completely might not recover.

Fertilizer: as long as days are above 50 °F, apply a balanced, non-acidifying liquid fertilizer every two weeks, or a slow-release fertilizer. While wintering indoors, once a month is enough. Keep it moderate: too much fertilizer reduces flowering and fragrance. Still, watch for signs of nitrogen shortage, such as foliage turning pale, especially in a bark-rich mix.

Pruning

Rosemary is shaped mainly by pruning. On a young tree, regularly pinch the tips of shoots as soon as their first pairs of leaves are fully open: the tree branches out and fills in. Prune the top and outside harder, since they are naturally more vigorous, to send strength toward the inner and lower branches. In midsummer, cut the top foliage back to older wood: the lower, finer branches lengthen and thicken. A light trim after flowering keeps the outline compact.

Avoid drastic pruning: rosemary regrows poorly after hard pruning, and an old, bare plant is slow to fill in again. Remove only a small share of the foliage at a time and, apart from that targeted cutback of the top, avoid cutting branches back into old wood. For thick cuts, use a concave cutter and cover the wound with cut paste.

Wiring

Once hardened, rosemary wood becomes brittle: it is almost impossible to bend without splitting. The safest method is therefore pruning, choosing at each cut the shoot that grows in the right direction. Wire only young shoots that are still flexible, with aluminum wire about one third of their thickness, wound at about 45°. To lower an older branch, a guy wire anchored to the pot or to a strong root is less risky than coiled wire. Watch the wire during growth and remove it before it marks the bark, cutting it at every turn.

Upright varieties such as ‘Arp’ easily form a formal upright style. Prostrate or twisting forms such as ‘Blue Lady’, with very fine leaves, suit informal upright, cascade and windswept styles. Only the root-over-rock style suits it poorly. An old rosemary bought at a nursery, with a trunk that is already woody, makes a good starting point.

Repotting

Repot every other year, at the very start of spring, before new growth is well advanced. Never prune more than one third of the roots in a year, and do not combine repotting with hard pruning in the same season. A tree that is slightly pot-bound flowers more.

Propagation: take softwood or semi-hardwood cuttings in summer, from shoots without flowers. Strip the lower leaves, dip the base in rooting hormone and insert them in a sterile medium such as perlite: they root in two to four weeks. Seed is slow and erratic. Watch for aphids, spider mites, mealybugs and scale insects, rosemary beetle, and leafhoppers that mottle the leaves. As for diseases, root rot from poorly drained mix and gray mold are the main concerns.