How to Grow Rosemary in Australia

Rosemary (Salvia rosmarinus, formerly Rosmarinus officinalis) is a woody evergreen shrub of the mint family, Lamiaceae, grown for its resinous, needle-like leaves. It comes from the dry Mediterranean and thrives on Perth sand, and on Anzac Day a sprig stands for remembrance. It is slow from seed, but most plants that die are loved too much: rich soil and frequent watering rot the roots.

Three mistakes that matter

1. Giving up on the seed tray. Seed comes up slowly and unevenly, and not every seed germinates, so a bare tray at three weeks is normal. Sow several seeds per plant wanted and keep the tray going for at least three months.
2. Keeping the roots wet. Heavy soil, frequent watering and humid summers leave roots in warm, wet soil, where root rot takes hold. Plant into sharply drained soil, a raised bed or a pot, and water only once the top dries.
3. Cutting into bare wood. Old, leafless stems seldom reshoot, so a hard cut or years of neglect leave a bare, woody bush. Trim lightly each year after flowering, as Trimming and Shaping describes.

When to plant

Rosemary seed germinates in mild to warm mix, from about 15 to 27°C. Most zones sow in spring, and the tropics sow early in the dry season.

Late seedlings can stay potted under shelter through a first frost or wet season, then go out at the next window.

Site, soil and setup

A bush stays put for years, so rotation does not apply, but replace a plant lost to root rot in a different, better-drained spot.

Container growing

A pot suits rosemary in the tropics, the subtropics and heavy clay, because drainage and rain are easier to control. Use a pot at least 30 cm across, with drainage holes; a standard 30 cm pot holds about 13.5 L. Fill it with premium potting mix, with coarse grit up to a quarter of the volume. Seedlings vary, so pot the most compact one.

• Water: when the top 2 cm of mix has dried, soaking it until water drains out; empty any saucer.
• Feed: half-strength fish and seaweed blend, about 12.5 mL per 9 L, every fortnight from spring to late summer.
• Position: the sunniest spot available, pulled under the eaves through the wet season or a long rainy spell.

Sowing Seeds

Start rosemary in punnets or 10 cm pots of seed-raising mix, then plant it out once the seedlings are sturdy. Potting and seed-raising mixes can carry Legionella bacteria, so wet them down, wear gloves and a P2 mask, and wash your hands afterwards.

Starting under cover

• Sow 3 to 5 mm deep in free-draining seed-raising mix, with room between seeds so seedlings are not crowded.
• Keep the tray within the range When to plant gives, under a clear cover lifted once seedlings appear.
• Keep the mix just moist, never wet, by misting rather than pouring.
• Expect the first seedlings in 15 to 25 days, and some still coming up after three months.
• Prick out each seedling (move it to its own small pot) once it can be handled.

A seedling heat mat takes the chill off a cool spring, and bright light keeps seedlings sturdy.

Planting out

• From four true leaves, harden off (move plants outdoors a little longer each day to adjust to sun and wind).
• Start with 2 to 3 hours in light shade, adding 1 to 2 hours a day over a week.
• Plant out at 5 to 10 cm tall or more, with the root ball only just covered.
• Space plants 60 to 90 cm apart, as each becomes a sizeable shrub.
• Water them in with seaweed solution at half strength, 15 mL per 9 L.

Care by growth stage

Young plants need steady water, established ones hardly any. Over-feeding reduces the essential oils that give rosemary its flavour and aroma, so feed at half strength at most.

Where a plant in very poor sand stops growing, a light spring scatter of all-purpose controlled release granular (NPK around 14-5-10) can help.

Trimming and Shaping

A light trim each year after flowering keeps rosemary dense, and regular picking does part of the job.

• Tip prune young plants often, so they branch low and stay bushy.
• After flowering, shorten the leafy shoots, keeping every cut within green, leafy growth.
• In humid districts, open up the bush just before summer so air moves through it.
• Replace an old, straggly bush with a rooted cutting.

Taking cuttings

A cutting copies its parent, so it multiplies your best seedling. Take softwood cuttings (sappy green tips) in mid to late spring, or semi-hardwood cuttings (firm but flexible stems) from late autumn to early winter.

1. Cut 10 to 15 cm lengths of stem that are not in flower.
2. Strip the leaves from the bottom half and dip the base in rooting hormone powder.
3. Push a pencil into seed-raising mix, insert the cutting and firm it in; a 14 cm pot holds 8 to 10.
4. Water, then cover the pot with a clear plastic bag on a wire frame.
5. Keep it warm, out of direct sun, and moist but not wet; roots usually form in 4 to 8 weeks.
6. Pot the cuttings up singly once fine roots show through the drainage holes, then harden them off before planting out.

Companion planting

Rosemary suits neighbours that want sun and lean, dry soil, and its pale blue flowers feed bees.

Grows well with

• Thyme, oregano and sage, which share its need for full sun and fast drainage.
• Lavender, another Mediterranean shrub that prefers low fertility and sandy soil.

Keep apart

• Basil and parsley, which need the richer soil and steady water that rot rosemary roots.
• Mint, which spreads by underground runners and wants moist soil.

Harvesting Sprigs

Pick a few tips once the plant is well established, no sooner than 4 to 5 months, and more freely after a year.

• Snip soft tip growth rather than woody stems, shaping the bush as you go.
• Take no more than a third of the plant at a time.
• For drying, cut stems just before the plant flowers, when the oils are strongest.

Hang small bundles upside down in a warm, dark, airy spot for 1 to 2 weeks, longer where it is humid. Store the stripped leaves in an airtight jar away from light. Fresh leaves also freeze, finely sliced, in ice cube trays of water. Use them with roast lamb, potatoes and barbecued meat, and in bread.

Common problems

An empty seed tray. Slow, patchy germination is normal, so keep the tray going, as Three mistakes that matter explains. Read more

Seedlings that differ from each other. Seed-raised rosemary varies by nature, and Taking cuttings shows how to copy the best one.

Tall, pale seedlings. Seedlings reaching for a dim window grow thin and floppy, so give the tray the brightest spot available. Read more

Pests and diseases

Rosemary in a sunny, open spot has few pests; most trouble follows wet roots, humid air or hot, dry spells.

Two-spotted mite (Tetranychus urticae). Mites under 0.5 mm long feed under the leaves, leaving yellow stippling, and multiply fast in hot, dry weather. A hosing under the leaves knocks them back; skip broad sprays, which also kill the predatory mites that eat them. Read more

Aphids (superfamily Aphidoidea). Small sap-suckers cluster on soft shoots, which wilt or twist under sticky honeydew. Squash or hose off small colonies, and let ladybirds and lacewings eat the rest. Read more

Powdery mildew (Oidium and related fungi). A white, powdery film spreads over the leaves, worst in shade, crowding and humid nights. Open up the bush, water the soil, not the leaves, and spray 1 part full-cream milk to 10 parts water. Read more

Phytophthora root rot (Phytophthora species). This soil-borne water mould rots roots in warm, wet soil; tips brown, then the plant wilts and dies. Bin an affected plant with its soil and replant elsewhere.

Damping off (Pythium and Rhizoctonia species). Seedlings collapse where the stem meets the mix, most often in wet, crowded trays. Use fresh mix and keep it just moist. Read more

Prevention first

• Choose a sharply drained bed or a pot, and let the top dry before each watering.
• Give each plant full sun and its full spacing, so the leaves dry quickly after rain.
• In hot, dry spells, turn a few shoots over to look under the leaves for mites and aphids.
• Take cuttings only from healthy plants, and never move soil from a spot where a plant died of root rot.

Tips

1. Lift the skirt of an older bush: stems lying on the soil often root by themselves, ready to cut free and pot up. Layering does this on purpose.
2. Where frosts are heavy, keep a rooted cutting in a pot in a sheltered spot, in case the bush is lost.
3. Strip a straight, woody stem and sharpen one end for a barbecue skewer.