How to Grow Sweet Marjoram in Australia

Sweet marjoram (Origanum majorana) is a soft, bushy herb of the mint family, Lamiaceae, grown for small, velvety, grey-green leaves with a sweet, mild flavour. It is a separate species from oregano (Origanum vulgare), with softer, sweeter leaves and little tolerance of frost. Where winters bring frost, most gardeners grow it as an annual.

Three mistakes that matter

1. Burying the seed. The seed is dust-fine and needs light, so a normal covering of mix often leaves an empty tray. Press it onto the surface instead, as Sowing Seeds describes.
2. Planting out too early. Frost can kill young sweet marjoram, and cold, wet soil stalls or rots it. Plant out only after the last frost, around Melbourne Cup weekend in cool and southern districts.
3. Keeping the roots wet. Roots rot in soil that stays soggy or is packed with organic matter. Plant into free-draining soil or a pot, and water only once the top 2 cm has dried.

When to plant

Seed germinates in mix between 10 and 25°C, best at about 20°C, and it is slow. Start trays at least 4 to 6 weeks before plants can go out, which in most zones means after the last spring frost. The tropics sow as the dry season begins instead.

A late sowing in the tropics or subtropics runs into the wet season, when humidity brings rot.

Site, soil and setup

Sweet marjoram likes a little more comfort than oregano: warmth, sun and some organic matter, but never wet feet.

Put each new planting in a different bed, away from soil where plants have rotted.

Container growing

A pot suits sweet marjoram well, because it can be moved to shelter before frost or a long wet spell. Give one plant a pot at least 20 cm across; a standard 20 cm pot holds about 5 L. Fill it with a premium potting mix, with coarse grit added up to a quarter of the volume. Sweet marjoram is compact by nature: a mound about 30 cm tall that can reach 60 cm in flower.

• Water: until water runs from the drainage holes, and never leave the pot standing in water.
• Feed: fish and seaweed blend at half strength, about 12.5 mL per 9 L, fortnightly from spring to late summer.
• Position: full sun, moved before the first frost to a sunny windowsill or a sheltered, frost-free spot for winter.

Sowing Seeds

Raise sweet marjoram in a punnet or tray and plant it out, because the seed is too fine to sow evenly in open ground. Direct sowing works in warm, frost-free weather; thin those seedlings to 20 cm at two true leaves.

Starting under cover

• Mix the seed with a pinch of dry sand so it spreads evenly.
• Scatter it thinly over moist seed-raising mix and press it in, or cover it with no more than a fine dusting.
• Keep the mix at the temperature When to plant gives, under a clear cover, and mist it; lift the cover once seedlings appear.
• Expect seedlings in 7 to 21 days, and do not give up early, because the seed is slow.
• Once seedlings have several leaves, lift each by a leaf, never the stem, into its own 7.5 cm pot.

The sand trick for tiny seeds helps here, and a seedling heat mat holds the mix warm through a cool spring.

Planting out

• Harden off the young plants (move them outdoors a little longer each day so they adjust to sun and wind) over about a week.
• Start when they have four true leaves: 2 to 3 hours in light shade on day one, then 1 to 2 hours more each day.
• Plant them out after the last frost, usually 4 to 6 weeks or more after sowing.
• Space single plants 20 cm apart, or small clumps 15 cm apart.
• Water them in with seaweed solution at half strength, 15 mL per 9 L.

Care by growth stage

Over-feeding reduces the essential oils that give sweet marjoram its flavour and aroma, so feed at half strength at most.

If growth stalls in very poor sand, a light scatter of all-purpose controlled release granular (NPK around 14-5-10) in spring can help.

Pinching and Harvesting

• Pinch out the growing tips of young plants once they are growing strongly, so they branch.
• Cut stems back before the flower knots open, to push out fresh, bushy growth.
• Once the flowers fade, cut the flower stems back to keep the plant neat.
• On a plant kept into a second year, trim lightly in late winter, then again in late spring to remove dead or straggly stems.

Companion planting

Sweet marjoram suits warm, sunny neighbours, and stems left to flower draw bees and other beneficial insects.

Grows well with

• Sage, which wants the same sun and free-draining soil.
• Capsicums and chillies, which share its warm-season timing.

Keep apart

• Mint, which spreads by underground runners and wants moist soil.
• Parsley and coriander in a shared pot, since they want steadier moisture than sweet marjoram.

Harvesting Sprigs

Pick young leaves once plants are growing strongly, often 8 to 10 weeks after sowing; the flavour is best just before the flowers open.

• Pick early in the morning, before watering.
• Snip off shoot tips, then strip the leaves from the stems.
• Take no more than a third of the plant at a time.
• For drying, cut whole stems once the flower knots form but before the flowers open.

Hang the stems in small bundles, upside down, in a cool, shady, airy spot until the leaves are brittle. Strip them into a clear airtight jar kept in a dark cupboard; condensation inside means they need more drying.

Add fresh leaves near the end of cooking, with eggs, soft cheese, poultry, meat and vegetables. Scatter leaves and the small, edible white flowers over salads. Sweet marjoram can upset the stomach of dogs and cats that chew it, so keep pots out of their reach.

Common problems

No seedlings. Seed covered too deeply, dried out or kept cold rarely comes up, so see Three mistakes that matter and sow again. Read more

Tall, pale seedlings. Seedlings kept in too little light stretch towards it, so move the tray into bright light at once. Read more

Plants die over winter. Frost usually kills sweet marjoram in the open, so in frosty districts carry a potted plant under cover, as Container growing describes. A plant sheltered under evergreen trees can come through light frosts in the ground.

Soft, bland growth. Rich feeding and heavy watering give lush plants with less flavour, so feed and water as Care by growth stage gives.

Leaves stop coming once it flowers. Flowering slows new leaf and weakens the flavour, so cut the flower stems as Pinching and Harvesting describes.

Pests and diseases

Sweet marjoram is usually healthy in a warm, sunny spot, and most trouble follows hot, dry weather or wet roots.

Two-spotted mite (Tetranychus urticae). These mites, under 0.5 mm long, feed under the leaves and leave yellow stippling, then fine webbing. Numbers climb fast in hot, dry weather, so hose the leaf undersides and avoid broad-spectrum sprays that kill predatory mites. Read more

Aphids, including green peach aphid (Myzus persicae). Colonies on soft shoot tips stunt seedlings, distort leaves and leave sticky honeydew. Squash small colonies by hand, or spray leaf undersides with an insecticidal soap whose label lists vegetables and pot plants, following its label. Read more

Damping off (Pythium and Rhizoctonia species). Seedlings collapse where the stem meets the mix, most often when sown thickly into wet mix. Sow thinly into fresh mix, avoid overwatering and keep trays well drained. Read more

Root and crown rot (Rhizoctonia solani and other soil fungi). The stem base darkens and rots and the plant wilts, usually in soggy soil. Lift and discard it, and put the next plant in drier ground or a pot.

Prevention first

• Keep the spacing Planting out gives, so air moves between plants.
• Check under the leaves often in hot, dry weather.
• Grow flowers nearby for ladybirds, lacewings and hoverflies, which eat aphids.

Tips

1. In late summer or autumn, cut 8 cm shoots just below a leaf joint and strip the lower leaves. Root them in moist mix in bright shade, as taking cuttings explains, then keep the pot on a sunny windowsill through winter.
2. Look around the edge of an older plant for stems that have rooted where they touch the soil, and pot them up.
3. Freeze finely chopped leaves in water in an ice cube tray, ready to drop into a sauce.
4. Grow a plant in a hanging basket or window box by the kitchen door, where it is picked often.