How to Grow Marshmallow in Australia

Marsh mallow (Althaea officinalis), sold as Marshmallow, is a perennial of the mallow family, Malvaceae, grown for its velvety leaves, pink flowers and deep root. Its stems reach 1 to 1.5 m, and the late-summer flowers draw bees and butterflies. In the wild it lines ditches, stream banks and brackish marsh edges from Europe to central Asia, so give it a damp, sunny spot. It is not the sweet, and not the weedy small-flowered mallow also called marshmallow in Australia. Many first attempts fail in the seed tray, because the seed wakes slowly and unevenly.

Three mistakes that matter

1. Giving up on the seed too soon. This is the commonest failure. The seed is partly dormant, comes up unevenly, and in trials only about a third of untreated seed germinated. Scarify or chill it as Germinating the seed describes, sow several per plant, and keep the tray damp for four weeks.
2. Choosing a dry or stagnant spot. Marsh mallow wants deep soil that stays damp, yet it does not thrive in permanently boggy, stagnant soil. Choose ground that holds moisture but still drains, as Site, soil and setup describes.
3. Lifting the root in the first year. The root is ready once the plant is two years old, and lifting a young plant ends it. Wait until then, lift in autumn and take only part of the clump, as Harvesting Roots explains.

When to plant

Warmth decides sowing. The seed germinates best in light with a daily swing between about 20 and 30°C, and worst at a steady 20°C in the dark. Sow in spring once days are reliably warm, or give chilled seed that same warmth indoors. In cool and temperate gardens, autumn sowing outdoors around Anzac Day also works, because winter does the chilling. One sowing gives plants for many years.

In the first or last month of your zone's sowing window, sow in trays under cover rather than direct, so you control the warmth.

Site, soil and setup

A marsh mallow clump stays put for years and sends a deep taproot down, so choose its spot with care. A rain garden, a pond edge or the low corner where water gathers after rain all suit it, provided the ground drains between soakings.

As a permanent planting, it sits outside the vegetable rotation.

Container growing

Pots are second best, because the deep taproot makes the plant unsuited to them. Grow it in a pot only where no damp bed is available. Choose the largest, deepest pot you can manage: a standard 50 cm pot holds about 52 L and stands about 39 cm tall. Fill it with quality potting mix.

• Water: deeply in the morning until it runs from the base, then again whenever the top few centimetres feel dry.
• Feed: worm casting liquid diluted to weak tea colour, fortnightly from spring to late summer.
• Position: full sun, sheltered from strong wind, because tall stems can tip a pot.
• Renewal: lift, divide and repot the clump every second autumn, when you take roots.

Germinating the seed

Marsh mallow seed has two brakes on it: a coat slow to let water in, and an embryo that is often dormant after harvest. In Serbian seed tests, the freshest seed was the most dormant. Dormancy faded over three years of storage, while the share of dead seed rose.

Untreated seed in those tests averaged about a third germinating, and earlier Serbian work recorded 9.5 to 37.7 per cent. Seedlings appear unevenly over several weeks, so sow three or four seeds for every plant you want.

Growers disagree about cold: some sow it warm after sandpaper with no chilling, others chill it first. Scarify one batch and chill another, then keep sowing whichever comes up better. Seed treatments are explained further in soaking, scarification and cold stratification.

When nothing comes up

• Sown too deep. Seed germinates worst in the dark, so sow again on the surface with only a dusting of cover.
• Too cool, or too steady. The seed wants the daily swing When to plant gives, not one steady temperature. Move the tray somewhere warm by day and cooler at night, or onto a seedling heat mat by day.
• Stratification bag dried out or went mouldy. Use sand that is only just damp, check it weekly, and bin mouldy seed.
• Old seed. Seed kept for years carries more dead seed, but fresh seed is the most dormant. A new packet is no shortcut: scarify or chill what you have and sow more of it.
• Given up too soon. Keep a warm tray damp for at least four weeks, and leave autumn-sown pots outside until spring before judging them.

Sowing Seeds

Start scarified or chilled seed in trays under cover, where you can give it the warm days and cooler nights When to plant describes.

Starting under cover

• Fill a tray or small pots with fresh seed raising mix and water it before sowing.
• Press the seed onto the surface and barely cover it, because light helps it germinate.
• Sow thinly and water from below, standing the tray in shallow water until the surface darkens.
• Prick out seedlings into their own pots once they are large enough to handle.
• Plant them out while young, before roots circle the pot and the taproot is cramped.

Sowing direct

In cool and temperate gardens, sow three or four seeds at each spot in autumn, 45 to 60 cm apart. Press the seed onto the surface, barely cover it, and water with a fine rose so nothing washes out. Mark each spot with a stake. Thin to the strongest seedling once they appear in spring.

Transplanting

Harden off seedlings (get them used to outdoor sun and wind over about a week of lengthening half days) before planting; see hardening off. Plant 45 to 60 cm apart at the depth they grew in the pot, and water in well.

Care by growth stage

Steady moisture decides how well marsh mallow grows, far more than feeding does. Give it deep soil, compost and water in dry spells, and it asks little else.

If growth stalls in poor soil, a light application of all-purpose controlled release granular (NPK around 14-5-10) scratched in at planting can help. Tall stems catch the wind in rich, damp soil. In spring, drive a stake of 1 to 1.5 m beside each clump. Tie the stems loosely to it with soft ties as they lengthen.

Cutting Back

Cut back twice a year: after flowering to tidy the clump, and after it dies down to clear old growth and any rust.

• Cut flowered stems back once the blooms fade, which also limits self-sown seedlings.
• Trim tatty foliage after flowering to encourage fresh, leafy growth.
• Once stems die back in autumn, cut the dead growth to the ground any time before spring.
• Rake up and bin the old leaves from around the base instead of composting rusty ones.
• Every few years, lift and divide a crowded clump in autumn or spring, and replant the pieces.

Companion planting

Marsh mallow shares a bed best with other long-lived plants of damp ground, and worst with its own relatives.

Grows well with

• Comfrey, another deep-rooted perennial that stays in one permanent spot away from dug beds.
• Mint, which likes the same moist soil, grown in a sunk pot because its runners take over a bed.

Keep apart

Hollyhock rust passes between plants of the mallow family, so the worst neighbours are marsh mallow's own relatives.

• Hollyhock, which carries the same rust.
• Mallow weeds, which harbour it and spread it to marsh mallow.
• Lavender and thyme, whose roots want dry, fast-draining soil that marsh mallow dislikes.

Harvesting Roots

Lift roots once the plant is two years old, in autumn after the stems have died down, when the root is plump with stored food.

• Loosen the soil well out from the crown with a garden fork, because the taproot runs deep.
• Lift the clump, cut off part of the root, and replant the rest at once.
• Scrub the roots clean with a vegetable brush.
• Leave thin roots whole and slice thick ones lengthwise so they dry faster.
• Dry them on a tray in the oven at its lowest setting, door ajar, for 6 to 8 hours.
• Stop when the pieces are hard and brittle.

Store dried root in labelled, airtight glass jars in a cool, dry, dark cupboard, where dried herbs keep for about six months to a year. Condensation inside a jar means the pieces are not fully dry: take them out and dry them longer. Leaves can also be cut just before the plant flowers and dried in small bunches in a warm, airy place out of bright light.

Traditional use and history

The genus name comes from the ancient Greek althaia, a word linked to the long medicinal record of the genus. The species name officinalis means sold in shops. Marsh mallow simply describes its home in wet ground and its place in the mallow family. The French call it guimauve, and Western Australia's organism list also records the name sweet weed.

The plant grows wild across Europe, North Africa and western and central Asia, and has escaped gardens in parts of Canada and the United States. Kew's extinction risk predictions rate it not threatened.

Safety

Marsh mallow is sold as seed for growing, and nothing here is advice to eat or take any part of the plant. Anyone thinking of using a plant should first seek advice from a qualified health practitioner.

Small-flowered mallow (Malva parviflora), the weed commonly called marshmallow in Australia, grows in every state and territory. It is an annual or biennial with round, lobed leaves and small white or pink flowers, mostly from autumn to early summer. Marsh mallow is a perennial with velvety grey-green leaves and pale pink flowers about 4 cm across in mid to late summer. The weed can also reach 1.5 m tall, so height does not tell them apart. Hollyhock, a close relative, flowers on tall spikes of 1.5 to 2.5 m. Check the botanical name before buying seed or plants.

Common problems

Seedlings collapse at soil level. Damping off (seedlings rotting at the base) thrives in wet, airless, crowded and cold trays, so sow thinly in fresh mix and water from below. Read more

Stems lean or snap. Tall stems on rich, damp soil catch the wind, so stake them in spring as Care by growth stage describes.

The plant vanishes in winter. Marsh mallow is herbaceous and dies back each autumn, then shoots again from the root in spring, so mark its spot before it disappears.

Leaves wilt on hot afternoons. Usually the root zone has dried out; soak it thoroughly twice a week in dry spells and mulch to slow drying.

A clump that sulks in wet ground. Roots run short of air in stagnant, waterlogged soil, so move the clump to ground that drains between soakings. Read more

Pests and diseases

Marsh mallow has few serious troubles, and rust is the one to watch.

Hollyhock rust (Puccinia malvacearum). Yellow or orange spots appear on the upper leaf surface, with reddish-brown pustules beneath, worst in wet summers. Remove affected leaves early, cut the clump to the ground in autumn, and pull nearby mallow weeds.

Two-spotted mite (Tetranychus urticae). Leaves turn yellow, mottled or stippled, mostly on the undersides, and numbers climb fast in hot, dry weather. Keep the plant watered and hose the leaf undersides. Read more

Common garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). Both feed at night and after rain, and young seedlings need the most protection. Hand-pick them after dark and protect trays set outside. Read more

Prevention first

• Space plants 45 to 60 cm apart so air moves through the leaves.
• Water at the base in the morning so the leaves dry quickly.
• Check bought-in hollyhocks and other mallow-family plants for rust before planting them nearby.
• Check leaf undersides each week in hot, dry weather.

Tips

1. Use the damp patch at a sunny downpipe outlet, where many herbs sulk.
2. Save seed only from rust-free plants, because rust spores can travel on the flower parts that come away with the seed.
3. Start a second clump a year after the first; lifting each one every second autumn then gives roots every year.