How to Grow Portulaca in Australia

Portulaca, or moss rose (Portulaca grandiflora), is a low, succulent annual in the purslane family, Portulacaceae. It is grown for bright, rose-like flowers on sunny edges, rockeries and pots, and it shrugs off heat and dry spells. The step most gardeners get wrong is burying the seed, which needs light to germinate.

Three mistakes that matter

1. Burying the seed. Portulaca seed needs light to germinate, so seed raked in or sown deep often never appears. Press it onto the surface and add no more than a 1 mm dusting of fine mix, as set out in Sowing Seeds.
2. Sowing into cold soil. The seed wants warm soil and young plants are killed by frost, so early sowings sit, rot or stall. Wait for your zone's window in the table under When to plant.
3. Rich, wet soil. Stems and roots rot in soil that stays wet, and heavy watering or feeding can kill plants outright. Choose a sunny, free-draining spot, water only once the surface dries, and keep feeding light.

When to plant

Soil temperature decides it: portulaca germinates best in soil of about 21 to 30°C, so sow once the soil is warm and frosts are over. One sowing usually flowers right through summer and into autumn, so most gardens need only one.

Where frost lingers, start seed in cells under cover 4 to 8 weeks before the last frost. Plant out once it passes, traditionally around Melbourne Cup weekend in Canberra and on the tablelands.

Site, soil and setup

Sun is the whole game. Give portulaca at least 6 hours of direct sun a day, because the flowers stay shut in shade, on cloudy days and overnight. It thrives in lean, sandy, gravelly or rocky ground as long as water drains away fast. It grows in acid to neutral soil, so most gardens need no pH correction.

The organisms behind damping off live on in soil and old plant debris. Do not sow where seedlings collapsed the season before.

Container growing

Portulaca suits pots, troughs and hanging baskets. Double Flowered Mixed grows 12 to 18 cm tall, with fully double 5 cm flowers, and trails over the rim. Choose a pot at least 20 cm wide and deep with drainage holes; a standard 20 cm pot holds about 5 L. Bagged potting mix can carry Legionella bacteria: wet it before handling, wear a mask and gloves, and wash your hands afterwards.

• Watering: let the top of the mix dry out, then water thoroughly until it runs from the drainage holes.
• Feeding: from the first flowers, give fish and seaweed blend at 25 mL per 9 L every fortnight.
• Position: the hottest, sunniest spot you have, such as a north-facing step or a sunny balcony rail.

Sowing Seeds

Sow portulaca where it is to flower, because seedlings dislike being moved; use cells only where frost or snails make that hard.

Sowing direct

• Rake the soil fine and water it, then mix the dust-fine seed with dry sand so it spreads evenly. Sowing tiny seeds shows the method.
• Scatter the mix on the surface and press it in with a flat hand. Cover with no more than 1 mm of fine seed raising mix, or leave it bare.
• Mist the surface to keep it damp until seedlings appear, usually in 1 to 2 weeks.
• Thin seedlings to 15 to 25 cm apart once they have a few true leaves, snipping the extras at soil level.

Starting in cells

• Fill small cells with seed raising mix, sprinkle a pinch of seed on each and press it in without burying it.
• Stand the tray in a warm, bright spot and mist it to keep the surface damp.
• Snip all but the strongest seedling in each cell.
• Harden off the seedlings (stand them outdoors for a few hours, then longer each day, until they take a full day of sun).
• Plant out at 5 to 10 cm tall with the root ball whole. Water in with seaweed solution at half strength, 15 mL per 9 L.

Common purslane (Portulaca oleracea), a leafy weed of disturbed ground, often comes up among a sowing. Leaves and flowers tell the two apart.

Care by growth stage

Portulaca is a light feeder. Lean soil and full sun keep it compact and flowering; rich feeding and wet roots bring soft growth and rot.

If plants stall in very poor soil, scratch in a little all-purpose controlled release granular (NPK around 14-5-10) at the label rate.

Deadheading

Faded flowers turn into seed capsules, and a plant busy making seed makes fewer buds. Regular tidying keeps portulaca flowering and limits self-sown seedlings.

• Faded flowers: pinch them off, with any swelling capsule beneath, as you pass.
• Straggly plants in midsummer: shear the longest stems back lightly to bring on fresh, bushy growth.
• Handling: the stems snap easily, so steady each one as you pinch.
• End of the season: pull out plants once they die back, leaving a few flowers to seed first if you want to save it.

Companion planting

Good partners share the hot, dry spot and differ in height or leaf.

Grows well with

• Nasturtiums: bold round leaves beside fine, needle-like ones, in the same sun and lean soil.
• Zinnias: taller summer flowers set behind a low portulaca edge.
• Spring bulbs: portulaca covers the bare ground once the bulb foliage dies back.

Keep apart

• Vigorous trailing vines, such as ornamental sweet potato: they overrun low portulaca.
• Sunflowers planted to the north: their shade keeps the flowers closed.
• Lettuce and other thirsty crops: the steady watering they need keeps portulaca too wet.

Harvesting Blooms

Portulaca is harvested mostly for seed and cuttings. Collect seed as the small round capsules ripen, because ripe capsules split and spill it.

• Check the plants every few days once the first capsules form beneath faded flowers.
• Snip stems carrying ripening capsules straight into a paper bag, which catches the dust-fine seed.
• Shake the bag to free the tiny grey to black seed, then pick out the husks.
• For cuttings, snip a healthy shoot tip and push it into damp potting mix; even broken pieces often root.

Store the seed in a paper bag inside a labelled glass jar, in a cool, dark cupboard. Seed saved from a mixture may not repeat its parents, so expect a mix of forms and colours. Cuttings rooted in small pots fill gaps in a bed or make a quick gift.

Common problems

Seed does not come up. Seed buried too deep or sown into cold soil, the first two points in Three mistakes that matter, is the usual cause. Read more

Seedlings collapse at soil level. Damping off, a rot chiefly caused by Pythium species and Rhizoctonia solani, strikes seedlings in wet soil or reused mix. Read more

Flowers stay closed. Shade is the usual cause, as set out in Site, soil and setup, so move pots into more sun.

Seedlings wilt after planting out. Portulaca resents root disturbance, so plant out small with the root ball whole, as set out in Sowing Seeds.

Seedlings appear where none were sown. Plants self-sow readily, and the volunteers may differ in colour or form from their parents; pull those you do not want while small.

Pests and diseases

Portulaca has few serious pests, and most trouble follows wet soil, soft growth or young seedlings.

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They chew seedlings to the ground after dark and leave shiny slime trails. Set beer traps near the sowing and fit copper strips around pots. Read more

Aphids (Aphidoidea). Clusters of small sap-suckers on shoot tips leave sticky honeydew, often with ants and black sooty mould. Hose them off with a strong jet, or spray insecticidal soap under the leaves. Read more

Mealybugs (Pseudococcidae). White, cottony insects settle in leaf joints and among the fleshy leaves. Dab small colonies with a cotton bud dipped in methylated spirits. Read more

Root and stem rot (Pythium species). Water moulds attack the fine roots in wet soil, and plants collapse from the base. Pull out affected plants, and lift the bed or move to pots as set out in Site, soil and setup.

Prevention first

• Choose free-draining soil and water only once the surface dries.
• Keep to the spacing in Sowing Seeds, so stems dry quickly after rain.
• Use fresh seed raising mix and washed cells for every sowing.
• Check seedlings with a torch after dark for snails and slugs.

Tips

1. Set pots where you pass in the morning and middle of the day, since the flowers close from late afternoon.
2. Tuck seed into cracks in a sunny rock wall or between pavers beside a path, where the plants will not be trodden on.
3. Give portulaca the hot spots that defeat other flowers, such as a strip beside the driveway or a bed against a sunny brick wall.