How to Grow Vinca in Australia

Vinca, or Madagascar periwinkle (Catharanthus roseus), is a member of the dogbane family, Apocynaceae, grown for flat pink, rose and white flowers above glossy leaves. It copes with heat and dry spells better than most summer annuals, but every part of it is poisonous if eaten. Most failures come from cold soil and too much water, not from drought.

Three mistakes that matter

1. Sowing into cold soil. Seed sits and rots in cool, damp soil, and young plants set out into it stall. Wait until the soil is properly warm, following the cue under When to plant.
2. Watering like a thirsty annual. Soil that never dries out invites root and stem rot, which can kill a bed of vinca in a short time. Let the soil dry well between waterings, as set out in Care by growth stage.
3. Leaving the seed in the light. Vinca seed germinates in darkness, so seed pressed onto the surface often fails to come up. Cover it with 2 to 5 mm of mix or soil, as Sowing Seeds describes.

When to plant

Seed comes up in 4 to 15 days in soil of about 21 to 28°C, and young plants stall in cooler soil. Sow from spring into early summer once frosts are over. In the cool and southern temperate zones, Melbourne Cup weekend is the traditional marker for planting out frost-tender flowers.

At the cool edge of a window, start seed in trays indoors and plant out once nights stay above about 13°C. Late sowings still flower, but plants slow down once autumn nights fall below about 16°C.

Site, soil and setup

Choose the hottest, sunniest spot you have: vinca grows in part shade but flowers far more freely in full sun. Free-draining soil matters more than rich soil, and sand, loam or well-drained clay all suit it. It prefers slightly acidic soil and tolerates slightly alkaline ground where drainage is good.

Root rot organisms live on in soil from one season to the next. Never replant vinca where earlier vinca or petunias collapsed and rotted; see waterlogged soil for fixing a wet bed.

Container growing

Vinca suits pots, window boxes and hanging baskets, and 'Little Mixed', at about 25 cm tall, stays compact enough for small spaces. Allow at least 10 L of potting mix per plant: a 30 cm pot holds about 13.5 L, and it needs drainage holes. Bagged mix can carry Legionella bacteria, so dampen it before opening, wear a mask and gloves, and wash your hands afterwards.

• Watering: wait until the mix is dry a few centimetres down, then soak it until water runs from the base. See watering pots in summer.
• Feeding: potted plants run out of food sooner than beds. From the first flowers, water on fish and seaweed blend at 25 mL per 9 L every fortnight.
• Position: full sun, off saucers, so no water sits around the roots after rain.

Sowing Seeds

Start vinca in trays for the earliest flowers and the most even plants, or sow it straight into warm garden soil.

Starting in trays

• Sow into fresh seed raising mix and cover the seed with 2 to 5 mm of mix.
• Cover the tray with a board or newspaper to keep it dark, and keep it warm; a seedling heat mat helps.
• Keep the mix moist but never wet, and lift the cover daily.
• Take the cover off as soon as shoots show and move the tray into bright light.
• Pot seedlings on or plant them out at 5 to 10 cm tall, once they have true leaves.

Sowing direct

• Wait for warm soil and settled weather, then rake a fine seedbed.
• Sow in small groups at the spacing in the table below, covered as for trays.
• Thin each group to the strongest seedling once true leaves open.

Transplanting

Harden off tray-raised seedlings first (give them a little more time outdoors each day for about a week). Plant them out after the last frost at the depth they grew. Settle each one in with half-strength seaweed solution, 15 mL per 9 L.

Care by growth stage

Vinca lives on warmth and lean, well-drained soil. Far more plants die from too much water than too little, so let the soil tell you when to water.

If growth stalls in poor, sandy soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched into the soil can help.

Deadheading

Spent vinca flowers drop cleanly on their own, so the job is shaping the plant and stopping seed rather than snipping every bloom.

• At planting out: pinch off the top pair of leaves for bushier plants with more flowers.
• Leggy plants: after a flush of flowers, cut stems back to just above a lower pair of leaves.
• Seed pods: pick off the slender pods, which form in pairs, while they are still green.
• Diseased stems: cut out blackened stems and put them in the bin, not the compost.
• Handling: wear gloves, because the stems bleed a milky sap and every part is poisonous, then wash your hands.
• Frosty zones: pull plants out once cold nights stop flowering.

Companion planting

Vinca pairs best with flowers that share its heat, full sun and dry feet.

Grows well with

• Zinnias: another heat lover sown in the same warm months, with taller flowers to stand behind vinca.
• Celosia: wants the same sun and summer warmth, and adds plumes and crests beside vinca's flat flowers.
• Marigolds: sown at the same time, they fill a hot bed edge with the same low, mounded habit.

Keep apart

• Petunias: both are prone to Phytophthora rot, so neither should follow the other in a bed where plants rotted.
• Lettuce and other crops watered daily: the steady moisture they need keeps vinca roots too wet.

Harvesting Blooms

Cut sprigs once most flowers on a stem are open, which usually begins about 12 to 16 weeks from sowing.

• Cut in the cool of the morning with clean snips, just above a pair of leaves.
• Take short sprigs, which suit posies and small jars better than tall vases.
• Strip any leaves that would sit under water and stand stems in water straight away.
• Float single flowers in a shallow bowl for a table.

Keep cut sprigs in a cool room out of direct sun and top up the water daily. Enjoy vinca in jars and posies only: the flowers are not edible and must never be used to garnish food.

Common problems

Bare trays or rows. Cold soil or seed left uncovered in the light explains most failures; see Three mistakes that matter. Read more

Seedlings topple overnight. Damping off (a stem rot of young seedlings, from Pythium, Phytophthora and Rhizoctonia species) strikes where mix stays wet. Read more

Yellow, dropping leaves in spring or autumn. Nights below about 13°C make leaves roll, yellow and fall, so plant later next time and let autumn plants go.

Pale new leaves in summer. Vinca struggles to take up iron in alkaline soil or mix, so check the pH as Site, soil and setup describes. Read more

Few flowers on long stems. Too much shade is the usual cause, because full sun brings the heaviest flowering.

Pests and diseases

Rot in wet soil kills far more vinca than insects do.

Phytophthora stem and root rot (Phytophthora species). Stems blacken near the soil, and plants wilt and collapse, often soon after heavy rain or overwatering. Pull out affected plants with the soil around their roots, bin them and replant elsewhere.

Aphids (family Aphididae). These soft sap-suckers, 2 to 3 mm long, crowd new shoots and leave sticky honeydew. Hose them off, or spray insecticidal soap onto the shoots and leaf undersides. Read more

Whitefly (family Aleyrodidae). Tiny white insects rise in a cloud when the plant is brushed, and leaves yellow. Hang yellow sticky traps nearby, and direct a soap spray at the leaf undersides. Read more

Prevention first

• Water at the base early in the day, so the leaves dry before night.
• Keep to the spacing in Sowing Seeds, so air moves between plants.
• Start every tray sowing in fresh mix and a scrubbed tray.
• Check new shoots and leaf undersides weekly in hot weather.

Tips

1. Vinca has naturalised in eastern Queensland, the NSW north coast, northern and western WA and the Top End, where it is an environmental weed. Pull self-sown seedlings and keep plantings away from bushland and dunes.
2. Plant out in mild weather or late in the day, because young plants establish poorly in extreme heat.
3. Grow vinca where small children and pets will not chew it, and never use it as a home remedy. If anyone eats part of a plant, call the Poisons Information Centre on 13 11 26.
4. In frost-free gardens vinca is a short-lived perennial, so a healthy plant can flower on into a second year.