How to Grow Phlox in Australia

Annual phlox (Phlox drummondii) is a low, bushy flower in the phlox family, Polemoniaceae, grown for bedding, pots, edging and small posies. It gives northern gardens colour through the dry, mild months and southern gardens colour from late spring, usually about 10 weeks from sowing. The step most gardeners get wrong is leaving the seed uncovered, because it needs darkness to germinate.

Three mistakes that matter

1. Leaving the seed on the surface. Phlox seed needs darkness to germinate, so seed scattered and left uncovered often never comes up. Cover it completely with 3 mm of fine soil or seed raising mix, as set out in Sowing Seeds.
2. Moving big seedlings. Phlox dislikes root disturbance, and seedlings teased apart or moved late often wilt and stall. Sow where the plants are to flower, or in single cells, and plant them out small with the root ball whole.
3. Sowing into the heat. Flowering slows in hot weather, and crowded plants in warm, humid conditions catch powdery mildew. Sow inside your zone's window in the table under When to plant, so the plants flower in mild weather.

When to plant

Seed germinates best in soil of about 16 to 24°C. Southern gardens sow from late winter into early summer. Tropical and subtropical gardens sow from autumn through winter into early spring, so plants flower before the wet-season heat and humidity.

Where frost lingers, sow in cells under cover about 6 weeks before the last frost and plant out once it passes. In Canberra and on the tablelands, that is traditionally around Melbourne Cup weekend.

Site, soil and setup

Full sun suits phlox best, although it copes with part shade. It wants free-draining soil enriched with organic matter, and it grows in acid, neutral or alkaline soil, so most gardens need no pH correction. Soil that stays wet brings crown rot.

The organisms behind damping off survive on old plant material in the soil. Do not sow phlox where seedlings collapsed the season before.

Container growing

Pots suit all four varieties, and the 20 to 25 cm 'Twinkle Dwarf Mixture' suits the smallest. Choose a container of at least 10 L with drainage holes, such as a 30 cm pot, which holds about 13.5 L. Bagged potting mix can carry Legionella bacteria: dampen it before handling, wear a mask and gloves, and scrub your hands afterwards.

• Watering: keep the mix evenly moist but never waterlogged, and water at the base of the plants.
• Feeding: container plants often need more feeding than beds. From the first flowers, give fish and seaweed blend at 25 mL per 9 L every fortnight.
• Position: full sun, or a spot with at least 3 hours of direct sun a day.

Sowing Seeds

Sow phlox where it is to flower, or in single cells so each seedling goes out without disturbing its roots.

Sowing direct

• Rake the soil fine and level, then sow the seed 3 mm deep and cover it completely.
• Firm the surface and water gently. Keep the soil moist, never wet or dry.
• Seedlings usually appear in 1 to 4 weeks.
• Thin seedlings to 15 to 25 cm apart once they have their first true leaves.

Starting in cells

• Fill cell trays or small pots with seed raising mix and sow two or three seeds 3 mm deep in each.
• Cover the tray with newspaper until the seedlings emerge, then move it into bright light.
• Snip all but the strongest seedling in each cell at soil level, which leaves its roots undisturbed.
• Harden off the seedlings (stand them outdoors for a few hours, then longer each day, until they take a full day of sun and wind).
• Plant out when the first true leaves appear, keeping the root ball whole. Water in with seaweed solution at half strength, 15 mL per 9 L.

All four varieties are annual phlox, sown and grown the same way; they differ in height, colour and petal shape.

Perennial garden phlox (Phlox paniculata) is a different plant: an upright perennial to about 1 m that flowers from summer into autumn. Moss phlox (Phlox subulata) is a mat-forming rockery perennial that flowers in spring. The sowing and care here are for annual phlox only.

Care by growth stage

Annual phlox is a light feeder: compost dug in before sowing carries it most of the way. Even moisture at the roots and dry leaves keep it flowering and free of mildew.

Where poor soil stalls growth, a little all-purpose controlled release granular (NPK around 14-5-10) at the label rate can restart it.

Deadheading

A plant left to set seed soon dies back, so pinching early and deadheading often keeps annual phlox in flower for longer.

• Young plants: pinch out the growing tip with the first set of leaves, and several flowering side shoots grow in its place.
• Every 5 to 7 days: remove faded flower clusters before seed forms.
• How to cut: snip just above a set of leaves with sharp snips or secateurs.
• End of the season: leave a few heads to ripen if you want seed, then pull out plants once they die back.

Companion planting

Annual phlox pairs best with other sun-loving flowers of a similar season and a taller or lower habit.

Grows well with

• Violas: sown in the same autumn window in the subtropics, they fill the front of a cool-season bed.
• Sweet alyssum: a low edging in front of phlox, sown in many of the same months.
• Celosia and zinnias: summer flowers of the same subtropical sowing list, set behind the lower phlox.

Keep apart

• Sunflowers planted to the north: they throw shade across a plant that wants full sun.
• Pumpkins: their runners and broad leaves roll over low bedding plants.

Harvesting Blooms

Cut stems for the vase when about half the flowers in each cluster have opened.

• Cut the longest stems you can, just above a set of leaves, so side shoots keep flowering.
• Strip the lower leaves and stand the stems in a clean bucket of water straight away.
• Use the short stems of the dwarf varieties for posies and small jars.
• For seed, pick the capsules as they turn from green to light tan, before they split open.

Let the capsules finish drying in an open paper bag, then shake out the seed. Keep it in a named envelope in a cool, dry, dark cupboard. The fragrant clusters of 'Beauty Blue' bring their scent indoors in a small vase.

Common problems

Seed does not come up. Seed left uncovered, the first point in Three mistakes that matter, is the usual cause, followed by soil that dries out or is too cold. Read more

Seedlings collapse at soil level. Damping off (a rot of seedlings at soil level, chiefly caused by Pythium species and Rhizoctonia solani) is most likely in wet soil or mix. Read more

Seedlings grow tall and pale. They are reaching for light, so move trays into full light the day the newspaper comes off. Read more

Plants rot at the base. Crown rot follows poorly drained soil; lift the bed, as set out in Site, soil and setup, or grow phlox in pots.

Plants wilt after planting out. See the second point in Three mistakes that matter.

Pests and diseases

Annual phlox has few serious problems; snails take seedlings, and mildew and mites follow crowding, heat and dry spells.

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They chew seedlings and soft shoots after dark and leave silvery slime trails. Hand-pick them by torchlight about two hours after sunset, set beer traps and put copper bands on pots. Read more

Powdery mildew (Golovinomyces species). White powdery patches spread over the leaves, often from the lower ones up, in crowded plants. Remove the worst leaves and spray 1 part full-cream milk to 10 parts water. Read more

Two-spotted mite (Tetranychus urticae). These tiny sap-suckers leave pale, stippled leaves that yellow and drop, and they build up fast in hot, dry weather. Hose the undersides of the leaves, then use insecticidal soap there if needed. Read more

Grey mould (Botrytis cinerea). Ageing petals turn soft and brown under a grey mould in cool, damp weather. Remove affected flowers promptly and water at soil level.

Prevention first

• Keep to the spacing in Sowing Seeds, so rain and dew dry off the foliage quickly.
• Water the soil, not the leaves, and do it early in the day.
• Use new seed raising mix and washed cells for every sowing.
• Walk the beds with a torch after dark while the seedlings are small.

Tips

1. Grow 'Twinkle Dwarf Mixture' in a pot at eye level or beside a path, where its fringed, striped petals can be seen up close.
2. Plant the fragrant 'Beauty Blue' near a seat, a door or an open window.
3. Phlox can reseed itself, so check your council's weed advice before letting plants drop seed near bushland.