How to Grow Ageratum in Australia

Ageratum, or floss flower (Ageratum houstonianum), is a warm-season annual in the daisy family, Asteraceae. It is grown for fluffy flower heads in blue, white, pink and red. Low kinds edge beds and pots through the warm months, and the taller 'Red Flint' earns a place in the vase. The most common mistake is burying the seed, which needs light to sprout.

Three mistakes that matter

1. Burying the seed. Ageratum seed needs light to germinate, and seed buried more than 2 mm deep rarely comes up at all. Press it onto the surface and cover it with no more than 1 mm of fine mix, as set out in Sowing Seeds.
2. Planting out into cold weather. Seed sits idle in cool mix, and hard frost damages young plants. Wait until the soil is warm and the last frost has passed, following the months for your zone in When to plant.
3. Letting plants go to seed. Seed heads cost the plant new flowers, and the light seed blows into bushland and creeks, where ageratum is an environmental weed. Cut spent flowers off, following Deadheading.

When to plant

Ageratum seed sprouts best at about 21 to 30°C. Sow after the last frost, or start trays in a warm, bright spot 6 to 8 weeks earlier. Plants usually flower 12 to 14 weeks from sowing. Deadheaded, they keep going for months, so one sowing can cover much of the warm season.

At the cold edge of a window, raise seedlings under cover and hold them there until frosts finish. A late sowing can take over from tired spring plants for autumn colour.

Site, soil and setup

Give ageratum at least 6 hours of sun a day. Where summers are hot, it welcomes a little afternoon shade. It needs free-draining soil rich in organic matter, slightly acid to neutral, around pH 5.5 to 7.0.

Root rot and damping off fungi survive in soil and old plant debris, so plant ageratum in a different bed each year.

Container growing

One plant needs a pot at least 20 cm wide and deep, which holds about 5 L. A group needs 10 L or more. The compact 'Blue Mink', 'Ball White' and 'Ball Mixed' suit pots and window boxes best.

Potting mix can carry Legionella bacteria, so dampen it first, wear a mask and gloves, and wash your hands with soap afterwards.

• Watering: keep the mix lightly moist and never let it dry out; pots need extra water in hot spells.
• Feeding: fish and seaweed blend at 25 mL per 9 L, fortnightly from the first flowers.
• Position: full sun, with afternoon shade in hot inland and northern gardens.

Sowing Seeds

Trays are the surest start: the dust-fine seed is easy to lose outdoors, and it needs light, warmth and moisture together.

Starting under cover

• Fill a clean tray or punnet with fresh seed raising mix and water it well before sowing.
• Sprinkle the seed thinly over the surface and press it in lightly. Cover it with 1 mm of fine mix at most, or leave it bare.
• Water from below or with a fine mister, so the seed is not washed under.
• Keep the tray in a warm, bright place at the temperature in When to plant, moist but never wet.
• Expect seedlings in 1 to 2 weeks.
• When the first true leaves open, prick out the seedlings (move each into its own cell or small pot).

Sowing direct

In warm soil after the last frost, scatter pinches of seed on raked, damp soil and press them in without covering. Mixing the seed with dry sand shows where it has landed, as sowing tiny seeds explains. Thin to the spacing under Transplanting.

Transplanting

• Seedlings are ready at 5 to 10 cm tall, usually 6 to 8 weeks from sowing.
• Harden off the plants over one to two weeks (get them used to sun and wind by leaving them outdoors a little longer each day).
• Set each plant no deeper than it sat in its cell.
• Space plants 30 cm apart, or 40 cm for 'Red Flint'.
• Water them in with seaweed solution at half strength, 15 mL per 9 L.

Care by growth stage

Steady moisture and full warmth decide how long ageratum flowers. Feeding stays light, because a well-prepared bed carries most of the load.

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

Deadheading

Removing old flowers keeps new ones coming and stops the seed that lets ageratum spread.

• How to cut: snip each faded cluster just above a pair of leaves with sharp snips or secateurs.
• How often: go over the plants regularly while they flower.
• After a flush: trim the whole plant back lightly to bring on new growth and more flowers.
• Young plants: pinch out the growing tips for bushier plants; pinch 'Red Flint' at about 20 cm tall.
• End of the season: pull plants out before any seed ripens, lifting the roots too, because stems snap and the roots regrow.

Companion planting

Ageratum suits a sunny bed of other warm-season annuals.

Grows well with

• Sweet alyssum: a lower edge in front that suits the same sun and season.
• Petunias: they flower through the same warm months in the same sunny beds.
• Marigolds: a classic bedding partner, with yellow and orange beside the blue.
• Zinnias: tall kinds such as 'Violet Queen', at 70 to 100 cm, stand behind low ageratum.

Keep apart

• Tall sunflowers such as 'Giant Single', at 180 cm, on the north side: they shade low ageratum.
• Beans: two-spotted mite breeds on both, so mites can move between them.

Harvesting Blooms

Cut stems once the central cluster of flowers has opened and the side buds are showing colour.

• Cut just above a pair of leaves with clean snips, so side shoots follow.
• Picking works like deadheading, so regular cutting keeps flowers coming.

'Red Flint' stems cut at this stage can last up to three weeks in a vase. They also dry well and hold their colour for winter arrangements.

Common problems

No seedlings appear. Seed buried too deep is the usual cause (see Three mistakes that matter), followed by cool or dried-out mix. Read more

Seedlings topple over. Damping off (a seedling rot at the soil line, mostly from Pythium species and Rhizoctonia solani) thrives in wet mix. Read more

Plants wilt and collapse in moist soil. Root rot from soil fungi browns the roots and crown, so lift and discard sick plants and improve the drainage.

Flowering slows in midsummer. Extreme heat holds flowering back, so water steadily and give afternoon shade in hot zones.

Seedlings come up unsown. Seed has ripened and spread (see Three mistakes that matter), so pull them before they flower.

Pests and diseases

Mites and whitefly build up in warm, dry weather, while moulds follow damp, crowded plantings.

Two-spotted mite (Tetranychus urticae). Leaves turn yellow, mottled or stippled, starting on the undersides, and fine webbing can appear. Spray insecticidal soap under the leaves twice, 7 to 10 days apart. Read more

Greenhouse whitefly (Trialeurodes vaporariorum). Tiny white, moth-like adults gather under the leaves, and their sticky honeydew grows black sooty mould. Hang yellow sticky traps, and spare the ladybirds and parasitic wasps that attack them. Read more

Grey mould (Botrytis cinerea). Ageing flowers turn papery brown under fuzzy grey mould, mostly in cloudy, humid, wet weather. Remove spent flowers and keep water off the blooms.

Powdery mildew (fungi of the order Erysiphales). White spots spread across the leaves until they coat them. Remove the worst leaves and spray 1 part full-cream milk to 10 parts water. Read more

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They eat seedlings and hole leaves after dark, especially after rain. Set shallow dishes of beer as traps, and use copper tape on wooden or plastic pots and beds. Read more

Prevention first

• Keep to the full spacing in Sowing Seeds, so air moves between plants.
• Water the soil, not the flowers, and do it early in the day.
• Check leaf undersides often in hot, dry weather for mites and whitefly.
• Avoid broad-spectrum sprays, which kill the predators that control mites.

Tips

1. Plant a clump near the vegie patch, where the flowers bring in bees and butterflies.
2. Store leftover seed sealed, cool and dry; it usually stays viable for about four years.