How to grow Ageratum houstonianum

Ageratum houstonianum · flossflower, bluemink, Mexican paintbrush

Ageratum (Ageratum houstonianum) — a mounded annual 15–30 cm tall with fluffy clusters of blue, pink or white flowers from late spring until frost. Ageratum grows best in full sun and soil with pH 5–6. Water it about every 7 days while it grows.

Updated: November 4, 2026

When to sow ageratum?

Sow for seedlings about 8 weeks before planting out.

Plant out in open ground about a week after your last spring frost, once the soil is warm.

It flowers about 85 days after the shoots appear and keeps flowering for about 120 days.

Frost dates · When to plant

Quick facts

Type	Flower
Latin name	Ageratum houstonianum
Family	Asteraceae
Also called	flossflower, bluemink, Mexican paintbrush
Planted as	seed
How it is started	from seedlings
Life span	grown for one season
Planting season	spring
Light	Full sun
Soil pH	5–6
Frost	does not tolerate frost
Soil for sowing	from 15 °C
Germination	8 days
Pricking out	14 days after the shoots appear
Seedlings ready	48 days
First flowers	85 days after the shoots appear
Flowering lasts	120 days
Grown in	Open ground, At home

How long does ageratum take from sowing to flowering?

From sowing to the first flowers: about 93 days, or 13 weeks.

Step	Day
Sowing	0
Shoots appear	8
Pricking out	22
Seedlings ready to plant out	56
First flowers	93
End of flowering	213

Days count from the day of sowing or planting, by the figures of the card.

Ageratum care by growth stage

Open ground

Stage	Watering	Temperature	Feeding
Seedlings	every 2 days	—	—
Growth	every 7 days	—	—
Flowering	every 7 days	—	balanced (NPK), at the start of the stage

At home

Stage	Watering	Temperature	Feeding
Seedlings	every 2 days	—	—
Growth	every 2 days	—	—
Flowering	every 2 days	—	balanced (NPK), at the start of the stage

Growing notes

Light

Full sun; where summers are hot it tolerates partial shade.

Soil

Fertile, moist, well-drained soil with a pH of 5.0–6.0.

Care

Sow 6–8 weeks before planting out, pressing the seed onto the surface without covering it, because light helps germination; 24–27 °C suits it. Direct sowing is not recommended. Move the seedlings to cell packs when the first true leaves appear, harden them off and plant out after the last frost, 23–30 cm apart. No pinching or staking is needed. Keep the soil moist to prolong flowering, and feed once, 6–8 weeks after planting, if growth looks weak. Watch for powdery mildew.

Other plants of the Asteraceae family

Ageratum belongs to Asteraceae; 20 other plants of this family have a guide here.

Plant lists it is in

What to grow on a windowsill or balcony

Part of: Garden flowers · Annual flowers from seed · Long-flowering plants · Flowers to start from seedlings

Sources

Figures checked against these sources.