<em>Ageratum</em>, Mexican ageratum: sowing, planting, care

Description and botany

Ageratum houstonianum (syn. mexicanum), also called “Mexican ageratum”, “Blue ageratum” or “Flossflower”, is a slightly hardy herbaceous perennial grown as an annual of family Asteraceae, native to Central America, Mexico, Peru and the Caribbean.

The genus includes more than 40 species, the most cultivated being Ageratum mexicanum. Ageratum Conyzoidess, known as “woman’s herb”, is an aromatic species long used in traditional medicine in Africa and on Réunion for its many medicinal properties, notably anti-rheumatic and antispasmodic.

It occurs in many varieties whose flowers are most often blue to violet such as ‘Blue Ball’, ‘Blue mink’ or ‘Danube Bleu’, sometimes white (‘White Hawaii’), or pink to red (‘Faity pink’ and ‘Royal Hawaï’). F1 hybrids also exist.

Ageratum quickly forms a handsome, more or less dense clump, very ramified, rounded and bushy. Habit is both wide and bushy. The smallest varieties, such as hybrids derived from the ‘Hawaï’ cultivar, are dwarf and do not exceed 0.15 m in height, while the taller ones form erect clumps up to 0.60 m high with a spread of up to 40 cm.

Stems are pubescent, both erect and spreading. They bear a deciduous, villous foliage that persists if winter is mild. Leaves 2 to 9 cm long, covered with a fine velvety layer, are opposite, ovate, sometimes heart-shaped, with dentate margins and a puckered appearance. Colour ranges from light green to dark green or a rather dark bluish green.

Ageratum ‘Old Grey’

These dense leaf cushions are almost entirely hidden beneath the long flowering from spring to early autumn, sometimes until first frosts. Numerous small downy heads, gathered 30 to 50 together in terminal corymbs, open profusely above the foliage. This flowering lasts so long that it gave the plant its name ageratum, from Greek meaning “that does not age”.

Flowers appear as small bright heads 2 to 5 cm in diameter. The tightly packed ray florets, gathered around a velvety centre, sometimes white, have a delightfully feathery texture, making them resemble small pompom daisies or ruffled pompons. These mossy-ball flowers come in every shade of blue, from lavender blue to powder blue through bluish-violet and purple. Some varieties offer white or mauve-pink pompoms (‘Royal Hawaï’) or even purplish-red blooms.

These flower clusters, which renew continuously right up to the onset of winter, are nectariferous; visited by butterflies, bumblebees and honeybees, they encourage pollination.

Fruits are small achenes, barely visible, turning black at ripeness and containing seeds that rarely self-sow under our cooler climates.

Flowers of long-stemmed Mexican ageratum varieties have excellent vase life and make generous country-style bouquets with very fresh colours.

Very little hardy, ageratum is damaged by frost from −5°C and can only overwinter outdoors in mild climates and seaside gardens. It is a perennial most often grown as an annual in our gardens. It grows in full sun in humus-bearing, light, well-drained soil that stays cool throughout the season.

Mexican ageratum is indispensable in natural gardens and romantic gardens, where it always brings a bright, colourful note.

These compact, very floriferous clumps are ideal for charming foreground scenes in annual flower beds and for flowering borders, window boxes and containers on terraces or balconies.

Close-up of an Ageratum inflorescence

Main species and varieties with blue flowers

There are more than 40 species, including many small Ageratum with blue flowers, perfect for borders, and larger forms ideal for structuring bed foregrounds and creating cottage-style bouquets. F1 hybrids are also very common in cultivation.

Most popular

Ageratum Blue Danube F1

A vigorous variety, covered in soft lavender-blue flowers. Perfect for decorating front of beds, borders and pots!

• Flowering time July to October
• Height at maturity 20 cm

See the product sheet

Ageratum houstonianum Blue Mink - Bluemink

An established must-have variety! Its long flowering and ease of cultivation make it an ideal annual for structuring borders, containers and the foreground of beds.

• Flowering time July to October
• Height at maturity 30 cm

See the product sheet

Our favourites

Ageratum houstonianum Timeless Mixed - Bluemink

A selection of fairly tall Ageratum, whose fluffy flowers display three shades. Ideal for adding height to borders or for creating summer bouquets.

• Flowering time July to October
• Height at maturity 45 cm

See the product sheet

Sowing and planting Ageratum

Where to plant Ageratum or Mexican agerate?

From its South American origins, Mexican Ageratum has retained a marked sensitivity to cold. This perennial, not very hardy, unable to tolerate temperatures below -5°C, is most often grown as an annual in open ground or in a pot in our climate. In regions with mild winters or by the coast, it can remain in open ground all year.

It needs a very sunny position to flower abundantly and a light, well-drained soil, preferably calcareous. A good humus-bearing soil that stays cool and fresh will suit it perfectly. It dislikes overly dry soils and stagnant moisture.

It makes it easy to create country-style scenes in every corner of the garden, adding a touch of lightness. Tall varieties will structure beds and borders.

Dwarf varieties find their place in very sunny rockeries, pots and planters on a terrace or a balcony.

When to sow and plant Ageratum?

Ageratum seeds are sown under cover in warmth (15-18°C) from February to April, for pricking out into garden in May. Ageratum in buckets are planted in spring, generally in May, once frosts have definitely passed. Follow our expert advice in our guide to succeed with sowing annual seeds!

How to sow ageratum seeds?

• Sow the ageratum seeds in buckets filled with a good seed compost
• Cover seeds with a fine layer of compost
• Press lightly
• Place the sowing under a cloche or in a polythene bag
• Keep substrate just moist until germination at 20-30°C and in light
• Thin seedlings then pinch regularly the tips of the stems to encourage the plant to branch
• When seedlings are sturdy, prick them out into larger pots or plant out directly into garden when all risk of frost has passed

Sowing Ageratum

How to plant Ageratum?

In open ground

Space young plants 20 or 30 cm apart depending on variety. For good flowering, Mexican agerate will appreciate a light soil: in soils that are too compact, improve drainage with a few handfuls of river sand. For an impressive massed effect, allow 5 to 8 plants per m² depending on size at ripeness.

• Dig a hole two to three times wider than the rootball
• Spread a layer of gravel or clay pebbles at the bottom of the hole
• Plant the young ageratum
• Backfill with garden soil mixed with a little compost
• Firm gently and water well

In a pot or planter

• Space seedlings 15-20 cm apart
• Place a draining layer at the bottom of the container
• Plant in a compost-soil mix
• Water thoroughly
• Before putting pots outside, wait until risk of frost has passed and water regularly in hot weather

Discover our step-by-step guide to planting annuals successfully!

Care and Maintenance

Ageratumm mexicanum is an annual plant that is very floriferous without requiring much care, provided soil remains well drained and cool in summer.

To flower well, water regularly, especially in dry weather: water without soaking soil so it never dries out completely. Excess moisture causes crown rot at the plant base.

Mulch to retain moisture at the base and reduce frequency of watering.

In a pot, water more often: avoid leaving water to stagnate in saucers. Add liquid fertiliser for flowering plants to watering water throughout growth period.

Remove faded flowers regularly to encourage new flowering and pinch stems regularly to promote branching.

In regions with mild winters, cut clumps back to soil level after flowering then spread a dry mulch (dry leaves) at the base to protect from frost.

In colder regions where it is grown as an annual flower, at end of autumn, lift clumps of ageratum that will not survive severe frosts.

If you want to try to keep this frost‑tender perennial, bring pots in at first frosts into a bright conservatory or a lightly heated greenhouse. You can put them back out in May as soon as temperatures warm, bearing in mind that ageratum will be more vulnerable to parasitic pests when grown indoors.

Potential diseases and pests

Rarely ill, Ageratum can nevertheless be infested with red spider mites and aphids when soil is too dry : spray with water mixed with black soap.

Ageratum plants grown indoors in conservatories are often invaded by whiteflies, especially when atmosphere is too dry: keep atmosphere sufficiently humid and well ventilated. To get rid of them: at first signs of infestation, spray a soapy solution made with black soap mixed with vegetable oil or nettle manure.

Its young foliage is vulnerable to slugs and snails: discover our tips to combat their attacks!

In overly wet soil, plant roots can rot: good drainage is essential to prevent disease.

Propagation

Mexican ageratum is a frost-tender perennial that can be multiplied by propagation by cuttings, although success is not always guaranteed, we recommend instead raising young plants each year from sowing with our ageratum seeds (follow our advice above).

Taking ageratum cuttings

1. In September, cut 10 cm lengths of stem without flowers
2. Remove lower leaves from stems
3. Plant in individual pots of turf and sand
4. Enclose in a clear plastic bag to retain humidity
5. Keep protected from frost
6. Plant out in spring, after frosts, when cuttings have sufficient roots

Companion planting with Ageratum

In the garden, the light, bushy silhouette of Ageratum or Mexican ageratumallows many uses and combinations. This pretty annual plant thrives in gardens of all styles to create wild-looking scenes in a natural garden, to bring a touch of colour and softness to a romantic garden or a pink garden in mixes with other annuals and summer-flowering perennials.

One idea for a combination: white Ageratum (or blue, such as ‘Horizon Bleu’, for example), Antirrhinum majus (such as ‘Royal Bride’), dwarf dahlia ‘Gallery Art Fair’ and Rudbeckia (‘Goldsturm’ or ‘Early Bird Gold’)

Its tufted flowers most often display incomparable shades of blue, making it a prized plant in a blue garden.

In a very fresh composition, the large heads of ornamental Alliums, cleomes, cosmos and an Ammi visnaga or toothpick plant will rise majestically above dense, velvety cushions of Mexican ageratums.

Blue flowers of ageratum will contrast with dandelion-yellow blooms of marigolds, nasturtiums, coreopsis, annual rudbeckias or achilleas in blue/yellow combinations.

Its colours, both soft and deep, will make grey-foliaged plants such as nepeta or artemisia really stand out.

In a border or rockery, plant it with other low-growing plants such as salvia, lantanas, nemesia, alyssum, Peruvian heliotrope and marigolds.

In a pot, it is the classic companion of begonias, fuchsias, bacopas, petunias, lobelias and impatiens.

An idea for a pot combination: Ageratum ‘Blue Mink’, Helichrysum petiolare ‘Silver’ and Petunia ‘Cascadias Rim Magenta’

Flowering of asters and Alstroemeria will accompany it into autumn.