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

Description and botany

Ageratum houstonianum (syn. mexicanum) Ageratum houstonianum, also called “Mexican ageratum”, “blue ageratum” or “celestine”, is a perennial herbaceous plant, not very hardy, cultivated as an annual in the Asteraceae family, native to Central America, Mexico, Peru and the Caribbean.

The genus includes more than 40 species, of which Ageratum mexicanum is the most widely grown. Ageratum conyzoides, known as “woman’s herb”, is an aromatic species long used in traditional African and Réunionnais medicine for its many medicinal virtues, notably anti-rheumatic and antispasmodic properties.

It is available in numerous varieties whose flowers are most often blue to violet such as ‘Blue Ball’, ‘Blue Mink’ or ‘Danube Blue’, sometimes white (‘White Hawaii’), or pink to red (‘Faity Pink’ and ‘Royal Hawaï’). F1 hybrids are also available.

Ageratum quickly forms a handsome, more or less dense clump, very ramified, rounded and bushy. Habit is both broad and leafy. The smallest varieties, such as hybrids derived from the ‘Hawai’ 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 deciduous foliage that is also villous, which may persist if winter remains mild. Leaves 2 to 9 cm long, covered with a fine velvet, are opposite, ovate, sometimes heart-shaped, with dentate margins and a puckered appearance. Colour ranges from light green to dark or bluish green.

Ageratum ‘Old Grey’

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

Flowers appear in small, bright heads 2 to 5 cm in diameter. Ray florets are very closely packed around a velvety centre, sometimes white, and characterised by a delightfully feathery texture, making them resemble small tufted daisies or fluffy pompons. These mossy ball flowers display all shades of blue, from lavender-blue to powder blue, through bluish-violet and purple. Some varieties produce white or mauve pompon flowers (‘Royal Hawaï’) or even reddish purple.

These bouquets of flowers, which renew continuously right up to the onset of winter, are nectariferous; frequented by butterflies, bumblebees and bees, they encourage pollination.

Fruits are small achene, barely visible, turning black at ripeness and containing seeds that rarely self-sow under climates that are too cold for them.

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

Very frost-tender, Ageratum is damaged by frosts from -5°C and can only overwinter outdoors in mild climates and coastal gardens. It is a perennial usually grown in our gardens as an annual. It prefers full sun in a humus-rich, light, well-drained soil that remains cool through the season.

Mexican ageratum is indispensable in naturalistic gardens and romantic gardens, to which it always brings a bright, colourful note.

These compact, very floriferous clumps are perfect for creating charming scenes in the foreground of annual flower beds and for flowering borders, window boxes and pots 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 blue-flowered Ageratum. Small varieties are perfect for borders, while larger ones are ideal for structuring the front of beds and composing country-style bouquets. F1 hybrids are also very common in cultivation.

Most popular

Ageratum Blue Danube F1

A vigorous variety, covered with soft lavender-blue flowers. Perfect for decorating the 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

A timeless, must-have variety! Its long flowering and ease of cultivation make it an ideal annual for structuring borders, containers and the front of flowerbeds.

• 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 ageratum?

From its South American origins, Mexican Ageratum has retained a strong sensitivity to cold. This perennial not very hardy, not tolerating temperatures below -5°C, will most often be grown as an annual in ground or in a pot in our climates. In regions with mild winters or by the sea, it can remain in ground all year round.

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

It allows quick composition of country-style scenes in every corner of garden, to which it brings lightness. Tall varieties will structure beds and borders.

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

When to sow and plant ageratum?

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

How to sow ageratum seeds?

• Sow ageratum seeds in buckets filled with a good seed compost
• Cover seeds with a fine layer of compost
• Press down lightly
• Place sowing under greenhouse or in a polyethylene bag
• Keep substrate just moist until germination at a temperature of 20-30°C and in light
• Thin seedlings then pinch tips of stems regularly to encourage plant to ramify
• When seedlings have strengthened, transplant them into larger pots or directly to garden when all risk of frost is removed

Ageratum sowing

How to plant ageratum?

In ground

Space young plants 20 to 30 cm apart according to varieties. To flower well, Mexican ageratum will appreciate a light soil: in soils that are too compact, improve drainage with a few handfuls of river sand. For a good massed effect, plan 5 to 8 plants per m² according to size at ripeness.

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

In pot or in planter

• Space seedlings 15-20 cm apart
• Spread a draining layer at bottom of container
• Plant in a mix of potting compost and compost
• Water generously
• Before moving pots outside, wait until all risk of frost has passed and water regularly in hot season

Discover our step-by-step guide to plant annuals correctly!

Care and maintenance

Ageratummexicanum is an annual plant that can be very floriferous without demanding much care provided soil remains perfectly drained and cool in summer.

To flower well, it must be watered regularly, especially in dry weather: water without waterlogging the soil so it never dries out completely. Excess moisture causes root rot.

Mulch to retain moisture at the base and reduce watering.

In pots, water more frequently: avoid letting water stagnate in saucers. Add liquid fertiliser for flowering plants to the watering water throughout growth period.

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

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

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

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

Diseases and potential pests

Seldom affected by disease, Ageratum can, however, become infested with red spider mites and aphids when soil is too dry: spray water mixed with black soap.

Ageratums grown indoors in conservatories are frequently invaded by whiteflies (also called whitefly), especially when air is too dry: maintain a sufficiently humid, well-aerated atmosphere. To get rid of them: at first signs of attack, spray a soapy solution made from black soap mixed with vegetable oil or nettle manure.

Young foliage is vulnerable to slugs and snails: discover our advice on how to fight their attacks!

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

Multiplication

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

Taking cuttings of ageratum

1. In September, cut 10 cm sections of stem without flowers
2. Remove lower leaves from stems
3. Plant in individual pots of turf and sand
4. Place in sealed transparent plastic bag
5. Keep away from frost
6. Plant out in spring, after frosts, when cuttings have developed sufficient roots

Pairing ageratum

In the garden, the silhouette both light and bushy of Ageratum or Mexican ageratums allows many uses and pairings. This pretty annual plant thrives in gardens of all styles for creating wild-looking scenes in a natural garden, to bring colour and softness to a romantic garden or a pink garden in mixtures with other annuals and summer-flowering perennials.

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

Its pompom flowers most often display incomparable shades of blue; it is a valuable plant in a blue garden.

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

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

Its colours, both tender and deep, will make grey-foliaged plants such as nepetas or artemisia 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 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.