Ageratum is a member of the Asteraceae or Daisy family. The genus includes many perennial and annual species from the tropical regions of the Americas, particularly Mexico. The name Ageratum comes from the Greek ageratos meaning ‘not growing old’: a reference to the flowers which retain their colour for a long time.


Ageratum houstonianum was named after the Scottish botanist and physician, William Houston, who collected seeds in the early eighteenth century. This half-hardy annual, also known as the Mexican paintbrush or floss flower, has long been a popular bedding plant in the UK. More recently, taller varieties have become increasingly grown for cutting.

Floss flowers bloom from July to October, attracting a range of pollinators, especially butterflies. They are included in the 99 best butterfly nectar plants in Dr Margaret Vickery’s Gardening for Butterflies. Several varieties such as ‘Blue Danube’ and ‘Blue Mink’ are listed as RHS Plants for Pollinators.

These vibrant bedding plants are one of only a small number of true blue annuals, although ageratum can also have white, purple and pink blooms. The flower heads are soft and fuzzy with each rounded cluster comprising a mass of tiny florets. It is, however, worth noting that ageratum is toxic and can be harmful to humans, grazing animals and pets if eaten.

How to grow ageratum

Ageratum houstonianum flower in the garden
Ageratum houstonianum. Getty Images

Ageratum needs a sheltered, sunny spot, though it will also tolerate light shade. Providing soil is fertile and well-drained, this versatile annual will grow in acid, neutral or alkaline conditions. Bedding plants can be bought in spring and transplanted into the garden after the last frost. Many varieties are available to buy as seed.

Where to grow ageratum

050713 05072013 05/07/13 05/07/2013 7 7th May 2013 Early summer Kevin Smith Photographer Sarah Cuttle practical projects Container 1 maroon red Solenostemon Coleus annual foliage plant pale blue mauve annual bedding Ageratum Savia bedding sage in glazed pot complementary colours final shot and portraits
Coleus, ageratum and salvia in a container. Photo: Sarah Cuttle

Compact varieties of ageratum such as ‘Dwarf Ball Mixed’ are perfect for the front of borders, containers and rock gardens. They can also be grown in peat-free compost in window boxes and hanging baskets. Plants in pots need regular watering, particularly in hot, dry periods.

Taller ageratum such as Ageratum houstonianum ‘Blue Horizon’ and the perennial species Ageratum petiolatum and Ageratum corymbosum add an airy charm to mixed borders. Both perennial ageratum flower profusely until the first frosts, but need lifting and protecting over winter.

How to plant ageratum

Ageratum does best in fertile, moist but free-draining soil in full sun or partial shade. Plant as you would any other annual bedding.

  1. Ensure you have a suitable spot, preferably in a sunny part of the garden
  2. Submerge the potted plant in water and leave for a few minutes to soak thoroughly
  3. Let plants drain and plant with the top of the rootball level with the soil surface
  4. Firm in and water
  5. Continue watering plants regularly until established

How to care for ageratum

Ageratum houstonianum flower in the garden
Ageratum houstonianum. Getty Images

Ageratum are hardy down to around 1-5°C. They need regular watering in dry periods, particularly when grown in containers. Floss flowers are heavy feeders and plants in pots will benefit from the addition of slow-release fertiliser, or a weekly liquid feed that is high in potassium.

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How to prune ageratum

170712 17072012 17/07/12 17/07/2012 17th 17 July 2012 RHS Tatton Park Show Gardens 18 to 22 2012 and New Plants photographer Jason Ingram Summer perennials Design ideas colour contrast texture mauve annual bedding Ageratum
Ageratum houstonianum. Photo: Jason Ingram

Ageratum does not need pruning, but pinching out the tips of plants at an early stage encourages them to grow sideshoots and bush out. Floss flowers can be deadheaded to encourage prolonged flowering, but some varieties grow quickly enough to cover faded blooms and do not require deadheading.

How to propagate ageratum

050713 05072013 05/07/13 05/07/2013 7 7th May 2013 Early summer Kevin Smith Photographer Sarah Cuttle practical projects Container 1 maroon red Coleus annual foliage plant pale blue mauve annual bedding Ageratum in glazed pot complementary colours final shot and portraits
Ageratum houstonianum. Photo: Sarah Cuttle

Floss flowers are easy to grow from seed. Sow indoors in spring onto moist, peat-free seed compost and only cover thinly as they require light to germinate. Prick out seedlings when large enough to handle and grow on in individual pots. Harden plants off after the last frost and transplant them into their final positions in borders or containers.

Alternatively, take softwood cuttings in summer to provide new plants for the following year.

Pests and diseases

Ageratum have few problems with pests and diseases. Plants can be susceptible to powdery mildew if they are planted too close together or in a shady spot. Always water plants at the base and ensure there is good air circulation, particularly if mildew is an issue.

Advice on buying ageratum

  • Floss flower varieties range in size, so check height and spread
  • For a wider choice, try growing ageratum from seed
  • Check plants over to make sure they look healthy before buying

Where to buy ageratum online

Ageratum varieties to grow

Blue mink (Ageratum houstonianum), close-up
Ageratum houstonianum 'Blue Mink'. Getty Images

Ageratum houstonianum ‘Blue Mink’ – with its glorious powder-puff flowers of the softest blue, this half-hardy annual should continue to bloom until the first frosts. Another relatively compact variety, it is perfect for adding colour and texture to any sunny border.

  • Height x Spread: 30cm x 30cm

Buy Ageratum houstonianum ‘Blue Mink’ from Dobies

Ageratum houstonianum ‘Blue Danube’ – this compact floss flower forms a dense mound topped with large lavender-blue blooms. A fantastic edging plant and ideal for containers. Prefers full sun and soil that is reliably moist but well-drained. ‘Blue Danube’ has received the coveted RHS Award of Garden Merit and is also an RHS Plant for Pollinators.

  • H x S: 20cm x 25cm

Buy Ageratum houstonianum 'Blue Danube’ from Thompson & Morgan

Ageratum houstonianum ‘Blue Horizon’ – this attractive floss flower has purple-blue blooms on long, sturdy stems. Great for cutting.

  • H x S: 75cm x 45cm

Buy Ageratum houstonianum 'Blue Horizon' from Sarah Raven

Ageratum houstonianum ‘Dwarf Ball Mixed’ – a superb variety for containers and as edging for borders. Dense clusters of fluffy blue, pink, lilac and white flowers. Easily grown from seed.

  • H x S: 30cm x 30cm

Buy Ageratum houstonianum 'Dwarf Ball Mixed' from Thompson & Morgan

Ageratum houstonianum ‘Timeless Mix’ – for a colour blend of taller floss flowers, this lovely mix of whites, blues, pinks and purples works well in a pollinator border or as an elegant cut flower.

  • H x S: 50cm x 30cm

Buy Ageratum houstonianum ‘Timeless Mix’ from Suttons