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 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
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.
- Ensure you have a suitable spot, preferably in a sunny part of the garden
- Submerge the potted plant in water and leave for a few minutes to soak thoroughly
- Let plants drain and plant with the top of the rootball level with the soil surface
- Firm in and water
- Continue watering plants regularly until established
How to care for ageratum
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.
More like this
How to prune ageratum
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
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.
Ageratum varieties to grow
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
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
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
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
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