Growing Ageratum: A complete guide to beautiful blooms

A closeup shot of an Ageratum flower

Ageratum, commonly known as flossflower, is a charming and versatile half-hardy annual that brings a soft, fuzzy texture to gardens.

It has clusters of small, button-like flowers in shades of blue, purple, pink, and white and offers a useful contrast to many other garden plants.

Growing Ageratum is straightforward. It is a low-maintenance plant that blooms for a lengthy period, typically from late spring until the first frost.

Ageratum is particularly well-suited for borders, edging, and container gardens, where its compact growth habit and prolific blooms can be fully appreciated.

Ageratum pairs beautifully with other summer annuals such as marigolds, zinnias, and petunias, creating a vibrant and colourful display. Its ability to thrive in both full sun and partial shade makes it a versatile choice for various garden settings.

All about Ageratum

Ageratum is known for its unique, powder-puff-like flowers that give it a distinctive appearance in the garden. The plant’s leaves and stems are often covered in fine hairs, contributing to its overall soft texture.

Ageratum has a natural mounding habit, making it an excellent choice for creating uniform edges or filling in gaps between taller plants.

Ageratum Hawaii White
Ageratum houstonianum Hawaii White by Photo by David J. Stang is licensed under CC BY-SA 4.0

Key facts

  • Scientific name and synonyms: Ageratum houstonianum (syn. Ageratum mexicanum)
  • Meaning of scientific plant name: Ageratum comes from the Greek ‘a geras’, meaning ‘not growing old’, referring to the long-lasting flowers. Houstonianum comes from William Houston, the Scottish botanist.
  • Common names: Flossflower, bluemink, pussyfoot, Mexican paintbrush
  • Plant family: Asteraceae (daisy family)
  • Place of origin of the plant: Mexico and Central America
  • Type of plant: Annual flowering plant
  • Size: Typically 15-60 cm (6-24 inches) tall and wide, depending on variety
  • Foliage – Description of foliage: Oval to heart-shaped leaves, green to dark green, often with serrated edges and a fuzzy texture
  • Flower – Description of flower and flowering time: Small, fluffy, button-like flowers in clusters, blooming from late spring to first frost
  • Fruit/Seeds: Small, dry achenes with fine bristles, not ornamentally significant
  • Toxicity: Ageratum is considered mildly toxic to pets and humans if ingested in large quantities, due to pyrrolizidine alkaloids present in the plant (according to the ASPCA). All parts of the plant can cause gastrointestinal upset if consumed.

Care requirements

RequirementDetails
Light requirementsFull sun to partial shade
Water and humidity requirementsModerate water needs; keep soil consistently moist but not waterlogged
Temperature requirementsPrefers 18-24°C (65-75°F); hardy in USDA zones 10-11, UK zones H1C-H2
Soil requirementsWell-draining, fertile soil with pH 6.0-7.0
Feeding requirementsMonthly application of balanced, water-soluble fertiliser during growing season
PropagationMainly by seeds, also possible through stem cuttings
PruningLight pinching to encourage bushiness; deadhead spent blooms to promote continued flowering
Ageratum Blue Horizon
Ageratum Houstonianum ‘Blue Horizon by Satin66Flower is licensed under CC BY-SA 4.0

Best Cultivars

  1. ‘Blue Horizon’: Tall variety reaching 60-75 cm, with lavender-blue flowers; excellent for cutting.
  2. ‘Hawaii White’: Compact plant with pure white flowers, growing to 15-20 cm tall.
  3. ‘Blue Danube’: Medium-height variety (30-40 cm) with intense blue flowers.
  4. ‘Red Sea’: Unusual reddish-pink flowers on plants 15-20 cm tall.
  5. ‘Dondo White’: Large, pure white flowers on compact plants, 15-20 cm tall.
  6. ‘Artist Purple’: Deep purple flowers on mounding plants, 20-25 cm tall.
  7. ‘Aloha Blue’: Sky blue flowers on compact plants, 15-20 cm tall.
  8. ‘Tango’: Mix of pink, white, and blue flowers on plants 20-25 cm tall.
Ageratum Artist Purple
Ageratum Artist Purple by Les Serres Fortier is licensed under CC BY 2.0

How to grow Ageratum

Soil and Feeding

Ageratum thrives in well-draining, fertile soil. A mix of garden soil, compost, and perlite can provide ideal growing conditions. Aim for a slightly acidic to neutral pH (6.0-7.0).

Feed plants monthly during the growing season with a balanced, water-soluble fertiliser (e.g., 10-10-10). Avoid over-fertilising, as this can lead to excessive foliage growth at the expense of flowers.

Watering

Maintain consistently moist soil, but avoid waterlogging. Water deeply when the top inch of soil feels dry. Ageratum is somewhat drought-tolerant once established but performs best with regular watering.

Mulching can help retain soil moisture and suppress weeds. Overwatering can lead to root rot, while underwatering may cause wilting and reduced flowering.

Pruning

Pinch back young plants when they reach about 10 cm (4 inches) tall to encourage bushier growth. Regularly deadhead spent blooms to promote continuous flowering and prevent self-seeding.

If plants become leggy, trim them back by about one-third to rejuvenate growth. In late summer, you can cut back plants by half to encourage a flush of autumn blooms.

Propagation

The easiest method to propagate Ageratum is by growing ageratum from seed. Start seeds indoors 6-8 weeks before the last frost date:

  1. Fill seed trays with seed-starting mix.
  2. Sow seeds on the surface, pressing lightly into the soil.
  3. Keep soil moist and warm (around 21°C or 70°F).
  4. Seeds should germinate in 5-10 days.
  5. Once seedlings have several true leaves, transplant them into individual pots.
  6. Harden off seedlings before planting outdoors after the last frost.

Alternatively, take stem cuttings in late spring or early summer, rooting them in a mixture of peat and perlite.

Ageratum purple with small blossoms

Common Problems & Solutions

  • Powdery mildew: White, powdery coating on leaves. Improve air circulation and avoid overhead watering. Apply fungicide if severe.
  • Aphids: Tiny insects clustered on new growth. Remove with a strong spray of water or use insecticidal soap.
  • Root rot: Wilting despite moist soil, often due to overwatering. Improve drainage and reduce watering frequency.
  • Leaf spots: Brown or black spots on leaves. Remove affected leaves and avoid overhead watering. Use fungicide if necessary.
  • Whiteflies: Tiny white insects on leaf undersides. Use yellow sticky traps or insecticidal soap.
  • Leggy growth: Insufficient light or overcrowding. Provide more sunlight and proper spacing between plants.
  • Poor flowering: Often due to excessive nitrogen. Reduce fertiliser and ensure plants receive enough sunlight.
  • Frost damage: Blackened leaves and stems. Protect plants or bring containers indoors when frost is expected.
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