My Everchanging Garden

Gardening That Grows With Me

Growing Ageratum – Both Tall and Short

I grew Ageratum for the first time in 2023. Floss flower has a lovely soft purple colour that blends well with darker purples, deep and light pinks, whites, and silver foliage plants, so it was a great addition to my preferred container colour pallet. Ageratum is a long bloomer with little deadheading required. Deadheading shorter ageratum can be challenging as the flowers are very dense. Sometimes I just shear the plants back.

This is my personal growing guide. I use it to track seed sources, when to start seeds and if I should adjust growing conditions in my own greenhouse and garden. Please don’t consider this professional grower’s advice.


I grew both a tall (Blue Horizon) and a short variety (Aloha Blue) in 2023.

The taller variety looked cute in the centre of containers but tended to flop. I had to stake them once a few blooms opened. I’m wondering if it was the fact that I supplement containers with more fertilizer, and the extra fertilizer caused leggy growth. For 2024, I will try the taller varieties in the landscape instead. I’ll still grow the short Aloha Blue for containers.

Swallowtail Garden Seeds

  • Blue Horizon – flowers sit on top of very tall stems
  • Aloha Blue – 6-8 inches tall with dense flowers on top of short foliage
Hardiness blooms late spring to frost
Exposure full sun to part shade
Deer resistant yes
Bloom time & length Summer to fall,
Deadheading, care cut off blooms as they turn brown
Water & soil conditions dry to medium, drought tolerant
Height Aloha – 8 inches; Blue Horizon 24-30 inches
Spacing Aloha – 6 inches, Blue Horizon 12-18 inches
Fertilizer Likes rich soil but don’t over-fertilize tall ageratum or it will flop.

My own notes:

I tried a single tall Blue Horizon in the centre of some containers. It was just OK – plants tended to fall over and they did not have the wow impact you would want in a focal point. The flowers were more airy in the tall Ageratum.


  • 8 weeks before frost is Mar 25
  • Surface sow, press lightly – do not cover. Needs light to germinate
  • 75-80°F/24-27°C for 5-14 days.
  • Use warm water (70°C) to keep soil temp up.


  • 65-70°F/18-21°C 60-65°F/16-18°C night.
  • Transplant into 4-4.5 in/10-11 cm pots.
  • Set out 1-2 weeks after last frost – planting out too early stunts growth.

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