Getting it to bloom
Why won't my Ageratum houstonianum 'Blue Horizon' bloom? (and how to make it flower)
Also called Blue Horizon Ageratum, Cut-flower Floss Flower (Ageratum houstonianum 'Blue Horizon').
More about ageratum houstonianum 'blue horizon'
About Ageratum houstonianum 'Blue Horizon'
Ageratum houstonianum 'Blue Horizon' · also called Blue Horizon Ageratum, Cut-flower Floss Flower · flowering
Ageratum houstonianum 'Blue Horizon' is a tall, cut-flower floss flower bearing dense clusters of fluffy lavender-blue blooms on long, sturdy stems. An F1 hybrid grown as a warm-season annual, it flowers from summer to frost, attracts butterflies, and is prized for bouquets. It needs full sun to part shade, steady moisture and fertile, free-draining soil.
Plant type: flowering
Watch for — Wilting and stalled bloom: Drought stress quickly halts flowering. Keep soil evenly moist and mulch beds to buffer heat and dryness.
The reasons ageratum houstonianum 'blue horizon' isn't blooming
Almost every non-blooming ageratum houstonianum 'blue horizon' traces back to one of these, roughly in order of how common they are:
- Too little sun — most of these need full sun (or very bright light) to flower well; shade gives leaves, not blooms.
- Too much nitrogen feed, driving lush foliage at the expense of flowers (very common with general or lawn feeds).
- The plant has not been deadheaded, so it stops flowering once it sets seed.
- Irregular watering — drought or waterlogging at the budding stage makes buds abort.
- It is still too young or was checked by a transplant and is rebuilding before flowering.
Feeding ageratum houstonianum 'blue horizon' a high-nitrogen general feed and growing it in too little sun — you get a big leafy plant and almost no flowers.
The fix — how to get ageratum houstonianum 'blue horizon' to flower
- Maximise sun. Give ageratum houstonianum 'blue horizon' the sunniest spot you have — for most bedding and fruiting plants, more direct light directly means more flowers.
- Switch the feed. Move off high-nitrogen feeds and use a higher-potassium "bloom" or tomato-type feed as it comes into flower.
- Deadhead regularly. Remove spent flowers often to keep it producing more rather than stopping to set seed.
- Water consistently. Keep moisture even through budding and flowering — drought-then-flood swings make buds drop.
Light and feeding do most of the heavy lifting here. Dial in the spot with the light guide for ageratum houstonianum 'blue horizon' and get the feeding right with the ageratum houstonianum 'blue horizon' fertilising schedule — the wrong feed (too much nitrogen) is one of the most common silent reasons a healthy plant makes leaves instead of flowers.
Bloom season and what to expect
Ageratum houstonianum 'Blue Horizon' flowers across its growing season (mostly summer) and, kept fed and deadheaded, can bloom for many weeks or right up to frost.
Post-bloom care so it flowers again
Deadhead, keep feeding lightly, and many will rebloom; collect seed from the best plants at the end of the season if you want to grow them again.
For everything else this plant needs day to day, see the full ageratum houstonianum 'blue horizon' care brief and its watering schedule — a stressed, badly watered plant rarely has the energy to flower at all.
Ageratum houstonianum 'Blue Horizon' blooming — frequently asked questions
Why won't my ageratum houstonianum 'blue horizon' flower?
Ageratum houstonianum 'Blue Horizon' blooms on the season's growth given enough sun, warmth and the right feed — there is no cold or photoperiod trick, just good growing conditions and a bloom-leaning feed. The most common reason it is not happening: Too little sun — most of these need full sun (or very bright light) to flower well; shade gives leaves, not blooms.
How do I make ageratum houstonianum 'blue horizon' bloom?
Give ageratum houstonianum 'blue horizon' the sunniest spot you have — for most bedding and fruiting plants, more direct light directly means more flowers. Move off high-nitrogen feeds and use a higher-potassium "bloom" or tomato-type feed as it comes into flower.
When does ageratum houstonianum 'blue horizon' normally bloom?
Ageratum houstonianum 'Blue Horizon' flowers across its growing season (mostly summer) and, kept fed and deadheaded, can bloom for many weeks or right up to frost.
What should I do with ageratum houstonianum 'blue horizon' after it flowers?
Deadhead, keep feeding lightly, and many will rebloom; collect seed from the best plants at the end of the season if you want to grow them again.
What is the single biggest mistake stopping ageratum houstonianum 'blue horizon' flowering?
Feeding ageratum houstonianum 'blue horizon' a high-nitrogen general feed and growing it in too little sun — you get a big leafy plant and almost no flowers.
Keep reading
- Ageratum houstonianum 'Blue Horizon' care — the full brief (light, water, humidity, problems, pet safety)
- Ageratum houstonianum 'Blue Horizon' light needs — usually the first thing to fix for flowers
- Ageratum houstonianum 'Blue Horizon' fertilising — the right feed for buds, not just leaves
- Should I water my plant? The simple check
- Why is my plant wilting? Wet vs dry
- Underwatered plant — signs and rehydration
- Why won't my peace lily bloom?
- Why won't my jade plant bloom?
- Why won't my tomato bloom?
- All 2023 bloom guides in the Growli library