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Flossflower

Flossflower

Ageratum houstonianum
A compact, frost-tender warm-season annual from Mexico and Central America grown for its dense, fuzzy, thread-like ("flossy") flower clusters — most famously in soft true blue and lavender-blue, the trait that sets it apart from almost every other bedding annual, plus pink and white forms. It mounds into a clumping cushion 6-12 inches tall and about as wide, blooming from early summer until frost over soft, heart-shaped, slightly hairy leaves. It is easy and rewarding bedding, edging, and container color in full sun to part shade with steady moisture, but it is strictly look-don't-eat: the foliage and stems contain hepatotoxic pyrrolizidine alkaloids and are toxic if ingested.
Climate fit: moderate (60/100)
Border
Container
Filler
Light
Full sun / Part shade
Water
Consistent moisture
Mature size
6-12" tall · 10" apart
Lifecycle
True annual (one season)
Native in Illinois
No

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Strictly an ornamental and never for eating: the foliage and stems of Ageratum houstonianum contain hepatotoxic pyrrolizidine alkaloids, which damage the liver if ingested, so it is toxic to people and animals and should be treated as inedible.

Cold hardiness

These values are location-based: this location's current hardiness is the baseline, and the 2050 value is a projected future climate for this same location.
Now
Zone 6b
Plotwright
USDA Zone 6b
-5°F to 0°F
Well-suited
Zone 7a
Plotwright
0°F to 5°F
Well-suited
In plain terms: This location has cold winters. Its winters are projected to keep warming through 2050.
Well-suited today and still thriving in 2050.

Heat tolerance

Heat tolerance values are location-based too: heat days today are observed at this site, and the 2050 value projects this same location under a future climate.
Loading AHS heat-zone data for this location...

Similar plants

Browse lateral options with similar roles, light needs, size, or native-range overlap; these are not filtered for a better climate fit.
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Moderate water
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Low water
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Consistent moisture
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Climate: moderate
Border
Container
Filler
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Perennial
Full sun / Part shade
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Border
Filler
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Sources & citations

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For lesson plans, articles, or research that uses this page. To cite a single upstream fact instead, use its specific source listed below.
Plotwright. (2026, May 17). Flossflower (Ageratum houstonianum). Retrieved 2026, June 24, from https://plotwright.com/plants/ageratum-houstonianum
Sources for every fact
Every fact on this page traces to a source. 18 fields cited - 18 source-backed.
NC State Extension Gardener Plant Toolbox
University extension service
Backs 17 fields
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Hardiness
Heat zone
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