Botanical Name: Ageratum houstonianum ‘Blue Horizon’ 

Common Name: Floss Flower 

Hardiness Zone: Grown as an Annual in Most Climates (USDA Zones 2–11) 

Size: 18–24" Tall × 18–24" Wide 

Growth Habit: Upright, Well-Branched, Full Habit 

Sunlight: Full Sun to Partial Sun 

Soil: Well-Drained, Moderately Fertile Soil 

Water Needs: Moderate; Consistent Moisture Encourages Best Blooming 

Bloom Season: Late Spring Through Frost 

Fertilizer: Light, Regular Feeding During the Growing Season 

Features: Clear Blue Flower Clusters, Tall Stems, Strong Garden Presence, Pollinator Friendly 

Uses: Excellent for Annual Beds, Mixed Borders, Pollinator Gardens, and Large Containers 

Patent: Not Patented 

Propagation: ✔ Propagation Allowed

More About Ageratum ‘Blue Horizon’

Blue Horizon is the variety that finally changed my mind about using Ageratum for height. For years, I thought of Ageratum as a front-of-the-bed plant—useful, reliable, but always secondary. Blue Horizon breaks that mold completely. It grows taller, branches strongly, and produces those soft, airy blue flower clusters that float above the foliage instead of sitting tight against it. The effect is light and open, almost cloud-like, and it adds vertical interest without feeling stiff or top-heavy. The height feels purposeful, and the stems are strong enough to support the blooms even when the plant is fully grown.

Blooming begins in late Spring and continues steadily until the first frost, with no real gaps once it gets going. The flower heads are generous and open, which pollinators seem to love. Bees and butterflies are constant visitors, especially during the heat of summer when other plants start to slow down. It’s a great option when you want blue in the middle or back of a bed without relying on perennials or taller annuals that can feel heavy by midsummer.

Why We Like It (Our Trials)

In our testing, ‘Blue Horizon’ impressed us with its structure as much as its bloom. The stems stayed upright through heat and humidity, and the plants held a full, balanced shape without collapsing or thinning out at the base. With regular watering and light feeding, it never showed the legginess or floppiness that older, tall ageratum types were known for.

What really stands out to me is how versatile it is once you start designing with it. It slips easily into pollinator gardens, mixed annual beds, and even more naturalistic plantings where you want height without rigidity. The flowers add movement, the color stays soft and usable, and the plant doesn’t overwhelm its neighbors. When I need a blue annual that brings height, airiness, and pollinator value without turning into a maintenance problem, ‘Blue Horizon’ is an Ageratum that we will carry season after season, no matter what new varieties come out.

‘Blue Horizon’ vs. Other Ageratum

Compared to other compact Ageratum like Aloha Blue and traditional edging types, ‘Blue Horizon’ delivers significantly more height, broader spread, and larger, airier flower clusters that create real presence in the landscape. Compared to Aloha White, it delivers the same increase in scale but with a softer, more diffuse effect in the landscape, where White feels crisp and defined while ‘Blue Horizon’ adds movement and openness.

It's main rival, Monarch Magic™, was bred primarily for maximum pollinator attraction with bold, elevated blooms, but ‘Blue Horizon’ strikes a balance; upright structure, strong branching, and a lighter, more open flower display that reads beautifully from a distance. For gardeners who want a Floss Flower that adds vertical interest, steady color, and pollinator activity, ‘Blue Horizon’ is an excellent choice.