Floss flower (Ageratum houstonianum 'Blue Horizon')
A vibrant annual, Ageratum houstonianum 'Blue Horizon' (Floss flower) offers continuous lavender-blue blooms from June to frost in zones 2-11.
Complete Plant Information
Overview
The Floss flower, scientifically known as Ageratum houstonianum ‘Blue Horizon’, is celebrated by gardeners for its remarkably long season of vibrant color, offering continuous bloom from early summer until the first hard frost. This frost-tender annual, part of the Asteraceae family, produces fluffy, thread-like flowers, giving it the common moniker of Floss flower. Ageratum houstonianum ‘Blue Horizon’ is particularly valued for its stately height among annuals, making it an excellent choice for filling out mid-borders or for use as premium cut flowers.
This reliable bedding plant thrives in average to rich, moist, well-drained soils, preferring the energy of full sun but appreciating some afternoon relief in the hottest regions. Growing between one and two and a half feet tall, this cultivar commands attention with its characteristic lavender to purplish-blue flower clusters. Its relatively low maintenance, combined with excellent pest resistance, secures the Floss flower’s place as an easy-to-grow favorite for season-long texture and hue.
Fast Facts
- Plant Family: Asteraceae
- Plant Type: Annual
- Hardiness Zones: USDA Zones 2-11
- Size at Maturity:
- Height: 1-2.5 ft
- Spread: 0.5-1.5 ft
- Bloom Time: June to frost
- Bloom Description: Lavender to purplish-blue
- Sun Exposure: Full sun to part shade
- Water Needs: Medium
- Maintenance Level: Medium
How to Grow
The Ageratum houstonianum ‘Blue Horizon’ thrives when planted after all danger of frost has passed. Start seeds indoors in late winter for the longest display, or sow directly into the garden after the last frost date, though this shortens the bloom period. Optimal planting involves rich soils that offer excellent drainage, as consistent moisture is required; plants wilt quickly if allowed to dry out completely. Taller varieties like ‘Blue Horizon’ should be spaced approximately 12 inches apart to ensure adequate air circulation, which is crucial to preventing potential mildew issues.
Consistent watering is key throughout the growing season, supplying medium water levels, especially during dry spells, since the Floss flower dislikes drying out. While deadheading the spent blooms is optional, as they typically fall away naturally, tidiness can promote a fuller, uninterrupted succession of flowers. Providing light afternoon shade in very hot summer climates can help the plants maintain vigor and prevent scorching.
As an annual, the structure is generally removed after the first autumn frost kills the plants. However, if you grew Ageratum houstonianum ‘Blue Horizon’ from seed, establishing good air circulation and avoiding overhead watering are the primary preventive measures against the occasional risk of powdery mildew in humid conditions.
Landscape Uses
The versatility of the Floss flower makes it a reliable component in various garden designs. Taller selections, such as Ageratum houstonianum ‘Blue Horizon’, are perfectly positioned in the mid-border where they can stand behind shorter edgers without getting lost. They also serve exceptionally well as cutting flowers, providing that signature mauve-blue tone for indoor arrangements that lasts well after cutting.
For mass plantings, smaller Floss flower varieties create dense blankets of color along walkways or border fronts, offering a soft edge to hardscaping. Consider pairing the deep lavender blooms of the Floss flower with contrasting yellows or whites, such as marigolds or petunias, to maximize visual impact throughout the summer months. Their consistent height and color scheme make Ageratum houstonianum ‘Blue Horizon’ ideal for creating cohesive, repeating color blocks in formal bedding schemes.
Standout Features
Flower Qualities
- Showy
Noteworthy Characteristics
Ageratum houstonianum, commonly called floss flower, is a frost-tender annual of the aster family that provides continuous bloom for beds, gardens or containers from late spring to frost. Features fluffy flowers (basically asters with ray flowers absent) in flattened to slightly rounded clusters atop compact, mounded plants generally ranging from 4” to 18” tall. Some taller varieties (e.G., the triploid ‘Blue Horizon’) are more erect and grow to as much as 30” tall. ‘Blue Horizon’ features fluffy lavender to purplish-blue flowers.
Tolerances
- Rabbit
- Deer
Potential Problems
Ageratum houstonianum ‘Blue Horizon’ is generally robust, avoiding serious insect or disease issues. Gardeners should remain watchful for common pests like aphids and whiteflies, which can usually be managed with physical removal or insecticidal soap. The most frequent concern is powdery mildew, occurring particularly when humidity is high, soil remains too dry, and air movement around the plant foliage is restricted. To mitigate this, ensure proper spacing when planting the Floss flower and water deeply at the base rather than wetting the leaves. In areas with poor drainage, root rot can develop, so soil amendment for drainage is vital before planting.
Frequently Asked Questions
What hardiness zones is Floss flower suitable for?
The Floss flower, Ageratum houstonianum ‘Blue Horizon’, is best suited for USDA Zones 2 through 11, functioning as a dependable annual bedding plant across a vast range of climates.
How large does Ageratum houstonianum ‘Blue Horizon’ grow?
This specific cultivar typically reaches a height between 1 and 2.5 feet tall, spreading moderately between 0.5 and 1.5 feet wide.
When does Floss flower bloom?
Ageratum houstonianum ‘Blue Horizon’ begins blooming in June and continues producing its distinctive lavender to purplish-blue flowers continuously until the first autumn frost arrives.
What sun exposure does Ageratum houstonianum ‘Blue Horizon’ need?
This plant performs best in full sun but will tolerate light shade, often benefiting from partial afternoon shade in regions experiencing intense summer heat.
Conclusion
The Ageratum houstonianum ‘Blue Horizon’ provides outstanding value to the garden through its season-long performance and unique, fluffy texture set against rich lavender-blue coloring. As a reliable annual, the Floss flower brings vertical interest to beds and containers alike. Before planting, ensure your site offers medium moisture and amend for good drainage to guarantee a spectacular show from this esteemed cultivar.
Companion Planting
When designing mixed borders using Ageratum houstonianum ‘Blue Horizon’, look for companions that complement its medium-maintenance needs and similar moisture requirements. Plants that enjoy rich, evenly moist soil and similarly benefit from a bit of afternoon relief, such as Impatiens or Tuberous Begonias, make excellent neighbors in shadier locations. For sunny borders, pair the Floss flower with heat-tolerant annuals that offer complementary textures. Consider combining the fine texture of blue Floss flower plumes with the coarser foliage or vibrant yellow hues of plants like Calibrachoa or Zinnias for striking, high-contrast displays.
Design Ideas
The upright habit of Ageratum houstonianum ‘Blue Horizon’ lends itself perfectly to formal garden designs where symmetry and crisp lines are desired. Plant them in uniform rows along the edge of a perennial border to create a soft, continuous ribbon of blue color that guides the eye. Because they are excellent in containers, cluster them in urns or window boxes, allowing the slightly mounded growth habit to spill softly over the edges, perhaps mixing them with trailing plants for an eye-catching multi-level effect.
