The genus Ageratum is comprised of ≈29 species in the Asteraceae that are native to the Americas (Johnson, 1971). Ageratum houstonianum (commonly called ageratum or floss flower) is the predominant commercially grown species and is marketed and sold primarily as a spring bedding plant and to a lesser degree as a cut flower (Stephens, 2006). Floss flower can be a short-lived perennial in frost-free areas (USDA cold hardiness zones 9 and warmer) and in practice is grown as a summer annual. Ageratum does well in part to full sun and in a range of soil types with moderate fertility and moisture. Most bedding plant cultivars have a compact plant habit (typically growing 10 to 25 cm tall) with capitulums held close to the foliage. Cut flower cultivars have been selected for taller-growing plants (typically growing 40 to 60 cm tall) with strong stems and longer internodes that more prominently elevate and display the inflorescences.
Ageratum houstonianum is native to Mexico and Central America and has escaped cultivation and naturalized in multiple warm regions of the world (Johnson, 1971; Stephens, 2006). Wild collections and most cultivars are diploid (2n = 2x = 20), whereas some cultivars are polyploid (Johnson, 1971; Sakata® Ornamentals, 2013; Stephens, 2006). Sporophytic self-incompatibility is present in this species and is the result of a single locus with a linear dominance series of alleles (Stephens et al., 1982). Self-incompatibility and also nuclear controlled male sterility provide breeders with effective tools to facilitate controlled crosses (Stephens, 2006) and are particularly useful because individual florets are very small and tedious to emasculate.
Ageratum houstonianum and its relatives produce terminal inflorescences with capitula containing only disk florets and are typically arranged in a compound cyme. Disk florets in a single capitulum open in concentric rings from the perimeter to the center over ≈1 week. The corolla of an individual floret is funnelform with typically a white base and colored tip. The corolla ends in five deltoid lobes and the adaxial and abaxial sides are typically similar in color. However, in some genotypes, lobe color can be markedly different on either side leading to a bicolored effect to the capitulum between open and unopened florets. The style is prominent and forked and typically the same color as the adaxial side of the corolla lobes. Plants produce oppositely arranged ovate, deltoid, or slightly cordate leaves on vegetative growth and transition to alternate arrangement as the stem produces reproductive tissue.
Ageratum houstonianum is generally easy to grow, floriferous, and it does not typically have significant pest issues, all of which contribute to its popularity. Currey et al. (2011) list several annual bedding plant species categorizing them for floral induction, and the ageratum cultivars surveyed were all facultative long-day plants and supplemental irradiance did not impact induction. Scheduling ageratum to produce flowering and salable plants is therefore straightforward and relatively easy for growers. Periodic challenges that can be encountered when producing ageratum include root rot diseases in overly saturated soils and infestations of thrips and whiteflies in the greenhouse. Outdoors in amenable soil with adequate light, water, and drainage, ageratum is typically a very dependable ornamental. Ageratum is also quite resistant to herbivory including Japanese beetles, a problematic insect in the eastern United States that feeds veraciously on a wide host range of landscape plants (U.S. Department of Agriculture, 2004). Ageratum cultivars are available in a range of flower colors including lavender–blue, lavender, pink, white, purple, and mauve, but the most popular color of ageratum is lavender–blue. Lavender–blue cultivars are particularly popular because that is a difficult color to find in other flowering bedding plants, especially ones adapted to flowering continually in regions with long warm summers.
In the past decade, elite genotypes of primarily compact growing A. houstonianum have been introduced as vegetatively propagated cultivars and are included in popular branded plant lines. Examples include the Artist® (Proven Winners®) and PatinaTM (Syngenta®) series of ageratum, which are widely available in independent garden centers and box stores as bedding plants in the United States in spring. Typically sold in ≈10-cm pots or in mixed patio containers, vegetatively propagated ageratum cultivars are typically grown to a relatively large size and tend to be more expensive than the seed grown cultivars typically sold as smaller plants in packs. However, exceptional performance for traits like heat tolerance, strong continual bloom, or very symmetric plant habit in many vegetatively propagated ageratum genotypes provides added value and helps justify the extra expense.
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