Technical Abstract: Hydrangea macrophylla cultivars are known to be diploid or triploid, but none of the cultivars examined appear to be tetraploid (Jones et al, 2007; Zonneveld, 2004). Increasing ploidy in woody ornamental plants has been shown to alter the morphology and enhance some ornamental characteristics such as shorter internodes, sturdier stems, and larger flower petals. Some of these traits, along with elongated stomatal guard cells and larger pollen, appear moderately increased in triploid cultivars such as Oregon Pride (Jones et al, 2007). Thus, synthetically producing tetraploid plants by applying anti-mitotic agents to meristematic tissue of known diploid cultivars may be a productive breeding strategy to produce enhanced cultivars. Moreover, tetraploid hydrangeas have potential as breeding lines to produce more triploid cultivars or to better combine genetic backgrounds with wide hybridizations experiments (Reed et al, 2008). Thus far, an effective, rapid method for doubling chromosome number in Hydrangea macrophylla has not been published.