ARS | Publication request: Hydrangea macrophylla Sun Tolerance for the Deep South

Submitted to: Southern Nursery Association Research Conference
Publication Type: Proceedings
Publication Acceptance Date: September 22, 2008
Publication Date: November 1, 2008
Citation: Rinehart, T.A., Reed, S.M., Owings, A., Bi, G. 2008. Hydrangea macrophylla Sun Tolerance for the Deep South. Southern Nursery Association Research Conference 53:198-200.

Interpretive Summary: Tree loss during hurricane Katrina in 2005 dramatically reduced the amount of shade in Southern Mississippi landscapes, particularly near houses. Trees removed during or after the hurricane are not expected to be replaced due to safety concerns. In this study we used full sun conditions that are commonly found in post-hurricane landscapes and evaluated the performance of 26 Hydrangea macrophylla cultivars that were listed as tolerant to full sun, at least according to anecdotal evidence (1, 2). None of the literature suggests that tolerance to sun enhanced tolerance to drought. It is widely accepted that even cultivars planted in sufficient shade require afternoon watering to keep foliage from wilting. In 2006 we initiated a sun tolerance trial of 26 H. macrophylla cultivars to determine if cultivars described as “tolerating full sun” would survive the extreme heat and humidity of the deep south. Our hypothesis was that most anecdotal evidence for sun tolerance applies to more temperate zones and no cultivar would tolerate full sun exposure in USDA Hardiness Zone 8b, or American Horticultural Society Heat Zone 9.

Technical Abstract: Although thoughts of hydrangea in full bloom are associated with hot summer days in the south, there is a lack of published information regarding hydrangea performance in the deep-south which includes USDA cold hardiness zones 8 and 9. Landscapers, retailers, and consumers rely on information gathered from vastly different sources including promotional literature. Replicated studies on Hydrangea macrophylla (Thunb.) Ser. are only associated with disease tolerance in the deep south, not necessarily sun or heat tolerance. Results from this study will help professionals and consumers in the selection of more sun tolerant varieties of Hydrangea macrophylla for warmer climates.