A Fine Line between Phytotoxicity and Blue When Producing Hydrangea macrophylla in a Nursery at a Low Substrate pH

Hydrangea macrophylla exhibiting blue sepals (versus purple or pink) have improved marketability; however, little research has been conducted to evaluate aluminum (Al), the element responsible for bluing, on crop growth, effectiveness of bluing sepals, and characteristics flower clusters in an outdoor nursery. This study compared substrate Al availability, color, number, size over a 56-week period two locations. A polymer coated (90-day release) ground sulfate [Al2(SO4)3; water soluble] was either incorporated into non-limed pine bark substrate, applied surface as top dress, routinely Al2(SO4)3 drench (low concn.) once (high concn.). In general, application increased plant foliar concentration, but also decreased pore-water pH electrical conductivity (EC) with varying effects based product’s solubility subsequent longevity. Aluminum potential phytotoxicity negatively affecting root morphology creating undesirable rhizosphere electrochemistry due being continually acidic, <4, EC temporarily >1.5 mS·cm?1. These suboptimal conditions resulted lower quality smaller plant. No plants exhibited clear, deep cluster sought by consumers. Neither effect pore could, alone combination, account lack vigor when concentrations were adequate flowering H. macrophylla. More is needed investigate electrochemical properties, possible mineral nutrient co-factors that provide synergisms toxicity protections, holistic health ensuring coloration vigorous