A fine line between phytotoxicity and blue when producing Hydrangea macrophylla in a nursery at a low substrate pH

Authors: PIETSCH, GRACE - University Of Tennessee; BRINDLEY, JULIE - Virginia Tech; Owen Jr, James - Jim; FULCHER, AMY - University Of Tennessee

Submitted to: Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/2022
Publication Date: 7/30/2022
Citation: Pietsch, G.M., Brindley, J.C., Owen Jr, J.S., Fulcher, A. 2022. A fine line between phytotoxicity and blue when producing Hydrangea macrophylla in a nursery at a low substrate pH. Horticulturae. 8. Article 690. https://doi.org/10.3390/horticulturae8080690.
DOI: https://doi.org/10.3390/horticulturae8080690

Interpretive Summary: Hydrangea macrophylla exhibiting blue sepals (versus purple or pink) have improved marketability and subsequent sales; however, little research has been conducted to evaluate aluminum (Al), the element responsible for bluing, on crop growth, effectiveness of bluing sepals, and characteristics of flower clusters in an outdoor nursery. This study compared substrate Al availability, crop growth, flower color, number, and size over a year in two locations. None of the treatments yielded blue flowers; however, all treatments except the control achieved foliar Al levels commensurate with those found on blue flowering plants. There seems to be no definitive, singular reason that explains why the hydrangeas in the present study were not a “true blue”, nor is there a clear definition of what a “true blue” is in consumers eyes’ or the literature. Similarly, the issue of root system “pancaking” a known phenomenon in H. macrophylla production and observed in these experiments, has not been investigated. Some treatments caused severe plant damage or death. Growers are advised to carefully consider the role of lime and the balance between fostering Al availability and maintaining a substrate pH conducive to H. macrophylla health and growth. Observing the role that Al application may have in raising the pH at which damage occurs is of particular significance. Future treatments exploring different application timings or forms of Al, in combination with lime and silicon treatments would complement this research. More research is needed on predicting flower color in outdoor production systems and the potential to incorporate nonsulfate-based Al sources such as clays pre-charged with Al and possibly incorporation of wollastonite to simultaneously deliver Al and supplemental silicon to promote blue flowers at a higher pH, thus avoiding the risk of an extremely low pH, anaerobic environment, or Al toxicity.

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