This month, the Southern Nursery Association (SNA) released the proceedings of their 63rd annual SNA Research Conference, and there is a lot of interesting research to sift through. Over the upcoming issues of Country Folks Grower, I’ll be recapping some of the relevant studies for our readers.
First up: the Container Growth Plant Production papers.
Diagnosing Aesthetic and Growth Disorders in Hydrangea Plants Under Commercial Nursery Production by Raul I. Cabrera, Eric Petit and Brad Moran at Rutgers University
After growers reported stunted growth and foliage chlorosis and reddening in hydrangea cultivars in nurseries in New Jersey, the research team delved into the issue. They noted “the severity of chlorosis symptoms and growth reductions were associated with reduced nutrient levels and substrate pH levels approaching 6.0.” They determined over-applications of liming amendments was a likely cause for the chlorosis, along with irrigation management practices that affected root growth (overwatering, root rot and low iron uptake).
The soil/substrate pH for hydrangeas is often a major consideration for flower color development. Unfortunately, there is only generic information regarding the most suitable soil/substrate pH range for the flowers with respect to the availability and uptake of mineral nutrients, their assimilation in plant tissues and aesthetic quality.
The team also noted that while bigleaf hydrangeas grow normally in full sun, it is reported that shading is favorable to their growth. They found higher shading densities of more than 60% can significantly reduce the incidence of the common cercospora leaf spot afflicting this species. Some commercial nurseries now grow some hydrangea cultivars under shade for this reason.
As for stunted growth, the research team visited a nursery which had reported a serious problem with growth and foliage chlorosis and reddening in several hydrangea cultivars – and attempts to mitigate the damage through a fertilization program were unsuccessful. Samples of both healthy and afflicted plants were taken and studied. They discovered the issues were tied to specific plant blocks and species, and that uncovered plants were smaller in height and size than those growing under shade (most likely due to higher rates of transpiration and reduced photosynthesis). “It is hypothesized the reddish coloration in sun-grown plants is likely an overproduction of anthocyanin pigments in young leaves as a photoprotective response to a higher UV-radiation incidence compared to the plants growing under shade,” the study states.
Other varieties had higher incidences of chlorosis growing under shade, however, so shading hydrangeas needs to be done according to what is best for each cultivar. Overwatering caused both growth and chlorosis issues.
Response of Spiraea and Buddleia to Controlled-Release Fertilizer With or Without Phosphorus by Danielle Haynos of Kansus State University and Tom Yeager of University of Florida
In this study, Spiraea japonica and Buddleia davidii were grown at Saunders Brothers Nursery in Piney River, VA, with 100% pine bark substrate. The plant growth data showed lower application rates of fertilizers applied to the surface with longevities of three to four months resulted in similar or more growth (versus the standard woody plant fertilizers with longevities of eight to nine months used at the nursery) after 13 weeks. However, application of fertilizer without phosphorus and three to four months’ longevity inhibited the growth of the two species (Japanese meadowsweet and summer lilac/butterfly bush). The study noted that previous research showed phosphorus leached readily from pine bark substrates.
The change in the growth index during the study was highest for Spiraea when Fertilizer 1 (N 13, P 5.7, K 10.8) and Fertilizer 2 (N 17, P 2.6, K 10) were applied, with no difference seen between the two. For Buddleia, F1, F2 and Fertilizer 4 (N 15, P 3.9, K 10) resulted in similar growth. A lower growth index change for Fertilizer 3 (N 43, P 0, K 16.6) for both plants shows that insufficient phosphorus was available.
The end results showed that during the 13-week evaluation, Buddleia that received fertilizer with three to four months’ longevity reached marketable size while Spiraea reached approximately 91% of marketable size (and the research suggest the growth needed to achieve marketable size probably could have been obtained by planting a few weeks earlier).
“Despite the fact that Spiraea was slightly smaller than marketable size, these data indicate that fertilizers with three to four months’ longevity are a viable option for the nursery when growing Spiraea japonica … and Buddleia davidii,” the research team concluded.