Root zone temperature effects on potted Dahlia production

Dahlias (Dahlia × hybrida) have been a popular greenhouse crop in recent years, as increased dahlia breeding and selection has led to a new generation of dahlias, with enhanced disease resistance, leading the way for a renewed interest in the horticulture market. Vegetative or clonal dahlia production has become more popular with many new dahlia series available to producers and consumers (e.g. ‘XXL’, ‘Karma’, ‘Gallery’, ‘Dahlietta’, ‘Dalaya’, etc.). It has been observed and reported that periodic significant crop loss (complete crop loss in some cases) of vegetative dahlia crops during greenhouse production in North America in late spring, shortly before market. It is hypothesized that elevated root zone temperatures (RZT) may be to cause, rather than disease problems. The main objective of this research was to evaluate and characterize elevated RZT effects on vegetative dahlia production on different dahlia lines and cultivars. Elevated RZT treatments were applied using a hot water bath. Dahlia plants (6 replications) were subjected to one of 5 RZT treatments; 18 (control; ambient), 35, 40, 45, or 50°C. Plants were treated 5 weeks after plugs were transplanted, when significant rooting was observed. Treatments were considered ‘applied’ when the temperature in the root zone of the pot (~2.5 cm from the edge of the pot; ~5 cm from the top of the media). Root ratings based on a predetermined scale (0=brown, dead; 4=white, active) were collected before treatments were applied and weekly after treatments. Our results showed that temperature affects were cultivar dependent. One week after treatment, with the exception of the controls, for most temperature treatments, root ratings decreased in ‘Dalaya Shiva’, ‘XXL Sunset’, and ‘Dalaya Yogi’. The cultivar XXL Veracruz did not respond to treatments. For ‘Karma Naomi’, ‘Karma Sangria’, ‘Melody Gypsy’ and ‘Melody Sincerity’, RZT treatments of 45 and 50°C resulted in decreased root ratings, one week post treatment. After 2 and 3 weeks, some root ratings increased, indicating root recovery across the treatments. These results provide insight into the effects of elevated RZTs. However, treatments did not result in complete crop losses, suggesting additional factors need to be considered to explain the phenomenon.