How to Keep Delphinium Alive More Than One Season: An Example of Research

What is research? In this instance, it is the repeated attempts to find the answer to a problem. As you will see, most of the remedies tried were unsuccessful, or inconclusive, but finally the most obvious solution seems to be the best.

The problem in this case is trying to keep delphinium plants alive more than one season. We typically transplant seedlings from the greenhouse in spring, but watch with dismay as the plant stand dwindles. In our first variety trial in 2005 using eight elatum hybrids of the ‘Aurora’, ‘Guardian’ and ‘Candles’ series, we had a plant stand of 34% in mid-August, which dropped to 10% in early September, clearly an unacceptable performance. We could not pin this decline on a particular disease organism, and  decided to see if altering growing conditions might rectify the situation.

Thinking that perhaps cooler soils might improve plant stand, we conducted an experiment in 2007 with mulches of straw or silver plastic reflective mulch, compared to bare ground. The overall decline in plant survivorship this case year was not as steep as previously, but neither of the mulches improved plant stand, which had declined to 47% in early October. There were only small differences in stand among the ‘Aurora’, ‘Guardian’ and ‘Candles’ lines used in this trial.

Work with vegetable seedlings at that time indicated that beneficial microbes in compost could counteract the bad effects of root pathogens, so we decided to test that theory. Seedlings of ‘Aurora Mix’ delphinium were raised in greenhouse artificial mix amended with 20% compost from 3 sources, compared to standard mix. Plants in the compost treatments were also given a cup of the same compost in the transplant hole. That was the year (2008) in which even the untreated control plots had nearly perfect plant stands, so we could not conclude anything about the compost treatments. See why I call this REsearch?

Finally, a glimmer of hope arrived in 2009, when a routine variety trial of five entries revealed two of outstanding vigor and stand: ‘Centurion White’ and ‘Magic Fountain Cherry Blossom’ (Fig. 1). 

To make sure this was not a fluke, we planted ‘Centurion White’ and ‘Guardian Mix’ in 2010, testing to see if we could improve plant stand by treatment with Trichoderma T-22 (commercial name ‘Rootshield’) applied to the seedlings at transplanting. The results were simple, and outstanding: plant stand of ‘Centurion White’ was 92% on September 15, whereas ‘Guardian Mix’ had nearly disappeared (24% stand). Trichoderma treatment made no difference (Fig. 2).

So in this case, choosing a vigorous variety seems to be the best option. Variety trials provided us with an answer to a vexing problem. That may not be the solution in every instance, especially if there are no differences in susceptibility to the mysterious malady among varieties.

Chris Wien

Professor

Chris Wien is recently retired Professor of Horticulture at Cornell University. Contact him at [email protected]