PGRs on perennials: Agastache spp.

Another group of herbaceous perennials that attract the hummingbirds is Agastache spp. Popular Agastache flowers come in a variety of species and cultivars that are commonly called the giant hyssops or hummingbird mints. And regardless of the color, these tubular flowers held high on the plant are great at attracting hummingbirds. But, like the Monarda I recently wrote about, homeowners have no idea how difficult it is for growers to get these very vigorous “weeds” to the garden center looking good. So, I’d like to give you a quick summary of our research results using PGRs for growth control of a few different cultivars of Agastache over the years.

By Joyce Latimer, Virginia Tech

Agastache is a very vigorous early grower that has been responsive to many of our PGR options. The problems are cultivar variability and variable responses to growth retardant applications (Table 1).


Table 1. Effect of growth retardants on plant height of Agastache cultivars. The percent height reduction relative to untreated plants is presented with the time it was measured in weeks after treatment (WAT). The persistence of the growth regulator, the time at which height control was still evident, also is given in weeks after treatment.

As with most of our other herbaceous perennials, there are significant differences in cultivar responses to the PGRs. However, growth regulation of Agastache requires starting with early growth retardant applications using the PGR recommendations below:

If you can make multiple applications to achieve moderate reductions in growth, try Dazide at 2 to 3 week intervals depending on the amount of growth regulation desired. However, be aware that Dazide-treated ‘Purple Haze’ plants had significantly fewer flowers than untreated plants (Photo 1). ‘Tutti Frutti’ was much more sensitive to Dazide with early stunting with a single application. However, the plants grew out normally finishing at 25% shorter than untreated plants with no delay in flowering and no difference in the number of flowers.



Cultivar response to Piccolo or Piccolo 10 XC were also significant. Spray applications were much less effective than drench applications, but drench rates varied with cultivars as well. These drench applications did not delay flowering or affect the number of flowers as compared to the untreated controls.

Concise applied as a single ~30 ppm foliar spray was generally effective in height control through most of the production cycle for ‘Purple Haze’ but somewhat excessive with ‘Blue Boa’ which finished at 38% shorter than untreated plants. Concise did not affect flowering in either cultivar.

These studies included a limited number of cultivars and PGR rates but hopefully this will give you a starting point for your own trials.

Source: Fine Americas

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