PGRs on Perennials: Monarda didyma

By Joyce Latimer, Virginia Tech

Ahhh, it’s hummingbird time! Everyone loves those tall, bright beebalm flowers attracting those adorable birds to the home garden. However, homeowners have no clue how tough it is for growers to get those vigorous “weeds” to the garden center looking good. So, I’d like to give you a quick summary of PGR options for beebalm (Monarda didyma) with research results from several different cultivars over the years.

Beebalm 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). Not only do cultivars vary in their response to different growth retardants, the same cultivar responded differently in subsequent trials. Persistence of the growth regulation also varied between different trials at the same PGR rates.

Table 1. Effect of growth retardants on plant height of Monarda 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.

Growth retardant Rate (ppm) Application method Cultivar Height reduction / time Persistence
Dazide 5000 x 2 Spray Marshall’s Delight

24% / 3 WAT

3 WAT*

5000 x 2 Spray Mahogany

25% / 8 WAT


Dazide / Citadel 5000 / 1500 Spray Marshall’s Delight

17% / 3 WAT

3 WAT*

Piccolo /
Piccolo 10 XC
120 Spray Jacob Cline

23% / 4 WAT


160 Spray Jacob Cline

16% / 4 WAT


6 Drench Raspberry Wine

40% / 4 WAT


8 Drench Jacob Cline

46% / 9 WAT

10 WAT

16 Liner soak Raspberry Wine

31% / 4WAT


Concise 15 Spray Marshall’s Delight

25% / 3 WAT

3 WAT*

30 Spray Jacob Cline

34% / 6 WAT


30 Spray Blue Stocking

38% / 4 WAT


30 Spray Jacob Cline

37% / 6 WAT


40 Spray Blue Stocking

36% / 4 WAT


45 Spray Mahogany

18% / 4 WAT


*No additional data – plants were transplanted to the landscape for evaluation.

So, growth regulation of Monarda didyma 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 or the Dazide/Citadel tank mix at 2 to 3 week intervals depending on the amount of growth regulation desired.

Spray applications of Piccolo or Piccolo 10 XC were less effective, but again we expect that multiple applications would give moderate growth regulation. However, the rates for multiple applications would likely be in the 120 to 160 ppm range. Substrate drench applications of Piccolo / Piccolo 10 XC were very effective with early growth regulation [3 weeks after treatment (WAT), Photo 1] that persisted throughout the crop production (6 WAT, Photo 2). These drench applications did not delay flowering but reduced the number of flowers as compared to the untreated controls. Test lower drench rates under your own conditions. Liner soaks at 16 ppm Concise resulted in height control through 4 WAT, but additional or more persistent control would require higher soak rates or an additional PGR application.

Concise applied as a single ~30 ppm foliar spray was generally effective in height control through most of the production cycle (Table 1). Strive for consistent applications across cultivars and time to reduce the variability between applications.

Watch for a video blog in mid-July with additional results and pictures of these Monarda growth regulation trials.