Early spring paclobutrazol applications to treat annual bluegrass

Paclobutrazol is a plant growth regulator used to inhibit annual bluegrass (Poa annua) growth.  This suppression makes the desirable turfgrasses more competitive, allowing them to crowd out the weedy annual bluegrass.

When warm February weather occurs in Minnesota, it can cause damage to annual bluegrass.  We partnered with a MGCSA member to investigate whether warm February temperatures, in combination with an application of paclobutrazol, would give us the opportunity to push annual bluegrass over the edge and eliminate it from putting surfaces.

Research questions

Will paclobutrazol be taken up by annual bluegrass roots at this time of year? Are there any negative consequences to the desirable grass (creeping bentgrass)?


Higher paclobutrazol rates will result in a greater reduction in annual bluegrass populations.

Study location

Rush Creek Golf Club (Superintendent – Dale Hibert, Assistant Superintendent – Matt Cavanaugh)

Applying paclobutrozol in early spring to golf course
Figure 1. Application of paclobutrazol treatments at Rush Creek Golf Club on March 27, 2017.
  • 6 oz/ac of Trimmit 2SC
  • 16 oz/ac of Trimmit 2SC
  • Untreated plot (control)
Plots after paclobutrazol treatment
Figure 2. Treatment plots on the day of application. From left to right – 6 oz/ac of Trimmit 2SC, untreated, and 16 oz/ac of Trimmit 2SC.


  • Little influence on growth of either annual bluegrass or creeping bentgrass
  • Likely too cold for uptake


Based on these results, we would not recommend a very early spring application of paclobutrazol in Minnesota. We suggest to delaying spring PGR applications such as paclobutrazol until growth is initiated.

Contact Maggie Reiter, Extension Educator - Turfgrass <[email protected]> for more information on this program.