Our objective in using PGRs on poinsettias is to control height with minimum effects on bract size. As a general rule, all PGR applications reduce poinsettia bract size compared to plants grown without PGRs. We rarely want the large bracts of a nontreated plant in mass-market retail situations. These plants can be softer and suffer more physical damage during shipping and handling by customers. It is important, however, to avoid reducing bract size to the point that it is noticeable. Also, for large wholesale poinsettia production, reducing costs and/or labor in PGR use is important.
The figures and pictures accompanying this article illustrate the use of drench applications early in the crop when sprays are normally used. There are four potential advantages of the early drench strategy:
Last fall was warmer than normal, which made height control difficult on most of our crops. In all of these examples, late drenches at the higher concentrations were required to finish the crops at the desired size. The use of a late drench is an option, if needed, with the early drench strategy just as with traditional sprays.
‘Snowcap’ is a relatively vigorous variety and usually requires more PGR than most other varieties. Figure 1 shows the results of using the early drench strategy on ‘Snowcap’ last fall. Paclobutrazol (Bonzi) was applied as a drench at 0.1 ppm four times, and then a late drench at 1 ppm was applied on Nov. 6. The process of deciding when to make a drench application was the same as deciding when to apply a spray. Shoot elongation started out strong after the pinch.
The first and second drenches were effective in slowing elongation and bringing height back to the curve by mid-October. Then, however, temperatures remained high, and elongation did not slow down as we had expected. Two additional early drench applications were made in late October before the late drench was applied.
The plants in Figure 3 are a comparison of ‘Prestige Red’ grown last fall with early drench applications compared to doing traditional sprays. Both strategies produced plants at the desired height of 16 inches, but the three early drenches provided better early control.
The ‘Monet Twilight’ in Figure 4 is an example how effective the Topflor (flurprimidol) early drenches were on another very vigorous variety. One of the consequences of heavy spray applications is delayed color development, as seen in Figure 5. Notice that the early drench strategy provided the needed height control without the effect on color development.
Now, let’s look at two examples of the early drench on less vigorous varieties. Figure 6 is Topflor on ‘Prestige Early Red’ that was delayed to finish at the same time as natural day ‘Prestige Red’. Figures 7 and 8 are paclobutrazol on the easy to grow variety, ‘Enduring Red’.
We made the decisions about when to apply an early drench similar to the way we made the decisions about applying sprays. The concentrations we used for these lower-vigor varieties were same as for the more vigorous varieties, but fewer applications were made.
In traditional sprays, there is a gap in October when we avoid sprays because of their potential for affecting bract development. A major benefit of using early drenches is that there is not a cutoff date, and they can be used after we prefer to stop using sprays. This late-October application option was used in the ‘Snowcap’ (Figures 1 and 2) and ‘Prestige Red’ (Figure 3) examples. Poinsettia crops can be controlled with sprays when they are easiest and switched to drenches when that is the better option.
It is important to recognize differences in chemical concentration for the early and late drench. The late drench is designed to hold plant height at the end of the crop, and we use 0.5-1 ppm of paclobutrazol or Topflor. We use much less in the early drench because we do not want to stop elongation. For most varieties, such as Freedom, Prestige and Orion, suggested concentrations of paclobutrazol or Topflor is 0.1-0.15 ppm. For more vigorous varieties, like ‘Monet Twilight’ and ‘Snowcap’, suggested concentrations of paclobutrazol or Topflor are 0.1-0.2 ppm. Growers in most of the country should start at 0.1 ppm and make the applications as needed. The higher concentrations can be used by growers, mostly in warm climates, who know they need more control.
I will end with a warning: The most common problem I have encountered with this technique is letting the decimal point slip — and applying 1.0 rather than 0.1 ppm.