Do PGRs control the growth of herbaceous groundcovers?
While most PGR use in perennials is to control the height of tall, vigorous plants, many growers have asked about using PGRs to control the creeping growth of groundcovers like Phlox subulata, Veronica repens or Vinca minor. These creepers manage to invade nearby pots, making a nightmare of order pulling.
Remember that PGRs control growth, not height. Using them to control the elongation of groundcover runners and stems is no different than using them on upright perennials. However, less research information is currently available on these types of crops.
In our research, Phlox subulata was not very responsive to multiple applications of daminozide (B-Nine) at 5,000 ppm but was responsive to a single application of the tank mix of 5,000-ppm daminozide with 1,500-ppm chlormequat chloride (Cycocel). Single foliar spray applications of uniconazole (Sumagic) at 15-30 ppm, paclobutrazol (Piccolo) at 120 ppm or flurprimidol (Topflor) at 30-45 ppm also gave effective growth control in Virginia. Veronica repens was very responsive to all of the PGR treatments with excessive growth reductions at very low rates of uniconazole (test less than 15 ppm), paclobutrazol (test less than 30 ppm) or flurprimidol (test less than 15 ppm). We’ve done much less research with Vinca minor but plan to test rates similar to those used for Phlox subulata and make multiple applications as necessary.
Remember, PGRs regulate growth, regardless of the direction of that growth.
I do not seem to be able to control the height of caladiums with sprays of any growth regulator. What should I be doing?
First, paclobutrazol (Piccolo, Bonzi or Paczol) or flurprimidol (Topflor) products are the chemicals to use on caladiums. In fact, several years ago we found that caladiums are one of the very few crops where paclobutrazol is considerably stronger than uniconazole (Concise and Sumagic). We still do not understand why.
Second, the PGR should be applied as a drench rather than a spray. When applied as a spray to any plant, these chemicals do not move (translocate) out of the leaf to the growing point where they work. With caladiums and other similar crops that have a compressed stem and only send leaves above ground, sprays do not work well. I can control caladium size by spraying twice a week and get each leaf as it emerges by directing the spray at the base of the plant and not fully expanded leaves.
The drench application timing is important. If applied soon after planting (before roots develop), the effect is reduced. Once the first leaves emerge and expand, it is too late to get the best results. Caladiums are very irregular in starting growth, so my suggestion is to make the drench application when 10 percent of the plants just start to emerge (spiking). This may be 2-4 weeks after planting depending on variety, temperature and other variables.
Look at individual products to ensure they are labeled for caladiums. The drench rates for paclobutrazol are 2-16 ppm and 4-16 ppm for flurprimidol. The optimum rate depends on variety vigor and desired control. It is common for growers to ship caladiums as soon as possible before many leaves emerge and they become too large. However, with a PGR drench, they can be held to allow a couple more leaves to expand and produce a much higher quality plant.
In my experience, the drench does not substitute for de-eyeing for the highest-quality large containers. In packs or other small containers, the drench does allow more leaves to emerge for a fuller appearance without de-eyeing. Preplant paclobutrazol soaks are approved on some product labels. The soaks are easy, but the results are more variable than with drench applications.