PGRs on Perennials: Verbena Canadensis

Homeowners love the spreading, trailing habit of Verbena canadensis. Cultivars like ‘Homestead Purple’ made a long-lasting splash in the garden center marketplace as a great groundcover with their spreading habit and heavy flower power spring through frost in the fall. However, growth management in the greenhouse or nursery can be a real challenge, especially in one-gallon pots. In fact, I’ve had a couple of calls about this crop already this year.

We have run a couple of tests over the last few years with both ‘Homestead Red Carpet’ and ‘Homestead Purple’ using the branching enhancer, Configure (benzyladenine, Fine Americas, Inc.) and/or the plant growth retardant, Concise (uniconazole, Fine Americas, Inc.) – with mixed results.

First, let’s talk about Configure which has been effective in increasing branching of a large number of our commercially produced herbaceous perennials. We tested Configure foliar sprays on ’Homestead Red Carpet’ at 0 or 600 ppm applied once about 2 weeks after size 72-cell plugs were potted into trade gallon pots and saw no improvement in plant branching. Of course, these crops branch pretty freely anyway. Finished plants had 25 to 30 branches with or without Configure.

So how about growth regulation? With ‘Homestead Red Carpet’ we tested Concise as 60 ppm foliar spray applied at label volume of 2 quarts per 100 sq.ft., an 8 ppm drench (10 fl.oz./trade gallon pot) or a  2 ppm liner soak (2 min soak). The liner soak was applied the day before potting. The spray and drench applications were applied about 2 weeks after potting. The measurement of interest, of course, was plant diameter. The spray application reduced plant diameter 26% at 2 weeks after treatment (WAT) and the effect persisted through 4 WAT where plant diameter was 24% smaller than that of the untreated control plants (Photo 1). The drench application reduced plant diameter 21% at 2 WAT but that dropped to 12% at 4 WAT compared to untreated control plants. Both the drench and spray applications tended to reduce the number of branches at 2 and 4 WAT but there were no differences at 6 WAT. The 2 ppm liner soak rate gave us no growth control. Higher rates may have been effective. The spray application delayed flower opening slightly. All plants were in flower by 6 WAT.

In another study with ‘Homestead Purple,’ we tested multiple rates (0, 15, 30, 45, 60 ppm) of Concise spray applications. All rates gave comparable growth control, about a 30% reduction in plant width, at 2 WAT. However, the plants were pretty much grown out of the growth regulator by 4 WAT. We should have planned on multiple applications!

So, our recommendation for growth regulation of Verbena canadensis is to start early with 15 to 30 ppm spray applications of Concise and be prepared to reapply as necessary. Watch for the resumption of rapid internode elongation and reapply to slow that growth. By the way, the finished ‘Homestead Purple’ plants were nearly twice the width of our ‘Homestead Red Carpet’ plants. And, as we’ve said before, PGR efficacy is affected by a number of factors. This paper reports the results of two individual trials. We would like to encourage you to conduct your own trials. Try the drenches or liner soaks at higher rates. Try Configure under your conditions. And, please share your results with us!!