Growth regulation of perennial trailing Verbena

Are you growing the trailing Verbena canadensis (now called Glandularia Canadensis), cultivars like ‘Homestead Purple’ or ‘Homestead Red Carpet’? Although gardeners like that spreading habit, it can be difficult to keep these plants tidy under greenhouse or nursery conditions, especially in one-gallon pots.

by Joyce Latimer, Professor, Horticulture, Virginia Tech.

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 branching. Although Configure has been effective in increasing branching of a large number of our commercially produced herbaceous perennials, it did not increase branching of ’Homestead Red Carpet’ with a single application of 600 ppm foliar spray. Of course, these crops branch pretty freely anyway. Finished plants had 25 to 30 branches with or without Configure.

What about growth regulation? We tested Concise on ‘Homestead Red Carpet’ with applications as a 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 or Sumagic (uniconazole, NuFarm/Valent USA) and be prepared to reapply as necessary. Watch for the resumption of rapid internode elongation and reapply to slow that growth.

By the way, pay attention to vigor as well, 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!!

Source: e-GRO