Summer PGRS: Herbaceous Perennials in the Nursery

As we get into the heat of the summer, you’re not growing many herbaceous perennials in the greenhouse. So let’s talk about the uses of plant growth regulators (PGRs) on perennials in the nursery or outdoor production areas. As you start potting up those summer and fall plants either for the fall market or those you plan to bulk up for overwintering, consider how PGRs can help you improve the quality of those plants for market.

If your summer plants are for the fall market, you can improve plant growth habit with branching agents and reduce the final size of these perennial plants with growth retardants. In general we want shorter, well-branched plants that are in balance with their container. Grower benefits of using PGRs to control plant growth include:

  1. reduced cost of production as less space per plant is required;
  2. decreased plant height meets more shipping height requirements;
  3. smaller size allows grower to ship more plants per truckload;
  4. less shrinkage or production losses because treated plants are more resistant to environmental and cultural stresses; and,
  5. enhanced shelf life, both in production and retail.

On the marketing side, we know that plant height impacts perceived quality. A plant in balance with its container is perceived to be of higher quality than an “overgrown” plant. To achieve this result, we must regulate growth during the period of rapid expansion.

However, we also do not want to reduce flower numbers or size or delay the flowering time. In general, making PGR applications early in the growth cycle minimizes the effects on plant flowering. Recognize that PGR effects on flowering in herbaceous perennials can be less predictable than with many of our annual bedding plants. However, the principles of “safety” are the same. For that fall market crop, try to treat before flower initiation. Consider the use of pre-plant liner soaks for your first PGR application to those vigorous crops (See Brian Whipker’s video on Pre-Plant PGR Liner Soaks)

As you start potting up those summer and fall plants that you plan to bulk up for overwintering, consider how PGRs can help you improve the quality of those plants for market next spring. Since we want the crop to grow, to bulk up, we want to be careful about growth retardant use. However, we also don’t want to be pruning, shearing or mowing the crop all summer to keep them in check. Yes, I know it also helps promote branching but we can generate some of that branching using Configure or Collate (see previous blog posts) while reducing some of those labor costs. Some growers use moderate rates of growth retardants like Piccolo/Piccolo 10 XC or Concise, not to eliminate, but to reduce the number of times they need to shear summer plants.

Photo 1_PP59 Miscanthus 8WAT Concise Liner 0 1 2 3 4 5 ppm2Photo 2_PP59 Calamagrostis 8WAT Concise Liner 0 1 2 3 4 5 ppm2One grower told me that using a 20 ppm Concise foliar spray on young ornamental grasses on a monthly basis cut the number of manual pruning events in half through the summer and fall. We have also found extended growth control with liner soaks on ornamental grasses like Miscanthus sinensis ‘Gracillimus’ (Photo 1) and Calamagrostis x acutifolia ‘Karl Foerster’ (Photo 2) that would reduce the frequency of pruning. When growth retardants are used in moderation, in other words at rates that do not cause excessive stunting of the plant shoots, root growth continues at a good pace and we actually see an increase in the root to shoot dry weight ratio. So we are getting good root fill in the pot while we reduce the number of shearing events. NOTE: there are some exceptions where drench applications cause a significant reduction in root growth. We’ll cover that in our next video.

In summary, using PGRs on herbaceous perennials in the summer nursery can improve plant quality and increase the salability of the plants for fall market. And, for those plants destined for spring sales, PGRs can assist in managing excessive growth thereby reducing labor costs for shearing, mowing or moving plants. And, don’t forget to test multiple applications of the branching agents during this season as well.