Three Ways Growth Regulators Can Change Your Shrub Game

By Brandon Gallagher Watson

Just over a decade ago, I got my first job in the science of arboriculture. Sure, it was a lowly internship as a research assistant but it got me out in the field, setting up trials and doing treatment evaluations. That first summer, my company was doing extensive rate development trials for a shrub growth regulator. The company had already found success developing a product for predictably reducing tree growth, and was interested in expanding that technology to shrubs. At the time, the focus was on better understanding the science behind how these treatments worked and why certain shrub species were responding strongly to the treatments while others seemed unfazed.

Now, more than a decade later, the functionality of growth regulation is well understood, and predictable rates have been determined for hundreds of different species. As a result, much of the focus on shrub growth regulators has shifted from a scientific inquiry to a business one. How can this technology be used by landscape maintenance professionals in ways that either add up to savings for service providers or a benefit to the clients? We’ll take a look at three different ways companies are implementing growth regulators and finding new uses everyday.

What’s your type?

First, let’s step back and revisit the science of how these products work to understand the uses or benefits that they can provide. At the time of my inaugural summer of scientific employment, plant growth regulators (PGRs) had been around for quite awhile. In fact, plant physiologists had already classified PGRs into two main categories based on how they accomplished growth control. The first group was known as Type I growth regulators. Normally, as a plant grows, it increases its total number of cells by dividing them through the process of mitosis. One cell becomes two, two cells become four, and so on. Type I regulators (mefluidide, maleic hydrazide, and others) work by essentially blocking this process. This technology proved very useful in certain applications, particularly in places such as fruit tree orchards, where they are frequently used to prevent sucker twig growth off a grafted rootstock.

Although they are very useful in agriculture, they have some issues when utilized in ornamental landscapes. Preventing mitosis can cause irregular growth patterns in the new tissue as it develops. This was not an issue for orchard use; but in landscape settings, where aesthetics are paramount, clients have been known to object to the sight of disfigured growth. Many Type I PGRs are closely related to herbicides, so although they will have a growth regulating effect, they can also have mild phytotoxic reactions such as a chlorotic yellowing of the developing tissue.

The understanding of plant hormones starting with the green revolution in the first half of the 20th Century led to the development of Type II PGRs. Rather than preventing cell division, Type II regulators (paclobutrazol, flurprimidol, and others) work by inhibiting the hormones that cause plant cells to elongate. The plant still produces the same number of cells, but those cells just don’t stretch out as far. This form of growth control is less stressful to the plant than Type I regulators, and even has some secondary health effects that, as we shall see, have some benefits to treated plants.

The hormone that is being blocked by Type II PGRs, gibberellic acid, is constructed from many of the same base compounds as other plant products — most notably chlorophyll and absicic acid. Absicic acid is known as the plant stress hormone, and has a role in helping plants through times of drought; thus, plants with a higher reserve of absicic acid are better equipped to withstand adverse growing conditions. Chlorophyll, of course, gives plants their green color, and plants treated with Type II regulators often exhibit a darker green appearance due to higher quantities of chlorophyll being produced.

Now, with some of the background info out of the way, let’s look at how these products are being implemented by landscape maintenance professionals.

Use #1: Trim or treat

The time from spring to early summer is extremely busy for everyone in the landscape maintenance business. The lawn is the growing, the weeds are coming up, the bedding plants need to get into the ground, and every client want it done at the sight of the first robin. Taking shrub work out of the peak-season equation can dramatically free up the labor to accomplish other tasks.

I recently spoke with Steve Sullivan, the director of technical services for Brickman Group and ValleyCrest Companies, which are becoming BrightView. Steve talked about how growth regulators are becoming an important part of their shrub maintenance programs. “First off, we try to take some shrubs where we can do more dormant pruning, we prune those [before the growing season starts] and then do more of a light tipping in the summer,” said Sullivan. “Other shrubs, where dormant pruning is not an option or might be a little more difficult, we are using shrub growth regulators.

“We are trying a combination of the two to take the total number of pruning hours, and instead of having them in a four to five week period, we are trying to spread that out. So we are using dormant pruning and Trimtect to reduce the need for shrub pruning during that peak spring/summer rush. It’s kind of a two-tiered attack to reduce our summer pruning.”

I asked Sullivan if he has ever had a client notice that he has used a growth regulator on a shrub. He replied, “People expect that they are paying for a certain look and that doesn’t happen, that’s when you will hear about it. They’re never really going to call up and comment, ‘Wow, this shrub looks really nice!’ That’s not going to be the kind of thing that you will hear in commercial landscapes. If you’ve done your job right, no one will notice.”

Use #2: Reduce pruning to reduce labor

Every trip to the site has costs. Fuel, wear and tear on the vehicles and equipment, and, of course, travel time. Work at the jobsite has costs too. This includes fixed costs such as labor and supplies, as well as opportunity costs. Every hour your crews are spending on one project is an hour not being spent on another project. Many companies are finding a place for shrub growth regulators in their maintenance programs by using them to free up labor for other work.

“We sprayed [a Type II PGR] on a privet shrub four days post-prune in June as a trial,” said Andrew Stith, production manager for Epic Landscape Productions in Oloathe, Kan. “We pruned again in August, and found significantly less top growth compared to the shrubs around it. The privet sprayed with [the regulator] held its shape through September — over five more weeks — nearly absent of escapes. This was far better than expected. It is conceivable that we would cut our prune time in half and reduce our clippings by more than half for the year.

“Expanded use of [PGRs] would ultimately reduce the workload of our prune crews, allowing them to assume responsibilities in other revenue generating departments,” Stith added. “With the results gained from trials this year, we will be expanding our use of PGRs next year.”

Use #3: Improving plant appearance

When it comes to their properties, clients often want the site to look perfect. There are many reasons for this, but often it boils down to the fact that the client sees the site as an extension of their brand. Traditionally, keeping up this image standard across multiple properties during the heights of the busy season can be challenging, at best. This is another area where landscape maintenance providers are utilizing shrub growth regulators.

It is well known that Type II regulators, like paclobutrazol, trigger secondary health benefits in treated plants. When vegetative growth is reduced, energy that would have gone towards shoot growth is now reallocated to other resources within the plant. As mentioned earlier, absicic acid and chlorophyll are increased, resulting in a darker green appearance that is more resistant to leaf scorch during drought conditions. Even in plants where the amount of growth reduction is not as significant, these benefits can still be seen. For example, yews Taxus spp.) only show about 15 to 20 percent growth reduction, but they are a visibly greener and more compact after treatment — so much so that many practitioners are using it solely for this purpose.

In flowering plants, the reduction in vegetative growth can result in an increase in flowers. There are two reasons for this; first, one of the ways energy is reallocated from growth reduction is into reproductive structures. Less shoot growth equals more flowers. Second, as the shoots are not growing out as far, the flower structures tend to not get covered by the foliage, giving the impression that the flowers are more prominent than in untreated shrubs. “We began using [this product] on our fastest-growing plant material, Elaeagnus pungens and Viburnum odoratissium. We were very pleased with the growth control we achieved, and were impressed by the improvement in overall appearance of the plants,” said George Kennedy from Terra-scape Enterprises in Tampa Bay, Fla.

Changing the game

New technology can change the way you are doing almost any service. Although growth regulators are nothing new for turfgrass and trees, traditionally there have been roadblocks to successfully integrating them for use of shrubs. With new developments in formulations, and a better understanding of the value they bring to landscape maintenance, their use has been increasing during the past few years in just about every market across the country. We only went into three different ways that this technology is being utilized in the industry, but there are many more. That’s the beauty of game-changing innovations — their best use is probably still waiting to be discovered.

Brandon Gallagher Watson is creative director at Rainbow Treecare Scientific Advancements.