growth regulators By Paul Pilon

The public has once again discovered the charm of perennials. Until recently, perennials were the victims of the “new every year” gardening attitude. But they have slowly crept back into backyard gardens, parks and roadside plantings over the past decade. As a result, commercial production of perennials has drastically increased. However, unlike annuals, less research has been conducted on growing perennials. This general lack of production information, the vast diversity of varieties and the tendency for many perennial species to grow like their weedy relatives has resulted in a less than flattering reputation for growing perennials (i.e., difficult, troublesome, and out-of-control).

To help tame some of the more wily perennial varieties, many growers have turned to growth regulators. The primary purpose of using these chemicals is to control plant height, but they are also frequently employed to maintain a high quality, shippable plant with a desired shape and size. If used correctly, growth regulators can also help increase plant uniformity, enhance rooting, improve plant color, produce better plant quality and give the plant a longer shelf life.

Fine Tuning the Art of Height Control

It is not uncommon for two greenhouse operations to use similar production techniques for a perennial crop and end up with completely different results. Many factors come into play when controlling height, making the use of growth regulators more an art than an exact science. Indeed, each time a grower uses a growth regulator, the results may vary slightly, even if the application rate never changed. Understandably, rates will vary between growers, but knowing this is not a negative. What it does mean is that experience is key. Individual growers must determine which chemicals, application techniques and rates are most suitable for their crops and growing operation.

Growth regulators should never be applied with the intention of stopping plant growth. They are valuable tools that, when used properly, will allow plants to finish at a desired size. Growth regulators should be used during active growth to control the rate of internode elongation, not as a last resort to stop growth before selling the plants. If you find that this is the position you are in, plant growth regulators are not the solution. In fact, growth regulators are less effective at reducing the rate of plant growth during this late stage of development than are other options. My recommendation to avoid this situation is to schedule height control chemicals into your crop production plans.

The Chemical Lineup

Some of the most widely used chemical growth regulators are A-Rest, B-Nine, Bonzi, Cycocel, Florel and Sumagic. All of these chemicals are effective, and all will work well with certain application techniques on selected plant species.

There are more methods of applying growth regulators than there are chemicals. In recent years, growers have become very innovative in finding new ways to use these height control tools. Some of these methods include, but are not limited to: sprays, drenches, seed soaks, bulb dips, media sprays, controlled residue and subirrigation. Unless otherwise stated, the techniques discussed in this article are based on applying foliar sprays.

Many plants are responsive to several different growth regulators. The amount of growth control achieved with each chemical varies widely based on the species of perennial, the plant’s developmental stage, the chemical and the rate used, as well as the application technique and uniformity of the application. Unless a plant is widely grown or has been researched extensively, it is difficult to know which is the best chemical to use or where to begin. Often with perennials, a trial and error process is used to determine the best growth regulators and the correct rates of application.

In our operation, Sumagic and B-Nine have been a good choice. These chemicals were the most effective for us on the widest variety of perennial species. Since there are often several plant varieties in the same greenhouse, these two products have produced the best results for our plants.

You could use different growth regulators for different plants, but using four or five different growth regulators on different varieties would be a time-consuming nightmare. Limit your options to one or two chemicals. This makes controlling the height of your perennial crop more practical.

The Rules of Application

Rule number one when using any growth regulator is to follow the directions. Always read the label on your growth regulator product and you will prevent potential headaches later on.

Rule number two for growth regulator application: less is better. It is better to apply growth regulators more frequently at a lower concentration than to apply a single application at a higher concentration. If you find that 10 ppm of Sumagic is the best rate for a desired plant, it is usually better to apply 2.5 ppm four times or 5 ppm two times rather than 10 ppm one time. This method produces a more gradual cumulative height control affect and will most often result in a more desirable looking plant.

The third rule of application takes a bit of observation. It is important to determine at which stage growth regulators will be the most beneficial for your plants. Once the plant canopy is overlapping, it is difficult to get good coverage on the stems and leaf surfaces. Rule number three is to apply growth regulators when the leaves are beginning to touch, but are not overlapping. Reapply chemicals in seven to 10 days if necessary. This is the stage at which it is most beneficial to know your plant, its habit and its rate of growth at a particular stage. You are trying to slow down a plant’s rapid growth, not stop its growth. Applying growth regulators too early could give you too much control too soon. Applying them too late may not give you the control you need.

The ideal rate for a plant growth regulator varies from grower to grower and from crop to crop. I cannot recommend a single rate that will work best for all growers in all regions; you must determine that on your own. In western Michigan, on most perennial crops, we apply five ppm of Sumagic and 2,500 ppm B-Nine. To achieve the best results, it generally takes two applications of each product approximately seven days apart. Some crops are more sensitive to growth regulators, so we apply them at half the above rates. Remember that once applied, you cannot easily reverse the effects of growth regulators, so it is always better to under apply; you can always spray again later if needed.

As I’ve said before, using growth regulators on perennials is not an exact science. Many growers may have had some bad luck with growth regulators in the past, primarily because of over or under control. But don’t give up. Growth regulators are only intimidating if you allow them to be. Be patient. Start using growth regulators on a small scale. As you gain more experience and become more comfortable, start using the chemicals more frequently and on a larger scale.

Alternative Methods for Height Control

Other than using chemicals, there are several ways to help control the height of perennials. Crop spacing greatly affects the final height of a plant and is one of the most important tools that can be used. Research has shown that most plants will generally grow taller at high-density spacing. However, wide spacing often does not always prove to be economical. Manipulating photoperiods and managing water and nutrient availability are other ways of controlling height without chemicals. Choosing short-growing varieties is also a strategy for controlling plant height. However, even after using these methods it is often still necessary to use chemical growth regulators to control the height of many herbaceous perennials.

With the increasing popularity of perennials, many growers are producing plants that they are unfamiliar with. Until more plant breeding work is done, many of today’s most desirable perennials are also naturally tall. Between the types of perennials growers are producing and the conditions they are growing them in, controlling the height on many varieties has become a necessity. Providing adequate plant spacing and using chemical growth regulators are the two best height control strategies a grower can use. When used properly, growth regulators can make a big difference on crop production, plant quality, and your bottom line.

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Paul Pilon

Paul Pilon is the head grower at Sawyer Nursery in Hudsonville, Mich.

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