Controlling Perennial Plant Height by “Watering In” By Paul Pilon

As perennials continue to enjoy popularity and staple status in retail garden centers, commercial growers are continually challenged to economically produce flowering plants for today's marketplace. To maximize impulse purchases, it is important for perennials to be in flower at the retail locations; the difficulty of this is that mature flowering plants are taller than the "green" non-flowering ones offered only a few years ago.

Growers most commonly attempt to produce these tall, flowering perennials at high densities to maximize profitability and sales per square foot of production space. Unfortunately, many of the systems and production environments we grow perennials in are not conducive to producing short, compact plants. Besides producing perennials at high plant densities, they are often grown within structures that minimize natural light levels in the early spring and subject plants to high humidity levels. All of these factors — the production of flowering plants, production at high plant densities and the production environments — often lead to conditions that promote undesirable stem elongation.

PGR Applications

To counteract the factors that lead to stem elongation and to produce plants of a desired shape and size, many growers routinely apply various plant growth regulators (PGRs), either by foliar sprays or media drenches.

Most commercially available PGRs are effective as foliar sprays, provided an appropriate volume of spray solution comes into direct contact with the plant tissues (leaves and stems) that absorb them. However, as most perennials grow and the crop canopy closes in, it is increasingly difficult to provide proper coverage to plant stems. Inadequate coverage often results in reduced efficacy of these applications, even when higher volumes are applied.

Several PGRs are absorbed through the roots. These PGRs, when applied as media drenches, usually provide longer lasting, more uniform control of plant height than do spray applications. Commercially, growers commonly apply media drenches using either traditional drench methods or the watering-in method. Traditional drench applications entail applying a precise and predetermined volume of solution to the surface of each and every container individually, whereas the watering-in method entails applying larger volume of drench solutions in a manner more consistent with routine irrigation applications. For controlling height of perennials using drenches or the watering-in method, commercial growers most commonly use PGRs containing flurprimidol, paclobutrazol and uniconazole.

The watering-in method is essentially a drench application that allows growers to apply PGRs to crops without the high labor costs associated with traditional pot-to-pot drench applications. It is difficult to apply drench applications uniformly as each pot is likely to receive differing volumes of drench solution. For example, growers aim to apply 4 ounces of drench solution to each 6-inch container; the reality is that every pot will likely receive a slightly different volume, resulting in a range of drench volumes being applied. This variability of drench volumes results in differing amounts of active ingredient being applied to each container, which leads to variable levels of height control achieved across the crop. In many instances, this amount of variability is commercially unacceptable.

Although the watering-in method may lead to variable application volumes and results, more consistent height control is generally obtained when the method is performed correctly. For best results, apply the watering-in method early in production, once the plants are established and just before their rapid-growth phase. When applied properly, there is usually no effect on flower size or timing and plant quality is usually improved.

As with all PGR application methods, there are essential application guidelines, and when followed will lead to more consistent applications and uniform results.

Understanding the "Volume Effect"

Although the watering-in method entails applying PGR solutions in a manner consistent with a grower's normal watering practices, most growers do not know the actual volume of solution being applied over a given area or to each pot. Similar to other application methods, growers must understand the effect of volume when applying PGRs using the watering-in method.

The volume of solution applied will have a greater effect on the results than the concentration of solution being applied. The amount of active ingredient (of any PGR) applied per container should be identical to the quantity applied with traditional drench applications.

For example, if a traditional drench application calls for 4 ounces of drench solution for every 6-inch container to deliver 0.12 milligrams active ingredient of uniconazole, then 0.12 milligrams of uniconazole should be delivered to every pot using the higher volume with the watering-in method. This is accomplished by decreasing the concentration of the PGR (uniconazole in this example) to compensate for the increased volume of drench solution being applied to each container.

Typically, the volume applied with the watering-in method is at least twice the volume of the traditional dosage method. In this example, applying twice the drench volume (8 ounces) per container would require decreasing the concentration of the drench solution by half to deliver the same quantity of active ingredient per container and obtain similar results. Ideally, no drench solution should leach out of the container following the application.

If the concentration of the solution (parts per million) applied remains the same, but the volume actually applied is doubled, then there will be twice the amount of active ingredient being applied in each pot, which will often lead to excessive height reductions and possibly non-saleable plants. Keep this in mind: Regardless of the concentration of the drench solution, the amount of active ingredient applied to each container increases as the volume increases. With the watering-in method, it is necessary to decrease the concentration of the drench solution being applied to compensate for the addition volume being applied.

Watering-in Calculations

To determine the quantity of PGR solution and the amount of growth regulator to apply using the watering-in method, it is best to make these calculations as if you were making a drench application.

Calculating application volumes. With drenches, growers must determine the total volume of solution that is necessary to properly deliver the desired amount of active ingredient to each container. The recommended drench volume varies by container size but is similar across the PGRs commonly applied to perennials.

The formula to determine the total volume of diluted PGR solution required is:

# Gallons PGR solution = # of pots x drench volume per pot 128 fl. oz. per gal

Example A. A grower intends to drench 3,000 5-inch pots using the labeled recommended drench rate of 3 ounces per container. Plug these numbers into the above formula.

# Gallons PGR Solution = 3,000 pots x 3 fl. oz. per pot 128 fl. oz. per gal

# Gallons PGR Solution = 9,000 ounces Ö 128 ounces per gal

# Gallons PGR Solution = 70.3125 gal

With the watering-in method, let's assume this grower is applying twice the volume of PGR solution as he would deliver using the drench (dosage) method. Using the watering-in method would require 140.625 gallons of diluted PGR solution when 6 ounces are applied to each container.

Conversely, many growers know how much irrigation they typically apply to a given area but do not know volume of water being applied to each container. To determine the application volume per pot, use the following formula:

Watering-In Volume per Pot = Total Gallons Water Applied x 128 fl. oz. per gal # pots in the area

Example B. A grower commonly applies 250 gallons to an area containing 5,000 5-inch pots. Using the formula above:

Watering-In Volume per Pot = 250 gallons water applied x 128 fl. oz. per gal 5,000 pots

Watering-In Volume per Pot = 32,000 ounces 5,000 pots

Watering-In Volume per Pot = 6.4 oz Remember, no leaching should occur following a watering-in application. If leaching does occur, you'll need to decrease the total volume applied to the area.

Calculating and Preparing Diluted Solutions

Determining the actual concentration of drench solution is critical because the height control achieved is a function of rate and volume. Once you understand the volume you will be applying, you can determine the amount of active ingredient you would like to apply per pot and determine the concentration of the drench solution to apply.

Currently, there are more drench rate recommendations available to growers than watering-in rate recommendations. As a guideline, anticipate watering-in rates to be approximately half of the rates used for drench applications because, typically, twice the volume of drench solution is being applied per container. If you'll be applying more than twice the recommended drench volumes, adjust the rate accordingly.

When determining rates, growers most commonly use parts per million (ppm) rather than milligrams active ingredient per pot. Keep in mind it is the amount of active ingredient that is delivered per pot that determines the amount of height control that will be achieved. For example, a standard 2-ppm solution of flurprimidol (or any PGR) can be delivered at any volume per container without altering the concentration of the solution applied, but the total milligrams active ingredient applied per pot is directly influenced by any changes in application volumes.

All PGR labels include dilution tables that help growers determine how to make diluted drench solutions at various concentrations. These tables usually list the ppm of the intended solution and specify the corresponding amount of concentrated PGR product to mix with a specific volume of water to achieve the intended concentration.

Let's use the two previous examples to determine how to mix a 3-ppm paclobutrazol solution to be applied using the watering-in method.

Example A. A grower intends to apply a 3-ppm paclobutrazol solution to 3,000 5-inch pots by applying 6 ounces per container. From the formula above, the grower determined it would require 140.625 gallons of solution. For simplicity and to account for any waste among the pots, the grower decided to mix 150 gallons of drench solution.

Using the table above, at 3 ppm it would require 0.096 ounces of paclobutrazol for every gallon of drench solution being made. This grower would multiply 0.096 ounces of paclobutrazol per gallon x 150 gallons to obtain a 3-ppm solution. After doing the math, the grower needs to add 14.4 ounces into 150 gallons. The grower would then apply approximately 6 ounces to every 5-inch pot using the watering-in method.

Example B. A grower commonly applies 250 gallons to an area containing 5,000 5-inch pots. To obtain a 3-ppm paclobutrazol solution, this grower would calculate the amount of paclobutrazol to add in a similar manner. After doing the math (0.096 ounces per gallon x 250 gallons), the grower must add 24 ounces of paclobutrazol into 250 gallons. As previously determined, with this grower's current irrigation practices, approximately 6.4 ounces of drench solution is applied to each pot.

Injection Equipment

Growers use various types of equipment to inject PGRs into the water source as they are being applied. This is a widely used and accepted practice. The difference with rate calculations is that the PGRs are mixed into a concentrated stock solution, which is then injected at a known ratio in to the irrigation water.

Expanding on examples demonstrated above, a grower would like to apply a 3-ppm paclobutrazol solution using an injector set at a ratio of 1 to 100. At this ratio, 1 gallon of stock solution is injected into 100 gallons of water. The final solution coming out of the hose needs to be 3 ppm. Using the dilution table above, a 3-ppm solution contains 0.096 ounces of a paclobutrazol product.

To make a 1-gallon concentrated stock solution (which will make 100 gallons of drench solution) which will be injected at a 1 to 100 ratio, the grower would need to calculate how much paclobutrazol needs to be in the 1-gallon solution of stock. If 1 gallon of 3-ppm solution contains 0.096 ounces of paclobutrazol, then 100 gallons would require 9.6 ounces (0.096 oz/gallon x 100 gallons = 9.6 oz). Therefore, the concentrated stock solution should contain 9.6 ounces of paclobutrazol and 118.4 ounces of water (9.6 oz + 118.4 oz = 128 oz [1 gallon]).


Apart from doing these calculations properly, here is the most important thing to remember regarding the watering-in method: The effectiveness of these applications is both a function of the rate and the volume being applied. Of these factors, volume is the determining factor and will greatly affect the amount of height control obtained. Using this method requires that the correct volumes be applied as uniformly as possible across all the plants being treated, as any variability may lead to inconsistent height control.

Conduct your own trials using this method to determine the best rates for your crops, growing conditions and application volumes. As forgiving as this method may seem, it is still easy to over- or undercontrol certain crops in certain situations. For this reason, use conservative rates and aim to provide a base level of height control (70 to 80 percent of the anticipated control required) knowing that you can either reapply using this method or tone the crops as needed using spray applications.

Paul Pilon

Paul Pilon is a horticultural consultant, owner of Perennial Solutions Consulting ( and author of Perennial Solutions: A Grower's Guide to Perennial Production. He can be reached by phone at (616) 366-8588 or e-mail at

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