by Elizabeth A. Tomlin

Dr. Brian Whipker, North Carolina State University; Chris Currey, Iowa State University; Joyce Latimer, Virginia Tech; and Lee Stivers, Extension Educator with Penn State Extension, discussed greenhouse production research in a recent webinar.

Stivers focused on accuracy when measuring and mixing small amounts of plant growth regulators (PGR) and pesticides.

“One of the key steps in making effective applications of crop growth regulators and pesticides is measuring and mixing the product accurately,” explained Stivers. “So, if a crop growth regulator or pesticide product is applied at a rate or concentration that’s either too high or too low, the results can be really disappointing and sometimes quite costly.”

Greenhouses frequently require small applications and the math behind calculating those minute amounts accurately can be quite tricky.

“We have to remember that liquid and dry ounces are not the same thing. A liquid ounce is a volume measurement in contrast to a dry ounce, which is a measure of weight.”

Dry ounces are most accurately and consistently measured with a digital scale, such as a postage scale.

Stivers recommends comparing your liquid ounce device with an accurate, graduated cylinder. Liquid measuring devices with pour-spouts are easier to handle. Tall, narrow devices, with easy to read measurements, are most accurate.

“For a really small amount, you should consider using syringes,” Stivers advised. Medical and veterinarian syringes without needles are available for purchase in a variety of sizes. “Remember that milliliter (mL) and ‘cc’ is the same amount of volume.”

Applying the proper amount of pesticide or PGR requires that you know how much volume per unit of area you are applying at a given speed and pressure (sprayer calibration) and that you mix the right amount of product and water in your tank (concentration of mix). Calculations for these involve “rates.”

Rates are one unit of measure per another unit of measure, such as 3.2 gallons per acre, which may be read as a fraction.

However, Stivers cautions, not all rates can be read as fractions.

Stivers emphasized, “If you follow units, you’re not going to make mistakes.”

Mix Master calculators are available at

Joyce Latimer reported on an annual PGR guide for both annuals and perennials, in alternating years. Crop reports, new research on tank mixing, PGR tips, rates, references and resources are all available in this guide at

“PGRs should be handled as production tools — like water and fertilizer. They should not be used as crutches for poor management of other cultural practices,” said Latimer.

Dr. Whipker explained that although PGRs are an integral tool used in floriculture production and that optimal rates help manage and enhance plant growth, excessive rates can stunt plant growth.

He presented information on recognizing PGR overdose symptoms and identifying diseases that cause distorted growth similar to overdose, while salvaging your crop when possible.

Whipker emphasized getting the rate right as the first concern, as was explained by Stivers.

“When you do mix right, you get nice even control,” said Whipker. “We want consistent results.”

Controlling plant stretch, producing plants proportional to pot size, creating tighter plant spacing and producing plants with darker green leaves that require less water are all positive factors resulting from PGRs that are used correctly.

However, overdose from spray drift onto non-targeted plants, or uneven drenches, may cause gnarly, stunted growth, lack of leaf expansion, flower delay/smaller in size and poor plant establishment in landscapes.

Products applied as foliar spray, substrate drench or chemigation, can help. However, Whipker cautions, begin with the lowest rate, start with a small trial, and wait seven days to evaluate results before reapplying. If a second treatment is required, use half of the initial rate. Excessive rates can result in excessive stem stretch, discoloration and upper leaf yellowing.

“Response rate can vary significantly by cultivar,” said Whipker.

Whipker advises following three steps when diagnosing plant symptoms.

“Know what disorders are potential, what information…you need to collect and then go through the problem solving. You really don’t want to shoot from the hip. Collect data, so you know you’re making the right diagnosis or send it off to the diagnostic lab.”

Information to consider includes; crop history, type of symptoms viewed, location of symptoms, initial pH/EC values, visual check of plant roots and percentage and pattern of crop affected.

Some symptoms are scattered, while others affect entire crops.

Whipker presented information on common diseases and disorders that also cause distorted growth, mimicking PGR overdose, including calcium deficiency; boron deficiency; broad mites; 2-, 4-D herbicide contamination; thrips and aphids.

Chris Currey presented updated information on the use of PGRs with bio-containers, which are increasing in popularity.

“The thing that changed greenhouse horticulture containers the most was the introduction of the petroleum plastic plant containers,” said Currey, explaining that there are a number of reasons why these containers have been so widely adopted in horticulture.

However, the downside to plastic containers in a more environmentally conscious society is in their disposal. This has resulted in looking for alternative containers for horticultural use, and commercially available bio-containers are being evaluated in research settings.

Bio-containers are made with bio-degradable material that will break down in the waste stream.

Some containers being studied and compared are peat pots, coconut coir-based biocontainers, paper fiber containers and rice hull containers. Bio-plastic containers are also being studied.

Because biocontainers are meant to break down, sometimes that action occurs during production, requiring adjustments in irrigation.

Since we know that differences in cultivars and plant substrate components need to be considered when using PGRs, do biocontainers also affect PGR efficacy?

In controlled studies, Currey reports that several biocontainers, including bio-plastics, rice hulls and paper fiber, produced results similar to growing with PGRs in plastic.

“So you don’t need to adjust your PGR concentrations for those containers.”

However, peat pots and coconut coir containers, both extremely porous containers, produced smaller plants in both treated and untreated containers.

“We believe that these smaller plants are likely attributed to substrate moisture. In our preliminary experiments, we saw that the plants grown in coir containers and the plants grown in peat containers required more frequent irrigation. It wasn’t that the plants were using more water, we believe that it was due to a quicker dry down of the soil substrate moisture. We all know that when we grow dryer, our plants don’t grow as big.”

Researchers do not believe coir or peat containers absorbed PGR or affected its function, but that plants were affected by mild moisture stress. When using fairly-porous containers, reducing PGR concentrations avoids over-regulating crops and producing smaller plants.