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▲Growing Knowledge

An ongoing series provided by Oregon State University in collaboration with the United States Department of Agriculture and in partnership with OAN

Putting the grower in controlResearchers look at using growth regulators to control tree heights in production

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In a nutshell, our research investi-gated three PGR products used on three tree species in two production situa-tions. These included two shade tree species — elms (three varieties) and Zelkova — as well as one Christmas tree species, the Nordmann fir.

Christmas tree methodologyFor approximately eight years,

we have been testing PGR use on Christmas trees. After many tests and a few damaged leaders, we have zeroed in on a narrow niche where PGR use appears to be cost effective and safe.

Our candidate trees need to be between 3–5 feet tall, and have an intact, uncut leader with a prior year’s growth of 14 inches or more. Our goal is to slow the tree down for one or two years and pre-serve the leader and bud structure.

Ch

al la

nd

gren

By Chal Landgren and David GosserTrees have a habit of growing in

ways and at rates that my not meet our production expectations. Producers then spend good money sending in crews to cut tops and reestablish leaders.

Our challenge was to investigate the use of plant growth regulators (PGRs) to stop trees’ upward growth at selected heights and to do so without damaging the crop.

A researcher uses a roller to apply a plant growth regulator to a Nordmann fir tree as part of a study.

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▲ PuttInG tHE GRowER In ContRol

In a perfect situation, the PGR would convert what might be an overly long, 18-inch leader, which will need to be cut, to an ideal, 12-inch, uncut leader with all the buds intact. The cost saving to growers involves not needing to train new tops using sticks and ties after overly long leaders are cut.

We have found that PGR use has one important limitation. The treat-ment is only effective if applied to newly emerging leaders while they are between 2–7 inches in length. In any given field, that window of time may last only 2–3 weeks.

The application method in Christmas trees utilizes two opposing small paint rollers. A worker applies the PGR to the roller pads to the point of saturation, then rolls the pads along the leaders.

In these trials we applied the PGR treatments to Nordmann fir Christmas trees. We used Nordmann firs in the study, because we have found that PGR use is much more consistent in this spe-cies than with noble or grand fir.

Three different sites were used, with around 200 trees per site per treatment. The application timing var-ied according to phonological growth, so treatments were made in the first three weeks of June. Only trees that met our treatment conditions (trees 3–5 feet tall, with leaders 2–7 inches long, and prior year leader growth of 14 inches or more) were evaluated and treated.

Shade tree methodologyThe approach for the PGR appli-

cation on the shade trees was more

broad-based, but only conducted at one location.

The preferred application method would be a broadcast treatment from a tractor mounted mist blower to large blocks of plants. However, few growers were interested in experi-menting with thousands of valuable plants, so a CO2 backpack sprayer was calibrated and used as a surro-gate and groups of shade tress were sprayed in rows/blocks.

In the case of elms, two year-old field grown plants were treated. The goal of the application was to hold the plants at a final desired height of around 85–100 inches. Since elms can continue to grow later in the growing season even with reduced water, plants growing too tall can outgrow the sup-port stakes. This can result in plants

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with curved and stiffened terminals, requiring additional labor to correct.

The Accolade™ variety was treated on July 27 at a starting height of 76 inches. ‘Princeton’ was treated on the same date, with an average starting height of 81 inches. Triumph™ was treated on Aug. 3, with a starting height of 68 inches.

Two PGR applications were made on each of the three varieties approx-imately two weeks after the initial treatment, with three measurement times to evaluate plant damage and height control.

Similar trials were conducted in Zelkova. However, rather than detail-ing this experiment, the bottom line here was that an unknown tip disorder impacted large blocks of plants, includ-ing our trial.

Products used in the studyWe used three PGRs in the study

— ProGib® (Gibberellic acid, for coni-fers only), Tre-Hold A-112® (NAA), and Florel® (Ethephon). Short descriptions of the current labeled uses for each are as follows.

Florel is used primarily to control undesirable fruit/nut development and mistletoe infestations.

The Tre-Hold product controls sprouts and sucker growth on apples, pears, olives, and ornamental woody plants and trees. It is used to control branch growth in orchards, residential areas, and areas where tree branch growth may pose a hazard, such as beneath power lines.

The ProGib product is used to con-trol fruit size and development and set in a variety of crops, to change dorman-cy and increase growth. In some seed orchard conifer trials, it has improved cone production and “stressed” vegeta-tive growth.

Study results for conifersThe leader lengths at the end of

the growing season are shown in Table

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Average Leader

Length (in.)

Floral®100 mI/L

Floral® 50mI/L

ProGib80 mI/L

Control Tre-HoldA-112®

3.8 mI/L

Three sites (avg.)

12” 15” 18” 17” 13

Table 1. Average leader length of Nordmann fir by treatment. Three sites averaged.

Treatment Name/ Rate Average of Results Measurements (in.)

Tre-Hold A-112® .095mI/l 113

Tre-Hold A-112® .19mI/l 107

Floral® 10mI/l 89

Floral® 20mI/l 78

Untreated Control 115

Table 2. Average final height for three elm varieties.

1 (next page). At the rates we used, ProGib modestly increased growth, and the lower rate of Florel only provided modest leader shortening.

The high rate of Florel and Tre-Hold appear to provide similar leader control. However, at one of the three sites, the Florel application at the high rate resulted in unacceptable leader bending/bud damage.

Study results for elmsThe leader lengths at the end of

the growing season for all three vari-eties are shown in Table 2. Yet, the results by variety are more nuanced. The Accolade™ and ‘Princeton’ variet-ies grew at about the same rate and treatments performed in a similar way. The Triumph™ grew more slowly and showed less height reduction relative

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to the control plants. Little if any plant damage was observed on the elms from any of the product/rate combinations

The Tre-Hold applications on the elms were largely not effective in reducing plant heights. It is unknown if a higher rate would be effective.

The Florel applications at the higher rate worked well at this site in this year. It was also apparent that only one application of the PGRs was not effective in limiting heights and it was only following the second spray that growth reductions were observed.

SummaryOn the elm trials we learned that

different varieties can respond differ-ently to PGR applications. We also observed that two applications of the PGR were needed to begin the slowing of leader growth.

The use of PGRs on shade trees is far from operational. We had a total of just over 500 shade trees at one site in a one-year trial. Producers investigating further use of PGR appli-cations on shade trees would be wise to look at small block trials over a number of years before beginning any large-scale applications.

On the Nordmann fir conifer trial, we have more years of experience and some level of comfort. Here too we have observed the two applica-tions are better at reducing leader growth, but even with one we do see leader height reduction.

With both shade trees and Nordmann fir, the application window for treatment is only “open” for around two weeks.

Chal Landgren is the OSU Extension Christmas Tree Specialist at the North Willamette Research and Extension Center (NWREC) in Aurora, Ore. He can be reached at chal.landgren@oregonstate.edu. David Gosser is a recent nursery intern at NWREC. His internship was funded by an ODA/OAN Research Grant. He is a student at Clackamas Community College.

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