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The Role of Plant Growth Regulators in Landscape Management

Alan C Beverly March 15, 2012

The use of plant growth regulators (PGR's) in horticulture is well established by growers of horticultural commodities, and much university research has explained the biochemical pathways which are the targets of the applied chemical. They all interfere with the synthesis of the Gibberellins, a primary growth hormone made in apical meristems.Thus deprived of the stimulus for growth the effects are a great reduction in new growth at the apical meristems of the plant. They are metabolized slowly, and have lasting effects. The plants' physiology changes when new growth ceases. There is more starch available for root tissues enhancement.

Formulations of PGR's for landscape use have been on the market for many years with variable acceptance by maintenance contractors, who continue to defy the supreme logic of their use for cost control and for the health of the clients landscape. The primary use is to reduce labor costs and unnecessary work which drives up the expense of a maintenance contract. The secondary and tertiary reasons for using PGR's are a direct benefit to the plants themselves; stronger roots, increased high and low temperature tolerance, and increased drought tolerance. The best PGR to use to achieve managements goals is Paclobutrazol. Three trade formulations are a 22% liquid, which is diluted to 1% and applied as a basal soil drench or injected below the soil surface. The dose of the ready-to-use 1% solution is very important to get correct. The species and its size are referenced to the correct dose. The chemical is poorly soluble in water, so it does not move within the soil profile. Feeder root uptake occurs when leaf actively transpires. The chemical appears at the apical meristems in a time which varies with the activity of the tree/shrub. Applications may be made at any time of year soil is not waterlogged or frozen. Three formulations, Shortstop, Profile 2SC, and Cambistat are all registered in California for this application. There are no downside or undesirable affects which have been noted. Once uptake occurs the chemical is effective for 3 years. This is the game changer factoid which makes the micro-economics of application a big deal,to the advantage of the property owner or municipality. The payback period after initial application works for three growth seasons, so the savings in pruning labor costs are tripled for one application of Paclobutrazol.

The savings are realized by the property owner only if expected maintenance work is not required in the future. The cost of the application is borne in the present in order to set up future latent value. If a maintenance contract is involved, it will need to be rewritten to specifically exclude the need to hedge trim or to prune trees, which are big expenses for any large property owner. This is the source of landscape cost reduction for property owners.

This strategy of using PGR's to reduce costs carries other benefits which should be heralded. The technique of hedge trimming plants for controlling size is well established by all landscape maintenance companies. Too bad for the plants' health though, because the negative effects are visually bad, and the plants' root systems are starved for starch by leaf removal, making these plants vulnerable to drought and heat stresses, insects,pathogens and creating a shortened lifespan. The owner must then pay the landscapers to replace plants which they killed softly. Stopping this unfair cycle of extra, unnecessary work is a cost savings to the property owner again. When a woody plant installed as screen (hedge) reaches the desired size it should be treated with Paclobutrazol to freeze it at this size, and for its' continued health.

Utility companies in the MidWest have discovered significant cost savings in keeping their high-voltage lines free of tree interference and damage. Once a tree under a utility line has been reduced by the standard ROW pruning technique by arborists, the trees are then treated with Paclobutrazol (by others) to suppress regrowth . The utility saves big $ for three years thereafter in what they aren't required to do, go back again and prune the same trees.The arithmetic of savings to be realized is bigger when the crew involves a bucket truck, two ground men and two climbers, far off the main road. When a hedge in a city Park is concerned the effects apply but the numbers are smaller. The carbon footprint of maintaining a hedge with a gas powered machine is greatly reduced also. The hedge trimmer is arguably the most dangerous piece of equipment on the landscapers truck, and is the plants worst enemy too. The use of PGR's dovetails well with everybody's desire to wear the label "GREEN".

Here's a photo of a plant commonly used in commercial plantings, Xylosma congestum. It is frequently hedge trimmed, causing latent adventitious buds below the cut to begin growth (which were suppressed by auxin hormones before the cut). The normal color is a pale yellow-green,with only the shaded leaf deeper green. I have treated this plant with Paclobutrazol continuously for 6 years to achieve the rounded top. This plant requires only 5 minutes each year to maintain, trimming by hand pruner some errant shoots. Needless to say, I do not fertilize this plant. Without Paclobutrazol the maintenance time would require 6x/year about 30 min each session using a ladder. The overall leaf color is a deeper green in full sun. Landscapers spend a great deal of time with this species, at great cost to the owners, which benefits only the contractor.

Xylosma congestum

The use of PGR's in landscape management, given current economic constraints, is a strategy which answers both the short and long term goals of restraining costs by reducing labor required to accomplish the goals of property owners and municipalities alike. The contractors know where their greatest profits arise, unspent labor dollars. It is time for the property owners and municipalities to begin to share in this also.

References:

Chaney, W.R. 2005 "Growth Retardants: A Promising Tool for Managing Urban Trees," FNR-252-W Purdue Extension Publications
Watson, G. 1996 "Tree Root System Enhancement with Paclobutrazol", JOA 22(5):211