Plant growth regulator.  That may sound scary to some, and counterintuitive to others.  Why would we want to intentionally stop a tree or shrub from growing, and how does that even work?  Believe it or not, there are many reasons why it may be beneficial to slow the growth of a plant, which include economic factors, environmental factors, and overall plant health factors.

Before we delve in to the specifics of plant growth regulators, or as I like to call them ‘plant growth managers (PGMs),’ it’s important to know that many of us come in contact with this technology quite often.  PGMs first became popular in the floriculture industry to get uniform plants that would be merchantable when they made it to the garden centers.  Have you ever noticed your house plants start becoming leggy and a bit yellow after they’ve been in the house for a few weeks?  That is because the growth regulator applied before you bought them is wearing off.

Early versions of PGMs, Type I growth regulators, were closely related to herbicides, and actually blocked cell division to accomplish reduced stem elongation.  While Type I growth regulators work very well at reducing growth there can be some unintended consequences of using them.  Namely, Type I growth regulators may cause leaf yellowing and distortion.  If applied above certain temperatures Type I growth regulators may also cause slight defoliation. 

Modern PGMs, Type II growth regulators, work within the plant to regulate the hormone (gibberellin) which is responsible for cell elongation.  This means the plant is still producing the same amount of cells, leaves, buds, etc. just those stems are extending 30%-70% less than normal.  This reduction in growth can lengthen time between pruning cycles for trees and shrubs growing in close proximity to infrastructure.  For example, the oak tree planted 10-feet from the corner of your executive building, the line of trees planted underneath those utility lines, or that hedge that needs to be trimmed a few times a year so you can see out of the first floor windows.  

Less pruning means less wounding for the plant (bonus for the plant), less time your crews need to spend managing your shrub and tree resources (bonus for labor), and also means less ‘green-waste’ your crews need to dispose of (bonus for the environment). In 2016, Rainbow Scientific, in cooperation with our partners, performed a series of trials to determine labor savings when incorporating Trimtect into their pruning operations.  Trimtect is a foliar spray-applied Type II plant growth regulator.  The sites selected contained highly manicured shrub hedges ranging in length from 50-180 feet long and 4-8 feet tall.  Over a twelve week period we found pruning time was reduced by an average of 62% when compared to shrub hedges not treated with Trimtect.  In another trial we found green waste removed from the site was reduced by 50% over a 12 week period.  Managers and crew leaders involved in the trials were unanimous in their positive response to the reduced need to prune.  Landscape crews were able to focus on getting more detail work accomplished (e.g. weeding, flower bed maintenance, trash removal, etc.), in addition to getting caught up on turf mowing and edging.  They also had less ‘call backs’ and complaints in areas where Trimtect was applied.

While plant growth managers reduce the amount of above ground growth by 30%-70%, they are also promoting responses in the plant that can encourage plant health. One positive side effect of growth control is the stimulation of another plant hormone, abscisic acid (ABA). ABA helps with preventing cell dehydration, and regulating leaf water loss by allowing stomata in the leaves to respond faster to drought conditions.  When PGMs are applied as a soil drench we see energy resources being diverted to the promotion of fine root growth. This allows a tree to mine more resources from the soil, and increases drought tolerance.  PGMs can also increase the amount of chlorophyll the plants produce.  Chlorophyll is, of course, what gives a leaf its green color, and plays a major role in photosynthesis.  Disease resistance to certain fungal leaf and canker diseases has also been recorded with the application of PGMs.  So, not only can you employ PGMs as a growth management tool, but they can also be employed as a plant health care tool.

The use of plant growth managers can often be overlooked, but when used correctly, can be a substantial tool to help reduce time spent on pruning while also benefiting plant health.  Next time you’re walking through your site think of those hedges, ground covers, trees, and vines that seem to need constant attention.  Now imagine, instead of the plants dictating your pruning schedule, you dictate the plants growing schedule.