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.

What's limiting your trees's growth?

What limits plant growth?

Have you ever experienced a tree or woody ornamental that just doesn't want to grow?  Overall plant health and vitality depend on several factors, and if just one is out of whack, you may end up with an under performing plant.

It begins in the nursery

Starting with a quality plant is key for long term success.  Occasionally there are breakdowns in how the plant is cared for in the nursery that can affect how it thrives in the landscape.  During production, if a baby tree is kept in a pot for too long it may eventually form a distorted root system.  If these conditions aren’t addressed, the deformed root system will remain with the plant for it's entire life.

One common root distortion that may form is circling roots.  Circling roots are roots that grow around the stem of the plant instead of perpendicularly away from the plant.  In the nursery this happens when young roots contact the side of the pot in which the plant is growing, and start growing along the outside of the container.  Circling roots can affect plant stability.  If these roots are in contact with the stem they can lead to a condition called girdling roots.  Girdling roots have the potential to cut-off portions of a stem’s vascular system, interrupting nutrient and water flow through the plant.

At planting

There is an old saying, “you only get one chance to plant a tree.”  While planting a new tree or shrub seems intuitively easy, there are some caveats to proper planting.  All woody plants should be planted so their root flare is  at grade level and exposed, not covered by excess soil or mulch.  The root flare is the transition zone between the trunk and the roots.  Buried root flares may result in bark damage which lead to vascular system issues.  Buried root flares may also lead to girdling roots.   Note, sometimes root flares are buried due to poor nursery practices.

It’s also important to dig a hole large enough for the tree/shrub.  Standards state digging a hole at least 1.5 times the diameter of the plant’s root ball. The larger the planting hole the better.  This is so newly emerging roots have an easy time of colonizing the native soil.  The more soil that can be loosened around a newly planted tree, the faster the tree will be able to become established.

Site, site, site

Matching the right tree to the site and site preparation is paramount to overall performance of trees and woody ornamentals.  Different species have unique light, soil, and water needs.  Let’s take our native flowering dogwood as an example.  According the USDA NRCS plant materials program: “Flowering dogwood is adapted to most upland sites but grows best on rich, well-drained soils on middle and lower slopes. It develops best as an understory species in association with other hardwoods.”  So, if we are to plant a flowering dogwood in a poorly drained clay soil in full sun, it goes without saying the tree will not be healthy vigorous specimen.

One major issue we run into, especially in our urban/suburban landscapes, is damaged soils.  During new building construction the top layers of soil, which forest trees roots thrive in, are removed.  What remain are compacted subsoils.  Compaction destroys soil pore spaces that air, water, and plant roots occupy.  This is direct stress on woody ornamentals, and one of the greatest reasons trees and shrubs fail to thrive in our landscapes.  Accompanying compaction in urban/suburban soils are also nutrient deficiencies, soil chemistry imbalances, and inadequate amounts of soil organic matter.

Another major issue for plants in our landscapes is water issues.  Too little or too much water is often the difference between green and growing plants or yellow and dying plants.  Combine improper watering with poor soils and you have a recipe for disaster.

All's not lost

Fear not.  While the issues we discussed may sound overwhelming the arboriculture industry has developed some innovative protocols and tools for mending these common problems.  Laboratory soil analysis, soil decompaction treatments, and root collar excavations utilizing compressed air-tools are just a few methods available to mend adverse plant growing conditions.  If you have a tree that just doesn't seem to be growing, call your friendly neighborhood qualified arborist for a thorough assessment.