Understanding Plant Growth Regulators (PGRs)

Great Western Tree Care – An Arborist’s Guide for Homeowners

When most homeowners think about tree care, they think about pruning, watering, fertilizing, or managing pests. But there is another powerful tool that arborists use to improve tree health—one that many people have never heard of:

Plant Growth Regulators (PGRs).

Plant growth regulators are professionally applied treatments that help trees grow more efficiently, handle stress better, and direct their energy into root development instead of rapid shoot growth. They are especially useful in Colorado, where trees face intense environmental stressors like drought, alkaline soils, compacted ground, and rapidly changing weather patterns.

This article explains what PGRs are, when they’re useful, and why they can be one of the best investments you make in the long-term health of your trees.

What Are Plant Growth Regulators?

Plant growth regulators are compounds that influence how a tree grows. Unlike fertilizers, which supply nutrients, PGRs act on a tree’s internal growth processes.

Most PGRs used in arboriculture are growth retardants, meaning they slow down shoot (top) growth. But slowing growth isn’t the goal—it’s the benefits that come with redirected energy that matter.

How PGRs Affect Tree Growth

Normally, a tree spends a large portion of its energy each year producing:

• long shoots
• new leaves
• canopy expansion

When we apply a plant growth regulator, something important happens:

The tree grows fewer, shorter shoots…

…and that unused energy gets shifted into:

1️⃣ Root growth

2️⃣ Defense chemicals

3️⃣ Stress tolerance

4️⃣ Leaf thickness and color improvement

Instead of forcing the tree to grow less, PGRs help the tree grow better.

Why Plant Growth Regulators Are So Useful in Colorado

Colorado’s Front Range is one of the most difficult urban environments for trees in the country. Trees here battle:

• intense sunlight
• rapid temperature swings
• prolonged drought
• compacted clay soils in new developments
• alkaline conditions
• limited rooting space
• pest pressures like borers, mites, and scale

These stressors drain a tree’s energy reserves.

By reducing unnecessary shoot growth and boosting root development, PGRs help trees:

• survive heat stress
• manage drought better
• access water more efficiently
• resist pests and diseases
• adapt to poor soils
• maintain better canopy density

PGR-treated trees often look healthier within weeks and remain that way for years.

How Plant Growth Regulators Are Applied

PGRs are typically applied through a soil injection around the base of the tree. The tree’s roots absorb the product and distribute it throughout the canopy.

A single application typically lasts 2–3 years, depending on the product and the tree’s growth rate.

Benefits of PGRs for Homeowners

1. Reduced Need for Frequent Pruning

Slower shoot growth means:

• fewer low branches to prune,
• fewer clearance issues over sidewalks and roofs,
• lower long-term maintenance costs.

This is especially helpful for fast-growing trees like:

• silver maple
• elm hybrids
• cottonwood
• honeylocust
• ash (if not yet removed for EAB)

2. Improved Drought Tolerance

With more energy going to the roots, trees:

• use water more efficiently,
• maintain leaf moisture levels longer,
• and show less stress during hot, dry months.

In Colorado’s climate, this is a major advantage.

3. Better Color and Thicker Leaves

PGRs increase leaf thickness and chlorophyll density. Homeowners often notice:

• deeper green leaves,
• fuller canopies,
• improved photosynthesis,
• less scorch in hot summers.

4. Reduced Pest Pressure

Stressed trees attract pests. PGRs:

• improve internal chemical defenses,
• thicken leaf tissues,
• and enhance overall vigor.

This makes the tree less appealing to insects like:

• aphids
• mites
• scale
• borers

5. Help for Trees in Poor or Compacted Soil

New subdivisions often have:

• stripped topsoil,
• heavy clay,
• construction compaction,
• buried root flares.

PGRs help trees adapt by accelerating root system development, giving them a better chance in challenging soil conditions.

6. Reduced Watering Requirements

While PGRs don’t replace watering, they do help trees:

• conserve internal moisture,
• manage drought stress,
• rely more on root efficiency.

This is incredibly valuable during watering restrictions or dry summers.

Which Trees Benefit Most from PGR Treatments?

PGRs are particularly effective for:

• Fast-growing shade trees
• Trees planted in poor, compacted, or alkaline soils
• Street trees and parkway plantings
• Trees under chronic drought stress
• Trees repeatedly pruned for size or clearance
• Trees suffering from nutrient deficiencies or root restrictions

Colorado homeowners use PGRs commonly on species like:

• Silver maple
• Linden
• Elm
• Honeylocust
• Ash (where appropriate)
• Burr oak
• Ornamental pears
• Crabapple
• Austrian pine

Are PGRs Safe for Trees?

Yes—when used by trained arborists.

Modern PGRs:

• do not harm the tree,
• do not stunt overall health,
• and are used widely by professional plant health care specialists.

They must be applied correctly and at the right dosage, which is why they are not DIY products.

When Should Homeowners Consider a PGR Treatment?

A plant growth regulator may be helpful if your tree:

• grows too fast for its space
• struggles with drought stress
• is newly planted in tough soil
• needs root recovery after construction damage
• suffers from repeated pest problems
• shows nutrient deficiencies
• faces chronic pruning demands
• needs improved vigor without stimulating excess growth

PGRs are often part of a well-rounded plant health care program, especially in challenging landscapes.

Final Thoughts

Plant growth regulators are one of the most effective yet underused tools for improving tree health—especially in Colorado’s harsh growing conditions. They don’t just slow growth; they rebalance the tree’s energy system so it can build strong roots, resist stress, and maintain a healthier canopy with less pruning and fewer problems over time.

For many trees, PGRs are the difference between simply surviving and truly thriving.