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Science-based gardening information for Colorado communities from CSU Extension, Denver Botanic Gardens, and Green Industries of Colorado.

1769 – Ash Care for the Do It Yourself Gardener   arrow

Ash trees in Colorado have 3 basic needs – water, insect control and pruning.


Consistent watering during the growing season and winter watering is a task a homeowner can readily accomplish.  Drought stressed ash are prone to native borer attack and winter kill.  Lawn irrigation is usually enough for ash in the growing season.  In the dormant season, ash do need to be winter watered every 4 to 6 weeks when there is a lack of snow cover.

Insect Control

Some insect control may also be accomplished by the home gardener.  You will need application equipment, personal protective gear and be willing to purchase, store as well as handle insecticides safely.

Tree size is important.  Grab your trusty measuring tape and wrap it around the tree trunk at 4.5 feet above ground level.  This measurement will give you the trunk circumference.  Divide the circumference by 3 and the result is an approximate trunk diameter.

Lilac ash borer

Lilac ash borer
This borer has been in Colorado for decades.  While lilac ash borer (LAB) doesn’t kill ash outright, it weakens tree vitality and structure.  LAB may attack any tree size, however is predominantly found in trees less than 8 inches in trunk diameter.  To treat for this borer, one spray per year in mid to late May with the active ingredient permethrin is required.   Soil applied insecticides are not effective.  Sprays, thoroughly wetting the bark, should be targeted at the tree trunk from the soil level and up to 3 feet above the first branches.  Smaller trees may be sprayed with a good quality pump up sprayer.

Emerald ash borer

Emerald ash borer
This one was discovered in Colorado in 2013. Now firmly established in Boulder County, emerald ash borer (EAB) will spread throughout the South Platte River Drainage.  EAB kills trees within 5 years of being infested.  Trees with trunk measurements less than 15 inches in diameter may be treated with consumer insecticide formulations.  Higher insecticide rates are required for trees larger than 15 inches in diameter.  These formulations are limited to commercial applicators and not found in retail outlets for consumer purchase.

EAB is treated systemically with soil applied insecticide.  Topical sprays are not effective.  To treat this insect, use an insecticide with the active ingredient Imidacloprid.  Apply once in the spring of the year just when you begin watering your lawn.  Mix insecticide and pour it slowly and evenly about the tree trunk base.  Take care not to let the insecticide run offsite into the sidewalk or gutter.


Pruning branches from the ground may be accomplished with hand or pole pruners.  If you need to climb a ladder or roof to prune, you should call a professional arborist.  Ash benefit from frequent pruning, removing dead and broken branches.  Ash also benefit from pruning to train and improve branch structure.

For more information on:

  • Landscape watering refer to message numbers 1621, 1719 and 1751;
  • Ash borer information refer to message number 1425;
  • And for tree pruning information refer to message number 1724.

For more information, see the following Planttalk Colorado™ video(s).

For a complete treatise on EAB in Colorado, visit the Colorado Department of Agriculture’s web page at