Besides the EAB, How Do I Keep My Ash Tree Healthy In Denver?

Fielding Tree EAB beetle Denver

With all the hype around EAB, it’s easy to forget ash trees face a variety of other threats. Learn what dangers ash trees in Denver are up against and how to help keep your ash tree healthy.

The Emerald Ash Borer has staked its claim on Denver’s ash trees, devastating the Front Range. EAB infestations are found in wooded lots, landscaped community parks, and well-kept residential areas. There is no safe space! To find out more about the EAB and how to fight the EAB, check out our EAB Resources.

That being said, don’t make the mistake of assuming that the EAB is the only threat to your ash trees. Learn about other pests and diseases that should be on your radar and how to protect your ash trees!

Ash Tree Species Commonly Found In Denver

Ash trees are a favorite pick for lawn, shade, or ornamental street trees in Denver. These deciduous trees are part of the olive (Oleaceae) family. In general, ash trees prefer moist soil and can tolerate warm and cold climates. Although there are over fifty species of ash trees around the world, there are just several different species that thrive on the Denver front range. Check out these detailed descriptions and see if you can identify any on your property!

Autumn Purple Ash

This fast-growing ash is well known for its spectacular purple fall foliage. It holds its color longer than most trees, all throughout the season. Standing 50-70 ft. tall the Autumn Purple Ash makes an excellent shade tree. Highly resistant to drought, this variety adapts well to many different soil types throughout zones 3-9.

Black Ash

The flexible wood of the Black Ash allows it to weave because it is so pliable. This hardy variety can thrive in cold temperatures and wet conditions. If you enjoy watching wildlife in your yard the Black Ash attracts birds and animals alike with their seeds. At maturity, it stands about 40 to 60 feet tall and grows well in zones 2-6.

European Ash

As its name implies, this species is native to Europe and also southeast Asia. In the U.S. this species thrives in zones 5-8, growing to be approximately 60-80 feet tall. Also called the common ash, the European Ash is distinguishable by its black buds; most other ash varieties have brown buds. This variety thrives in Denver because it does very well in highly alkaline soil. It’s beautiful full, rounded crown makes it a great centerpiece for any lawn.

Green Ash

This ash variety is one of the most commonly found in the U.S. because of its hardiness. It has the ability to resist pollution and salt damage, allowing it to grow well in urban landscapes. Capable of growing in a wide range of soil types, the Green Ash thrives in zones 3-9. At full maturity, this upright growing ash will reach 50-60 feet tall.

Narrow Leaf Ash

Known for growing well in urban areas and acidic soils, the Narrow Leaf ash is perfect for the Denver climate. Originating in southwest Asia, this ash variety grows gracefully upward with limbs that look like tendrils. The narrow dripping leaves turn a lovely purple in the fall. Standing 50-80 feet tall, the Narrow Leaf ash tree can be found in zones 5-8.

Blue Ash

Native to the Midwestern U.S., this species grows well in zones 4-7. The Blue Ash gets its name from its inner bark which turns blue when exposed to the air. It was actually used to make dye at one point in history! This species will tolerate wet, soggy soil and can thrive in areas that do not typically drain well. They also love alkaline soil, making them a great fit for the Denver landscape. Blue ash trees spread very little, growing straight up to roughly 50-70 ft tall.

White Ash

This shady ash tree has a spreading crown and towers at 80 ft tall. One of the most common ash varieties throughout zones 3-9, the wood from white ash trees are often used to make wooden baseball bats. In the fall the White ash tree becomes ablaze with gold and red color.

How To Identify Denver Ash Trees

All ash tree species have certain similar identifiable properties, including the branches, leaves, bark, and seeds. Spot the ash trees in your neighborhood or property easily with this expert guide from the Michigan State University Extension:

  • Branch and Bud Arrangement: Rather than being staggered, branches and buds are directly across from each. When looking for opposite branching in trees, please consider that buds or limbs may die; hence not every single branch will have an opposite mate.
  • Leaves: Composed of 5-11 leaflets, Ash leaves are compound patterned.  Leaflet margins may be smooth or toothed. The only other oppositely branched tree with compound leaves is boxelder, which almost always has three to five leaflets. 
  • Bark: The bark is gray and smooth on young trees. On mature trees, the bark is tight with a distinct pattern of diamond-shaped ridges. 
  • Seeds: When present on trees, seeds are dry, oar-shaped samaras. They usually occur in clusters and typically hang on the tree until late fall, early winter.

Ash Tree Diseases and Infestations

When it comes to ash tree pests, the EAB gets all the attention. However, Denver ash trees are plagued by several other pests and diseases. Luckily, you can protect your trees with proactive treatment. Colorado State University Extension offers these quick fact details on each pest:

Ash Sawfly

  • Green, worm-like larvae chew the leaves of ash trees in spring
  • Weather is an important natural control, because late spring frosts and strong winds may kill large numbers of sawflies
  • In early stages of an infestation, larvae make small “pinhole” wounds in the interior of leaves
  • If necessary, ash sawflies can be  controlled by garden insecticide, and many can also be dislodged by a strong jet of water from a hose

Oystershell Scale

  • Pests are brown and shaped like an oyster shell
  • Scales attach themselves to the bark of twigs, branches, and trunks
  • Scales cause harm to a tree by sucking plant juices, injuring or killing entire branches


  • Aphids are on almost all types of plants and a few species can cause plant injury
  • Some aphid species can curl the new leaves of some types of plant
  • Feeding aphids excrete honeydew, a sticky fluid that can attract other unwanted pests

Lilac/Ash Borer

  • Larvae of lilac/ash borer are cream colored with a dark head, while adults typically resemble a paper wasp
  • Larvae concentrate feeding in the lower trunk, particularly near the base of the tree
  • Borer tunneling activity produces gouging wounds under the bark

Banded Ash Borer

  • Adults are distinctive, with red and brown markings and rather long legs
  • Feeding is usually limited to trees that are in serious decline or have recently died
  • Damage is produced by tunneling patterns and oval-round exit holes

Ash Bark Beetles

  • Stout-bodied beetles, gray or brown in color
  • Ash bark beetles rarely attack healthy portions of the tree
  • Adult bark beetles cut chambers just under the bark
  • Common injury includes limb dieback and crown thinning

Cytospora Canker

Cytospora canker is a fungal disease that occurs on parts of elm trees that are stressed. The canker-causing fungi cause girdling of the plant, killing the plant above the canker.

Ash Tree Care And Treatment Options


Chemical controls can effectively control pests like beetles, ash borers, and aphids.

Treatment with Bacistat trunk injection: When your body has an infection, you take an antibiotic. Bacistat is an antibiotic for your trees! It can be applied yearly in April-May. We recommend treatment every 2 years to limit trunk damage. Bacistat will reduce only the twig stage, not the blossom stage of infection. Optimal results with Bacistat occur when applied prior to the onset of visual symptoms.

Ash tree fungal diseases, such as Cytospora Canker, develop in trees that are stressed or injured. Healthy trees can fight off infection! Here are our recommendations on best seasonal practices for keeping ash trees healthy and strong:

  • Watering: proper watering is an overlooked but very important tree care practice. Some ash trees are prone to fungi related diseases if their soil becomes too saturated with water, and other damage can occur if there isn’t enough water. We can provide expert consultation on how much water you should be giving your ash trees, and how often.
  • Fertilization: for many ash species, even the ones commonly found in Denver, the high alkaline soil can be a problem. The best way to neutralize this issue is with the right fertilizer application. Balancing the soil and getting your trees the nutrients they crave will keep them strong and healthy.
  • Tree trimming: annual pruning and trimming help your ash trees continue to stand tall and proud. Trimming damaged or slightly infected branches can save the whole tree!

Are you concerned that your ash tree is displaying symptoms of a pest infestation or disease? Contact one of our certified arborists ASAP! At Fielding Tree & Shrub Care we take a four-step approach for treating ash trees. Sending a picture in advance helps speed things up! We work to cure the disease or at least slow it down with the most viable treatment option available for your tree or shrub’s unique situation.

To learn more, schedule your complimentary on-site diagnosis today! One of our arbor experts will walk you through our four-step approach for treating ash trees and create a custom plan for the unique needs of your trees!