Tree Health Care

When a tree looks unwell

Trees show stress in a handful of ways: thin or yellowing leaves, leaves that scorch brown at the edges in summer, dead branches at the top of the canopy, oozing or cracked bark, holes in the trunk or clusters of shoots sprouting low on the trunk. Some of these point to insects or disease. Many point to drought, poor soil, construction damage to the roots or a tree planted too deep. Our arborist looks at the whole tree and its surroundings before recommending anything, because spraying a tree that is really short of water does not help it.

Emerald ash borer

Emerald ash borer is an invasive beetle that kills ash trees. Its larvae feed under the bark and cut off the tree's flow of water and nutrients. Since it was first found in Colorado in Boulder in 2013, it has spread to many Front Range cities, and Denver confirmed it in 2025. Signs include thinning at the top of the canopy, small D-shaped exit holes in the bark, S-shaped tunnels under loose bark, woodpecker damage and shoots sprouting from the trunk. Not every ash with a thin canopy has it, so an inspection matters before you decide.

Treat, or remove and replace

A healthy, valuable ash can be protected. The most effective treatment is a trunk injection of emamectin benzoate, which the tree carries up into its canopy. Denver's forestry office reports that treatment is around 90 percent effective when done correctly and must be repeated every 2 to 3 years for as long as you want to keep the tree. Small ash trees, and ash trees already in decline, are often better removed and replaced with another species. Colorado has limited the sale of many neonicotinoid insecticides, including imidacloprid, since July 2024, so most homeowner soil drenches are harder to find. Injecting a street tree in Denver needs a free permit from the Office of the City Forester, which we apply for, and Denver has run an ash removal and replacement program for some small street ash trees.

Other problems we see

Aphids and scale insects on many shade trees, iron chlorosis turning leaves pale yellow in our alkaline soils, and bark beetles such as Ips attacking pines and spruce weakened by drought are common calls. Fire blight on crabapples and other fruit trees, and canker diseases on aspen, show up in wet springs. Some of these need treatment, some need better watering or mulch, and some are cosmetic and best left alone.

Treatments and licensing

Applying pesticides for hire in Colorado requires a commercial applicator license from the Colorado Department of Agriculture, and our technicians hold one. We only treat when it makes sense for the tree, we follow the product label and we tell you in writing what we applied and when the next treatment is due.