Colorado trees are succumbing to the same insects ravaging forests. Arborists have tips to keep your backyard healthy.

Years of roller coaster weather in Colorado are taking a toll on our trees.

Besides frequent drought, the state experiences extreme temperature fluctuations, with day-to-night temperatures often varying by 30 degrees or more. Recall Dec. 21, 2022, when a 51-degree high was followed by a -10-degree low in the Denver area. (Take a stroll down a chilling memory lane with this National Weather Service review of the greatest fluctuations in temperature changes since 1872.)

“It’s tough being a tree in Colorado,” says Whitney Cranshaw, Ph.D., emeritus professor of Entomology at Colorado State University.

According to CSU Extension, 2018 and 2020 were two of Colorado’s driest years on record, and we’ve experienced many other hot, dry years. Recent summers have been brutal, and last summer was one of the worst, with last August registering worldwide as the hottest on record, according to the National Oceanic and Atmospheric Administration. In the Denver area, the average temperature in August was above normal — with 15 days at or above 90 degrees, according to the National Weather Service. 

Those are the background facts for this: While humans have intermittently frozen and sweltered, so have Colorado’s trees. And stressed trees become hosts for deadly pests and disease, primarily an ips beetle (there are 11 known species in Colorado) or pine wilt disease.

“All insects are advantageous, just like cancer is advantageous,” says arborist Troy King, owner of American Tree preservation service, based in Golden. “Don’t give them the advantage.” 

Keeping trees healthy requires mulching, watering and fertilizing not only pine and spruce trees, but all trees, from the ash, which are at severe and imminent risk of emerald ash borers, and red maples to birch, linden and the many ornamental and fruit trees.

Mulch

“Mulch is a wonderful thing,” says arborist Toni Smith, the Rocky Mountain territory manager for Rainbow Ecoscience, which provides chemical products and equipment to tree and lawn care companies. 

Mulch helps retain moisture and keeps the soil temperatures cooler around a tree. It provides a sturdy barrier against weeds (better than landscape fabrics, which Smith discourages using). As good-quality mulch degrades, it adds nutrients into the soil around a tree. 

Smith recommends using arborist wood chips, which includes any part of a tree that’s passed through a wood chipper. (See if you can obtain the material free via ChipDrop.) The next-best thing? Small bark mulch. 

Watering

Oftentimes, the difference between a living tree and a dead one is watering. Because of the years of drought and our sizzling summers, trees need to be watered year-round. 

On a late November afternoon, King pointed out the dead and dying trees in Arvada and Wheat Ridge neighborhoods. Behind a line of six tall, dead spruce trees on 42nd Avenue in Arvada stood the same variety, but healthy and living. What was the difference? Water. 

Trees are dying from thirst and they’re dying from the opportunistic pests and diseases that prey on stressed trees, King says. 

The ips is a bark beetle that infests pine and spruce trees, according to CSU. An infested tree dies from the top down. Avoid this — and pine wilt disease — with ample watering.

“The ips’ ability to attack a tree is greatly affected by the hydration of the tree,” Cranshaw says. “The tree’s primary defense is a good pitch (or resin) flow that drowns out the (pest).” 

Give trees a slow, thorough watering weekly or every two weeks during the summer, Smith suggests. During the winter months — November through April — that don’t receive at least an inch of precipitation as rain or snow, water trees an average of 10 gallons for every 2 inches of diameter. Or use a garden hose with a fountainhead sprinkler, set a timer to 20 minutes and slowly soak every tree on two sides, Smith said.

Don’t water trees when air temperatures hit freezing — it could kill them — but do water them when temperatures are above 30 or 40 degrees, said King, who, like some other arborists, will water your trees for you. Among the more than 3,000 clients his company serves annually, he estimates about 100 ask for help watering their trees during the crucial winter months. 

Fertilizing

Visit a reputable garden center for advice on fertilizer, Smith said, and avoid concentrations that lean heavily into nitrogens, which encourage growth. Instead, look for fertilizers with organic materials such as friendly bacteria and humic acids, which help trees absorb nutrients. 

“I’m not a big nitrogen fan,” Smith said. “Nitrogen has its place, but it pushes the tree. It makes the tree push quite a bit of new growth. If it’s not able to support that below ground (in its root system), you’re setting yourself up for failure.”  

She also recommends using a fertilizer with little or no phosphorus because of its overuse in the agricultural industry. Fertilizers high in nitrogen and phosphorus wash into storm drains, eventually reaching — and de-oxygenating — nearby lakes and streams, according to the National Oceanic and Atmospheric Administration. 

One more critical detail: Do not fertilize newly planted or stressed trees. 

“It’s like giving a sick person a shot of adrenaline,” Smith warned. 

If all else fails …

It’s time to call the professionals. If a pine or spruce looks under duress, call a tree care company. But know that insecticides are preventative, according to CSU Extension fact sheets for ips beetles and pine wilt disease. They cannot save a tree that’s already been infected by either. 

“The first sign of pine wilt is death,” King says matter-of-factly. Death can occur within three weeks of infection with the nematode, according to CSU research. 

Still, King will sometimes battle for a tree’s life. 

“I have the technology to save a tree, but I cannot save every tree,” he says. “I’m a preservationist. That’s what I do. I save trees. But if you don’t water your trees, even if you spray them (with insecticides), you’re probably going to end up with dead trees. You’ve got to water your trees.” 

CORRECTION: This story was updated at 12:10 a.m. Dec. 18, 2023 to correct the location of American Tree’s headquarters. It is in Golden.