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The 4 Most Dangerous Trees for Colorado?

As gardeners in Colorado may well know, our climate and altitude make for challenging growing conditions. This doesn’t mean we are completely immune from invasive exotic plants making an unwelcome home here. We do not have the burden that gardeners in wetter and warmer states may have at stopping the spread of invasive plants, but there are a few species you should be aware of so you can help stop these unwanted guests from gaining a foothold in our landscapes.

Why be concerned about invasive plants? They crowd out native plants, propagate uncontrollably, and may reduce forage for wildlife. Some may have a negative impact on your garden plants. And research suggests that some invasive species may pose dangers to humans through the increased risk of flooding due to damaged waterways, or increased fire danger.

The two worst invasive woody plants for Colorado and the surrounding region are the Russian Olive and the Tamarisk. Russian Olive (Elaeagnus angustifolia), is native to western and central Asia. It was introduced into North America in the latter part of the 1800’s. It is not related to true olive plants (Olea europaea) but its fruit is edible but not very palatable for human consumption. It out competes native plants because its seeds are irresistible to birds which spread the seeds far and wide. The seeds have a low mortality rate, germinate readily in poor soil (it can fix its own nitrogen in its roots), reach maturity quickly and thus outcompete native plants. In Colorado, they often begin setting a foothold in riparian areas, and then spread from there.

Russian Olives invading a wetland in New Mexico.

Tamarisk (Tamarix ramosissima), which is more of a shrub than a tree, similarly starts its spread in the riparian areas of the Colorado and other southwestern states. It has been such a successful invader of wet areas, that it has overtaken huge sections of the rivers of the southwest. However, it is not as tolerant of cold temperatures as is Russian Olive, which restricts its spread to the warmer, lower elevations of the southwest. Tamarisk is tolerant of many soil types, and thrives in full sun. One of the concerns about Tamarisk’s effect on native landscapes is how they out compete native vegetation, altering the nutrient cycles of riparian areas. They also consume large amounts of water, and secrete large amounts of salt, both items further slowing the success of nearby native plants. A massive amount of resources and manpower are being directed at the fight to stop the spread of this plant through the wildlands of the desert southwest.

…the Siberian Elm is “one of, if not the, world’s worst trees…a poor ornamental that does not deserve to be planted anywhere”.


Closer to Denver and the urban areas of Colorado’s front range, there are two trees that are common pests. These are the Siberian Elm (Ulmus pumila), and the Tree of Heaven (Ailanthus altissima). Both of these trees release a profuse amount of seeds, and these seeds are very adept at sprouting in disturbed areas. They proliferate along transportation corridors and abandoned lots. Both trees prefer full sun, but Ailanthus can tolerate some shade allowing it to invade some mature native forests.

Young Ailanthus altissima

Although not directly related, Ailanthus trees (a.k.a. Tree of Heaven) are often mistaken as sumacs. In springtime they produce many flowers that have a foul odor which some say is similar to cat urine. It rapidly out competes other tree species, releasing an allelopathic chemical which inhibits the growth of other plants. Some researchers have extracted this chemical from Ailanthus trees, successfully using it as an herbicide. Not only is this tree toxic to other plants, there have been anecdotal reports of it being slightly toxic to humans and livestock. Ailanthus trees can grow quite rapidly, leading to weak, unstable branches.

Siberian Elms also have structural problems, with many weak or dead branches that can break off in heavy wind. Siberian elms have a short dormancy period which leads to early flowering in spring, and late leaf drop in fall. In Colorado that means they can become dangerously loaded with wet snow in our common heavy spring snowstorms, or the occasional fall snowstorm, leading to heavy branches falling on roofs and vehicles. In addition to the dangers of breaking limbs, the trees do not have a very favorable appearance, with an awkward branching pattern. One doesn’t need to search too long before you find many Siberian elm seedlings sprouting up in unwanted spots in the urban landscape. The sprouts show up in shrub beds where they are difficult to remove by hand and hard to spray with herbicides without damaging neighboring desired plant materials. Siberian elms are susceptible to damage from elm leaf beetles which leave the leaves looking skeletonized, but it doesn’t seem to kill the trees.  Notable horticulturalist Michael Dirr says the Siberian Elm is “one of, if not the, world’s worst trees…a poor ornamental that does not deserve to be planted anywhere”.

To be fair, I should mention that some people look favorably upon these four tree species, and say that in Colorado’s high desert climate (that is naturally and predominantly tree-less east of the Rocky Mountains), an invasive tree is better than no tree, especially in urban areas. I would beg to differ, noting that these trees are too difficult to control and remove, to the detriment of native flora and fauna. Please become aware of these invasive trees and be considerate about whether you want to allow these in your landscape.


This is the official blog of Outdoor Design Group, Colorado Landscape Architects.  For more information about our business and our services, click here.

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Is Your Tree Next to Die? How Emerald Ash Borer Will Affect Our Urban Landscape, and What You Should Do About It

Green Ash tree killed by Emerald Ash Borer

2013 brought tragic news to urban tree enthusiasts in Colorado. In September 2013 the emerald ash borer was found in Boulder County. This find means that thousands of trees along Colorado’s Front Range could be decimated by this pest. And as Colorado State University Professor of Entomology Whitney Cranshaw tells us, the spread of the emerald ash borer (EAB) is dependent upon human activity. It is likely that EAB came to the United States via humans transporting contaminated wood crates from Asia, and it probably reached Colorado through contaminated firewood.


The emerald ash borer, Agrilus planipennis, is a beetle native to Asia. It is small, about the size of a grain of rice. Being a non-native insect in North America, it has no predators to keep its population in check. And unlike the Ash trees in Asia, our native Ash trees don’t possess the natural insecticides that control EAB. Adult borers eat leaves of Ash trees, then mate and lay eggs in crevices in the bark. The eggs hatch, releasing larvae which burrow beneath the bark, which disrupts the trees nutrient and water supplying layers. The tree becomes riddled with tunnels the larvae leave behind. The larvae then hatch in spring, burrowing their way to the surface, where they repeat the cycle of feeding on the leaves, breeding and laying eggs of new generation. They kill the canopy of the tree, limb by limb, with the entire tree dead in 3-5 years.

Adult EAB on a penny.

Colorado’s department of Agriculture estimates that there are 1.45 million ash trees in just the Denver Metro area alone.  The costs to spray so many trees will quickly mount.

Tree and insect experts agree that there is no point in spraying your Ash tree unless the tree is currently infested. “Since most EAB treatments provide control for one year or, at most, two years following application there is no benefit in treating a tree prior to when EAB is present”, Cranshaw writes in a recent report from Colorado State University.

In other words there is no treatment that will stop the EAB from attacking your Ash tree. Spraying before the tree is infested only wastes money and needlessly adds dangerous chemicals to the environment. In fact, many entomologists warn that even spraying an infested tree may have lethal impacts on beneficial insects such as bees and butterflies, and won’t completely rid your tree of EAB, and would probably only buy you a few extra years of life of an infested Ash tree.

If we take a serious look at the situation, spraying at all is mostly a futile and dangerous endeavor. Spraying gives tree care companies extra income but endangers the life of beneficial insects, could possibly have harmful effects on people and is very unlikely to ultimately save your Ash tree. As CSU’s Cranshaw has said, he expects in 5-10 years all Ash trees in the Metro Denver area to be infested by EAB.  Furthermore, Cranshaw writes: “Once established at a location emerald ash borer can be expected to survive in the area as long as any ash trees remain. Therefore some management of emerald ash borer will be required for as long as one wishes to maintain the tree”. It is our opinion that a better strategy to dealing with EAB is planning for the eventual replacement of your Ash tree, rather than treating your ash tree with toxic chemicals, year after year.

What can we do to slow the spread of EAB to the rest of Colorado?

1.)    Do not transport any ash wood in or out of your area.

2.)    Do not plant more Ash trees in Colorado.

If you have an Ash tree, you really should consider planting a replacement tree now. If you are unsure about what type of tree would be a good alternative to Ash, contact your local nursery, arborist, or contact us and we can help you with that decision.

This is the official blog of Outdoor Design Group, Colorado Landscape Architects.  For more information about our business and our services, click here.



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How to Tell if You Have Tall Fescue or Crabgrass in Your Lawn

Recently I had a client express frustration about his inability to kill the crabgrass weeds in his lawn.

It turns out that what he had was tall fescue rather than crabgrass.  Tall fescue is a spreading cool season grass that often appears in lawns.  It has a courser texture than bluegrass, and will sometimes “green up” in the spring earlier than the rest of the lawn.

How to Remove Tall Fescue

Crabgrass (left) has smooth leaves that often have small hairs. Fescue (right) has ridges on the leaves.

So here’s the thing about trying to remove it:  Selective weed killers that target weeds such as crabgrass will not kill tall fescue.

The only way to remove the tall fescue is to treat it with a non-selective herbicide such as Roundup.  This type of weed killer will also kill the lawn that is directly around it.  Which means you will need to spot-seed that area, or if you have a lot of fescue you may need to remove the entire lawn and start over with seed or sod.

Should You Remove it at All?

In my lawn, I have clumps of tall fescue but I don’t like to use many chemicals on my lawn.  I just leave the clumps in place- they are hardly noticeable once the lawn greens up.

Tall fescue emerging in a bluegrass lawn that is still dormant in early spring.

Fescues are actually pretty highly recommended as lawn grasses-  according to the Colorado State University Extension Office a fescue lawn can require up to 50% less water than a bluegrass lawn.  In fact, one of the popular sod blends we often specify for the front range is a “90/10 Fescue Bluegrass” blend.  That is, 90% fescue and 10% bluegrass.

So before deciding to use a bunch of chemicals on the fescue in your lawn, or doing a major lawn renovation – consider leaving it in place.  The fescue can be hardly visible at all from a distance, and may end up saving you money in the long term on watering.

This is the official blog of Outdoor Design Group, Colorado Landscape Architects.  For more information about our business and our services, click here.


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The Secret to Saving Your Trees and Shrubs this Winter

As you are all probably aware, we live in a semi-arid climate in Colorado. Unless you have a landscape comprised entirely of native plants, you need to be cognizant of the occasional need for winter watering to maintain healthy landscape plants. This can be a challenge due to the fluctuating temperatures we experience. And unless you want to re-winterize your irrigation system each time you winter water, you likely will be watering by hand.

Generally speaking, if there has been no natural precipitation for a month and the temperatures have been above normal for your region, trees and shrubs planted within the last year will benefit from receiving supplemental water. Only water when the air and soil temperatures are greater than 40 degrees Fahrenheit, with no snow cover.  Plants that are on the south or west side of your house are more likely to dry out before those on the east or north.  And plants that get reflected heat from buildings, walls and fences or that are in windy sites are more likely to dry out more quickly.

Recently planted deciduous trees are most in danger from winter dry spells. Established trees are somewhat immune from winter drought, unless the dry spell is extreme.  If you are uncertain if your tree is established, observe this advice from Colorado State University Extension Service: “Trees generally take one year to establish for each inch of trunk diameter. For example, a two inch diameter (caliper) tree takes a minimum of two years to establish under normal conditions.“   Water the newer and younger trees once a month with 10 gallons of water per each trunk diameter, measured at 6 to 12” above the ground. Let the water soak in slowly to a depth of 12”.

Similarly, recently planted shrubs are more susceptible to dry winter conditions than more established shrubs. For new shrubs, water them with 5 gallons, two times per month. Small established shrubs need 5 gallons once per month.  Large established shrubs need about 18 gallons per month.

Newly planted evergreen trees and shrubs are also very sensitive to winter drought, because they don’t drop their leaves or needles so they still are transpiring moisture through the needles all year long. Some arborists recommend misting the leaves/needles with water rather than focusing on the soil. Some of the water will trickle down and get into the soil. Consider doing this 3 times a month for those new evergreens. This should help your evergreens avoid the brown damaged needles that occur during drought stress. Evergreens that need special attention during winter drought are spruce, fir, arborvitae, yew, Oregon grape-holly, boxwood, and Manhattan euonymus.

Obviously, if we receive enough precipitation, no supplemental watering is needed. The rule of thumb here is if snow still covers the ground or if there was a significant snow fall (6” or more) at least once in the last month, you probably don’t need to water that month. But if we have a dry spell and you’ve not properly watered your landscape plants, they may become weakened, making them prone to insects and disease.

If you have a recently planted lawn, consider giving it ½” of water per month during winter dry spells. Established lawns shouldn’t need supplemental watering because bluegrass goes dormant in winter.

Do not neglect to properly mulch all your trees, shrubs and perennials. This will help to keep your plants from further drying out.

Once you have watered your landscape plants, don’t forget to unhook your hose from the hose bib so as to avoid frost damage to the pipes once temperatures drop back down below freezing.

This is the official blog of Outdoor Design Group, Colorado Landscape Architects.  For more information about our business and our services, click here.


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How Long Should You Run Your Lawn Sprinklers During the Drought?

Despite the recent rain across Colorado, much of the state remains in a drought.

The watering restrictions that Denver Water put in place for this year remain in place, having gone into effect on April 1st.

You may be aware that watering restrictions mean you can only water two days per week, but how long should you run your lawn sprinklers on those days?  Denver Water recently released this chart, which includes recommended watering times for different types of spray irrigation.  Note that due to the drought, these watering times have been reduced from their standard recommendations.

This is the official blog of Outdoor Design Group, Colorado Landscape Architects.  For more information about our business and our services, click here.


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