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.
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I have three old, big elms of some type on my old downtown Colorado Springs residential lot. They are indeed big pests. Branches break off all year around, twigs and sticks break off and litter the yard all the time, and their seeds try and succeed growing anywhere in the yard including inside the cracks in the sidewalks. They are not particularly attractive. I wish the new home owners of this property some 70 years ago picked a different tree, like a maple or oak.
Hi Tom,
It sounds like you’ve got Siberian Elms. They are quite a nuisance due to the proliferation of seeds and seedlings. Best of luck in dealing with them.
Sounds like Chinese Elm. Small leaves, right?
They grow like weeds. Sprouting up everywhere, especially inside the branches of other plants(rose bushes) and fences.
I have three old, big elms of some type on my old downtown Colorado Springs residential lot. They are indeed big pests. Branches break off all year around, twigs and sticks break off and litter the yard all the time, and their seeds try and succeed growing anywhere in the yard including inside the cracks in the sidewalks. They are not particularly attractive. I wish the new home owners of this property some 70 years ago picked a different tree, like a maple or oak. Its not native to the front range and its only value is that it grows very fast, so it gave instant gratification and long term problems.
One of the main reasons we sold our last home and moved out of Lakewood was the neighbor’s Siberian elm took over our yard. We spent so much time chopping those nasty, smelly trees! Hated them!
How can I get rid of my trees of heaven efficiency. I keep pulling them and they keep coming.
Thanks
If they are seedlings, you can continue to pull out the young seedlings, or try spraying them with an herbicide such as glyphosate (be careful not to get it on plants you want to keep). If they are seedlings, and you don’t have a mature “tree of Heaven” tree in your yard, the seeds are probably being blown on to your property from a tree(s) in your neighborhood. If you do have a mature “tree of Heaven” in your yard, you should consider removing it, or at the very least, cut it back in late Spring before it can flower and set seed.
If there are mature trees on your property that you’ve cut down and they keep sending up shoots, that is more challenging.
Spraying glyphosate on the suckers coming up probably won’t kill the mature tree. If there is a larger trunk of the tree that is alive, you can try giving it a fresh cut, and then soon after that, drilling hole(s) in the freshly cut trunk and pouring concentrated glyphosate into that hole. A search of the internet will provide instructions on how best to do this. If you are concerned about the safety of using glyphosate, there are directions found online for killing stumps using epsom salts.
I’m going to fall on the side of “any tree is a good tree”. Russian Olives do NOT crowd out natural fauna — that is just a lie. Take a look, and you will see the native plants growing quite happily right under a Russian olive. The lie is just to cover up what people don’t want to actually say — they don’t like non-native trees growing wild. Doesn’t matter if it is pretty or not, it is not native so they don’t think it belongs. Then they make up lies to try to get everyone else on their side.
As far as those Siberian Elms go — if you live in an over-crowded track development where broken lims are going to fall on your house (your neighbors house, your car, etc.)…well, they probably don’t belong there! But if you have a few acres and need a fast windbreak or screen, good luck with native trees!
Put the right tree in the right place is my point. Don’t call the tree “bad” just because a lot of people don’t know how to properly select or plant for their needs.
I agree.
SIberian elms are certainly a double edge sword. I have a small grove of them in the back half of my small acreage urban property which provide nice screening and forest habitat. Big limbs do frequently fall, but it all melds nicely in the wild part of my yard. Unfortunately, my property also has giant siberian elms in the front half too. Limbs crash down on my house, leaves clog my gutters year round, we have to move our cars every time the wind blows, I’ve spent over $7k managing/removing them and you’d barely notice, I’ve had to replace broken rafters in my home’s roof, the roots are invasive and never die, the roots invade clay sewer pipes and need to be roto-rootered every few years, the seeds sprout hundreds of saplings every spring in my landscape areasand you can’t easily yank them out with your hands.
For years I’ve had to deal with what I was told were “Chinese elms” on my 5 acres in Adams County, turns outthese are Siberian Elms planted by previous owners family in 1920’s. Awful tree as related herein, branches continually breaking off and falling in a tangled mess, good for nothing wood too, can’t be worked, doesn’t burn well. These trees are far bigger than average because they grow along a ditch lateral and have a constant source of water, height over 100’ trunk diameter 30”.
Finally had to remove one this weekend because it had spread over house roof and appeared weakened, limb, actually second trunk, weighed ten tons, scary job with pulley blocks and winch cables but now renters in the house are breathing easier.
I’ve got a huge issue with what looks like the “Trees of Heaven” here in Grand Junction, CO. I’ve cut them and all it does is spread them much worse. Last year they began spreading just outside my grass line and now are growing on my grass, which is suffering, as well as on the other side of my house which never had any of these trees before. They’ve been growing at an astounding rate and don’t know how to get rid of them without taking drastic measures