Imagine a shade tree that’s free, never needs water and will never die. Too good to be true? Yes, it is.
This is known as The Tree of Heaven (Ailanthus altissima) and it’s a widely distributed noxious weed. The ensuing text presents five noteworthy noxious weeds growing on Colorado’s Front Range. As evident with these five weeds, the public attitude towards weed management must evolve to reflect the staggering reality at hand – invasive plants have spread globally and some of these plants have very particular adaptations that are highly detrimental to native ecosystems, naturalized ecosystems and (even) human health. Landscape architects, property owners and the public must respect noxious weed classifications in order to confront the problem.
Bindweed in a recently installed commercial landscape. Westminster, CO. Credit in work cited (photo 1)
What’s a weed? The simple answer is that one man’s weed is another man’s treasure. This topic is stacked with semantic complexities. The term ‘weed’ is used to subjectively describe undesirable plants. This definition is flimsy – like a plastic bag blowing in the wind. An ecological interpretation contends that most weeds are ruderal species, which appear in high-productivity environments with high levels of disturbance (Beck, 2013). They pop up when conditions allow & devote all energy to seed production. These plants colonize disturbed land & are eventually overtaken by larger species. Weeds that mature into shrub or tree form are merely invasive. Invasive plants are introduced into an environment where they did not evolve. They are highly successful, to the point where they outcompete native plants. In other words, they found a new home with similar conditions to their old home. Invasive plants range from ruderal weeds to large shade trees and they are highly influenced by location & climate. What’s invasive in California will probably not be invasive in New Hampshire, though some noxious weeds are adaptable enough to span vast geographic regions. ‘Noxious weed’ is a legal term that cuts through layers of ambiguity. If it’s a noxious weed, then, yes, it’s a weed – and you should eradicate it! The State of Colorado considers noxious weeds to be:
Aggressive invaders that are detrimental to the economy & native ecosystems.
Plants that can poison livestock.
Carriers of detrimental insects, diseases & parasites.
Plants that are detrimental to the sound management of natural or agricultural ecosystems.
The spread of plant material by sea-faring Europeans during the 18th century was a landmark shift in global ecology. Previously, plant populations were very localized. Now days, mass disturbance and plant migration have imperiled native ecosystems – as demonstrated by the flowing weeds, which are all non-native, invasive & noxious But first, a few mentions are in order.
Noxious weeds are classified by the government as a matter of policy.
They are classified as type A, B, C, with A being the most harmful.
Noxious weed management requires specialized knowledge and techniques.
Noxious weeds are highly adaptable and competitive due to adaptations picked up from their native habitat over thousands of years.
They lack natural controls from their native habitats (insects, pathogens, etc.).
Noxious plants come in all forms: annual, perennial, vine, shrub & tree.
The remedy for non-noxious weeds is a non-lazy individual with two hands, two feet & the ability to visually differentiate.
Most non-noxious weeds DO NOT require an herbicide! Just pull them before they flower.
Type A noxious weeds are designated for eradication by the Colorado Department of Agriculture (CO DOA, 2024). These next level super weeds include Japanese Knotweed (Reynoutria japonica) – one of the most invasive plants in the world. It was taken from the slopes of a Japanese volcano and brought to Europe during the mid-1800s. It has spread worldwide and has been found in at least 12 Colorado counties. The plant is so prolific that it can grow through asphalt and even into buildings. Why? Because it adapted to the extreme conditions of its natural environment – volcanic islands. It has adapted to massive disturbance and is the first plant to recolonize the volcanic moonscape post-eruption! If you have this plant.. refer to expert advice for removal. Godspeed!
(Left, 2) Japanese Knotweed in its native habitat and (right, 3) its new habitat. Photo credits in work cited (photos 2 + 3).
Type B weeds are included in Colorado Department of Agriculture noxious weed management plans – which intend to eradicate/contain/suppress their continued spread. Russian Olive (Elaeagnus angustifolia) illustrates this category in Colorado. Like other invasive plants, it was installed with legitimate intentions, windbreaks in this case, but it quickly became detrimental to the native environment. Often found in riparian zones up to 8000’, Russian Olive is highly adaptable – it is tolerant of shade and poor soil. In Colorado, they stress native riparian zones, outcompeting cottonwoods and willows (CO DOA, 2014).
Type C weeds are included in state noxious weed management plans, but their elimination is not a goal. Instead, outreach and education are program goals. Prolific urban weeds in our region are typically Type C and include the Tree of Heaven, Field Bindweed and Poison Hemlock – among others. These plants are widespread and their complete elimination is impossible. If you live in the Denver metro area, you have seen them. The Tree of Heaven (Ailanthus altissima) is the quintessential urban trash-tree. Originally from China, this highly adaptable tree can grow 9 feet in a single growing season up to mature heights of 40-50’. If cut, the tree responds with a myriad of suckers (cut off one head and five grow back). It unsurprisingly produces thousands of seeds and also spreads by rhizome. It can be eliminated with the application of glyphosate to a freshly cut stump. Another worthy mention goes to European Field Bindweed (Convolvulus arvensis) – the ultimate yard pest. It is thought to have originated as an imported seed contaminant (NPS, 2009). Also known as morning glory, it is highly adaptable and aggressive. It’s ubiquitous white flowers can be found growing within your bluegrass lawn, flower beds and anywhere else with soil, light and water. It’s a perennial vine that reproduces via seed and creeping roots (NPS, 2009). Much like the Tree of Heaven, Bindweed does not like glyphosate and 2-4-D, but this method only works while the plant is actively growing. Best of luck getting rid of this one.. The final type C noxious weed killed Socrates and is known as Poison Hemlock (Conium maculatum). It’s highly adaptable, but prefers riparian areas, producing thousands of seeds from white flowers. There’s enough coniine within 6-8 leaves to invoke respiratory paralysis and death (USDA, 2018). Other mammals are also at-risk including canines and livestock. On the Front Range, it is found in thick stands with carrot-like leaves and white flowers. It can be eliminated by chemical and mechanical means. Be sure to wear PPE if interacting with this plant! It is highly toxic!
Tree of Heaven stand. Photo credit in work cited (photo 4).
So, what do we do about it? The prevention of noxious weeds by homeowners and commercial property managers requires knowledge and effort. If the government considers a plant noxious, it should be eradicated before it flowers and disperses 180,000 seeds that last for 100 years (Mullein!). It doesn’t matter how pretty you think it is. Weeds, regardless of their classification, thrive with neglect. At the most basic level, human development is disturbance and disturbance generates weeds. Limit disturbance as much as possible and preserve native habitats. Be careful with imported fill; it might be stacked with weed seeds! Pay attention and guide your landscape in the right direction. Learn how to eradicate these plants. It’s not as simple as you might think. Many of these plants have extensive root systems that will reshoot in perpetuity. Herbicides are massively overused, yet they are an extremely useful tool in this fight. Use them sparingly and safely! We’ve significantly altered the earth and it’s our responsibility to mitigate the damage! At the end of the day, just be glad you don’t have Japanese Knotweed growing through your floorboards.
Landscape architects (LAs) are part of the problem and the solution. LAs manage a larger scope than just plants, but typically develop plans to replace existing vegetation with proposed vegetation in accordance with code, client, budget, site, etc. Intentions are benevolent, yet ideas turn into plans, which turn into commercial shopping centers, homes, parks and so on. Landscape architects, in a sense, perpetuate ecological disturbance, opening the door for colonizer species (aka: Weeds). Of course there’s a plan to avoid this, which is one reason landscape architecture exists in the first place. It’s critical for LAs to identify existing noxious weeds on a project – so they can be eliminated before breaking ground. Many noxious weeds have deep roots, allowing them to survive extreme disturbance (like a construction project). Landscape Architects employ a myriad of techniques to manage plant competition and succession. They are highly trained individuals, but more can be done. They can advocate for limited disturbance to native landscapes. They can design native landscape zones on the site periphery. They can stipulate proper maintenance for native grass. With landscape architecture, this goes beyond noxious weeds. Are business incentives and ecological stewardship mutually exclusive?
* This was not written by an ecologist, policy maker or certified pesticide applicator. Nor was it written by an official landscape architect. Seek expert advice elsewhere before making any drastic decisions! Your local CSU Extension Office is a good place to start.
This is the official blog of Outdoor Design Group, Colorado Landscape Architects. For more information about our business and our services, click here.
Outdoor Design Group is based in Arvada, Colorado – this blog concerns the Great Plains and Western United States. A previous blog post – wonderfully written by Julie O’Brien – traced the origins of the “green carpet”. The ubiquitous American lawn. It’s important to note that the green carpet exists in multiple forms. It’s a literal thing and a cultural construct. Most Americans associate homes with green lawns regardless of the prevailing environmental conditions. The lower 48 of the USA receives between 5” – 120” annual precipitation. Established Kentucky Bluegrass (KBG) requires about 26” of precipitation during the growing season. Our collective perception of the green carpet must incorporate prevailing environmental conditions!
Within the confines of this blog, the term ‘native grass’ refers to grass species found on the Great Plains prior to western settlement; to understand the applicability of native grass, refer to the map below. The eastern half of the United States receives 30 – 70” per year; the western half gets 5-30” (with exceptions in the Pacific NW). Denver sits on the dry half with 15” annual precipitation. Landscape is all about place, and our place is pretty dry. Our native flora have adapted to the prevailing conditions, yet the same cannot be said for the green carpet!
Sown native grass shines in low-traffic applications. Once established, it needs no supplemental water or fertilizer. It is not soft and fluffy. It needs full sun and doesn’t tolerate heavy foot traffic. If planted in your front yard, there is a high probability that at least one neighbor will vehemently dislike. Yet it’s a solid solution for covering low-use space. After all, you need to do something with any given patch of earth – so weeds don’t run amok. Semi-arid climates such as ours render large-scale KBG nonsensical. It requires excessive irrigation, fertilizer and herbicides, while contributing zero ecological value. Native grass isn’t a silver-bullet; it’s a tool in the toolbox that’s applicable to many areas.
There are myriad factors to consider with native grass. First off, it doesn’t have to be native! Secondly, many species are sold by container. While many viable grass species are endemic to the Great Plains, other well-adapted varieties come from elsewhere in the world. Either way, native grass species have deep roots, grow relatively tall and are not available as rolled sod. They are typically sown by seed, and in some cases installed by plug. It’s difficult to establish and there’s an ongoing maintenance requirement. Ample moisture is required during the first season and an underground sprinkler system is recommended. Temporary (above ground) irrigation can also be used. Weed management is critical. Certain weeds (especially bindweed) must be addressed BEFORE installation! Ongoing weed management will be required! Native grass is not easy. If you’re willing and able, you’ll be rewarded.
There are many cool & warm season grasses to choose from – that differ in terms of height, texture & cold-tolerance. On the shorter side, buffalograss and blue grama are proven options. Taller options include little bluestem, big bluestem, switchgrass, western wheatgrass, sideoats grama & tufted hairgrass – in addition to many more. You might consider several varieties of water-thrifty turf as an alternative to native grass. Options such as Dog Tuff, RTF Tall Fescue & Tahoma 31 Bermudagrass.
This is not an installation guide. Contact your local agricultural extension for more information. Seed suppliers such as Pawnee Buttes and Arkansas Valley Seed are also great resources. Most western residential and commercial landscapes have a place for KBG – the key is scale; not overdoing it. Native grass isn’t for everyone, nor is it perfect for every landscape – yet it’s severely underutilized!
Written by Matthew Whiteman
This is the official blog of Outdoor Design Group, Colorado Landscape Architects. For more information about our business and our services, click here.
Denver Water customers received news last year that the pH of the water delivered to them by Denver Water would be adjusted from near neutral (pH 7.5) to slightly more alkaline (pH 8.5), which began in Spring 2020.
The motivating reason for this change is for improved human health, which is highly commendable (more alkaline water is less prone to leach lead from pipes into people’s drinking water; lead is toxic to human health).
But, this announcement has many in the Landscape Industry wondering what impacts this may have on irrigating landscape plants, as well as indoor plants, and how it may affect water conveyance equipment (irrigation systems, etc).
A cursory investigation via the internet seems to indicate that the possible impacts from higher pH water on irrigation systems and the plants that receive that more alkaline water are:
Increased mineral deposits on irrigation equipment as well as pots, planters, etc. For drip emitters this can be very problematic as the water emits at such a low and slow rate that calcium scale build-up could end up clogging the emitters.
Reduced absorption of minerals and nutrients by the plants.
Most of the soil pH in Colorado’s Front Range are alkaline, so it is possible that this increase in pH will have little or no effect on plants that are already suited or adapted to the native soils of our area. But for plants that don’t do well in alkaline soils, will this more alkaline water be the factor that pushes those plant over the edge? Is this such a bad thing if those plants aren’t suited for our soils (and climate) to begin with?
What are some actions you can take to mitigate the problems of more alkaline water?
Water more deeply and less often. Frequent and shallow waterings may cause more build-up of salts and minerals like calcium.
Use alkaline adapted plants in your landscape rather than plants that need acidic soils. Generally speaking, most native plants for our region (Colorado’s eastern Front Range) are well adapted to alkaline soils.
Check your irrigation equipment for hard water scale build-up. Clean or replace drip emitters that are clogged with scale to ensure proper water flow.
Add Acidifying materials to the soil, like granules available from garden stores? Some experts seem skeptical of this method, saying it is very difficult to have much impact on the total alkalinity of our native soils.
Add more organic matter such as compost? Aside from most xeric adapted plants as well as plants native to our region, many of the standard landscape plants benefit from added compost for multiple reasons. Adding compost to soil can lower the pH (more acidic) as well as aid in water retention, soil texture and provide nutrients to plants.
You may wonder if softening your water via chemical means is a good option. Several experts say this is not a good way to lower the pH of the water for plants because softened water has higher salt content, which is also problematic for plant health.
Some articles sourced on the internet mention lowering the pH of the water you irrigate your houseplants with, by adding vinegar. This may work for a small watering can used on houseplants, but for landscape plants this is simply impractical. Could an in-line fertigation system be used with an acidifier such as vinegar on landscapes? We here at ODG are uncertain about this option, but it seems like an intriguing, if also risky, option.
Time will tell how our landscapes react to the increased alkalinity of our water supply. We hope the effect is minimal, because gardening in our high altitude semi-arid climate is challenging enough as is!
This is the official blog of Outdoor Design Group, Colorado Landscape Architects. For more information about our business and our services, click here.
As Spring has arrived once more, we here at Outdoor Design Group have been thinking back to a past post we published, that ruminated on the importance of the annual Spring garden clean-up. We wanted to revisit that post and make some minor updates and add some extra information.
“Spring Clean-Up” includes the spring landscape maintenance tasks that are vital for a healthy and attractive landscape, especially for Xeriscape. This article is intended to give a simple, easy to follow guide covering the most important spring landscape maintenance tasks.
If you read our Fall landscape maintenance tips, you know that we are not a proponents of keeping the landscape perfectly tidy over the winter. Keeping a less tidy landscape over the winter is good for the health of the plants, and good for small wildlife like insects and birds. Now that spring is here, among other things you will need to clean your beds of those leftover leaves, and cut back your perennials and grasses.
Important Timing Dates
March to April: Cut back perennials and grasses. Clean up leaf litter and other debris.
Early May: Time to start up that irrigation system. Resist the temptation to start your sprinklers before May 1st. In Colorado, we generally get plenty of moisture in March and April so lawns do not generally need supplemental water yet, and it will go to waste. Many shrubs and perennials are only just beginning to emerge.
The average last frost date in the Denver area is May 5th.
The rule of thumb for planting new plants in the Denver area (and other Front Range communities) is to wait until Mother’s Day (middle of May) to plant new bedding plants. Consult sources such as NOAA to find the last frost date for your area.
If you are starting seeds indoors for warm weather vegetables, herbs or flowers, St. Patrick’s Day is a good date to do so.
Ornamental Grasses and Perennials
Woody perennials like Russian Sage may not completely die back
Cut back herbaceous perennials and ornamental grasses so that they will grow back bigger and better than before. For perennials, cut them back to about 2″ to 3″ above the ground. For woody perennials, such as some of the sages, some of the old growth will often begin to leaf out and you can leave more of the existing plant to re-grow, or you can choose to cut it all the way back to a few inches above the ground.
Most groundcover and other low growing perennials usually don’t need to be “cut back” but they may need to be cleaned up, with dead areas gently raked out or trimmed off.
When cutting back ornamental grasses, a rule of thumb that we like to use is to cut them back to 1/5th (or 20%) of their mature height. Feather Reed Grass, for example, grows to a height of 4-5′, so you would cut them back to 10-12″ height.
Cut grasses back to 1/5th of their full height
Some ornamental grasses look better if they are not cut back, namely Mexican Feather grass. It won’t hurt the plant if you do cut it back, but the form of the plant looks better if not cut back.
“Divide and conquer”! Divide up summer and fall-blooming perennials and grasses and transplant them throughout the landscape. This gives them time to grow this season, so that in the late summer you will have new plants to enjoy. With ornamental grasses, if the center of the plant is dead, cut this out and compost or discard.
Wait to divide spring blooming perennials until early fall.
Remove excess mulch and leaf scraps and other debris from around the bases of plants, along with any extra mulch that was placed around plants to protect them over the winter. Spring is the wettest time of the year along the Front Range, and the mulch is needed the least at this time. Leaving excess organic material around the bases of plants in spring can cause root rot, mold, and insect damage.
Consider saving any seed heads or pods from perennials you’d like to have spread around the yard. You may want to save the seeds and start them indoors in nursery pots, or you can just simply break up the seed pods and sprinkle them in the areas you might want to see new plants.
Planting Beds
Controlling weeds in bed areas is very important in the spring. You should to get a leg up on the weeds now, while the soil is soft and easy to work with and the weeds are small. A great time to pull weeds is when the ground is still moist from snowmelt or a spring rain. If weeds are allowed to “run wild” and set seed throughout the spring, you will have a maintenance headache later in the summer. Weed control can be a quick and easy task when done correctly from the start of the year through the end.
For bed areas between plans that may have become uncovered during winter windstorms, add a new layer of mulch.
Vegetable Garden Beds
Remove old/dead plant debris from last year. Compost this debris, unless this material was diseased. Tomato plants are very susceptible to diseases so we recommend never composting them, just dispose in the trash.
Consider having a soil test done. Many gardeners add more nutrients (in the form of compost) every spring thinking that their soon to be planted veggies will need that extra boost; but if its not needed, it can backfire and cause problems. Ideally, vegetables and some perennials prefer soil that is 4-5% organic matter. Native plants and cacti prefer soil that is only 1-3% organic matter.
If you want to add compost to your beds, the best time to do so is in the fall, giving the compost time to break down and ‘mellow’.
Lawns
Aerate your lawn in spring. Aeration allows for greater movement of water, fertilizer, and air which stimulates healthy turf. Aerating also increases the speed of decomposition of the grass clippings and enhances deep root growth. Compacted soil especially benefits from core aerating. Be sure to mark the locations of sprinkler heads with flags prior to aerating so that they will not get damaged.
It’s a good idea to rake the lawn areas once really well in the spring to clean up any leftover sticks, pine needles, etc. In the summer we use a self-mulching lawnmower that mulches and re-deposits the clippings in place rather than bagging them. However the first time we mow in the spring we like to attach the bagger to suck up any new clippings and other debris left over from the winter.
If there are bare spots in your lawn, consider broadcasting some grass seed after you have aerated, and before a forecasted snow or rain event. The cooler and wetter conditions of Spring are perfect for germinating many popular lawn grass types.
You may want to fertilize your lawn in the spring to give it a boost. Don’t over do it though, because fertilizer can wash off of your lawn and the runoff can be harmful to water supplies and wildlife. Consider timing your application of fertilizer right before a spring snow, so the melting snow can help the fertilizer percolate to the root zone of the grass.
Remove or kill any emerging weeds, like dandelions, as soon as possible before they set seed and spread around your yard and into your neighbors’ yards.
Early Spring is a good time to apply a pre-emergent herbicide on your lawn to prevent crabgrass. However if you have recently applied grass seed, do not apply a pre-emergent herbicide.
Look at the size and shape of your lawn, and how much water you used this year in the battle with our dry climate to keep it green (or, brown?). Consult with a landscape architect about reducing your lawn area and redesigning your landscape to make it more attractive, sustainable, and functional.
Trees and Shrubs
You can now remove tree wrap from trees. If you also have tree stakes and the tree is mature enough to not need them anymore, Spring is a good time to remove the stakes.
For most trees and shrubs, don’t prune them in the spring. They are budding out and are susceptible to more stress and damage at this time. I find that it can also be difficult to tell whether a branch is dead or alive without physically checking each one, since the leaves have not yet emerged. Wait until late summer or fall to remove any dead branches and to do any other pruning.
Some shrubs (butterfly bush, barberry, privet, Japanese spiraea) are best pruned in late winter or early spring. Other shrubs (lilac, forsythia, mock orange) and some trees (ornamental pear, redbud, pines and spruces) are best pruned in late spring or early summer. Generally speaking, you should not prune a plant until soon after it has flowered.
Trees that are considered “bleeders” (have a lot of sap) such as dogwood, elm and maple should be pruned after the leaves are fully formed.
But wait to prune young or newly planted trees and shrubs, until they have had some time to become established.
In most Spring times, there is adequate precipitation for landscape trees and shrubs. However, if it’s been a dry winter and continues to be a dry Spring, keep an eye on soil moisture around your plants. Before you start your irrigation system in May, remember to check the soil moisture, and give trees and shrubs extra water if needed.
Irrigation
Resist the temptation to start your irrigation system before May 1st. After May 1st, pressurize your irrigation system and check for and leaks and for proper spray distribution. Adjust the angle and throw of sprinkler heads. Check irrigation equipment for clogged nozzles and sprinkler heads for damage. Realign heads if necessary. Examine any drip irrigation to make sure there are no leaks or other issues, and plan to make regular check-ups on your irrigation system throughout the season.
Stake down and cover with mulch any exposed drip irrigation lines.
Spring Weather Considerations
Watch for those late spring snowstorms! Trees and shrubs that are flowering and/or leafing out can catch the heavy wet spring snow on their branches, often causing serious damage due to limbs breaking off. If you can safely do so, consider gently knocking off any accumulating snow during the wet spring snow events we typically get. This may save branches from breaking during the storm.
Other
Check any metal edging to make sure that it is has not heaved over the winter. If it has popped up out of the ground, you should repair it. Edging keeps the sod from creeping into landscape beds areas, which can be a maintenance nightmare if it gets out of hand. Metal edging can also be a safety hazard to pets and children, if not properly seated or capped. If you have a plastic protective cap on your edging, inspect it and replace it if necessary.
If you follow all of the above recommendations you will end up with a lot of organic material and cuttings as a result of your spring cleanup. If you have the space for it, consider composting this material. Or, rather than throwing the material away to go in the landfill, check to see if there is a local recycling center that will take the material to compost it.
If you compost at home, do not include any weed seeds. Home composting does not heat up enough to kill the weed seeds.
As you go about your landscape doing these Spring cleaning tasks, you will begin to accumulate quite a bit of organic material and cuttings. Most of it is very good organic matter that is great for composting. If you have space in your yard for it, consider composting it on site. This will provide you with a free soil amendment. Another option is to take it to your local recycling center where there might be a organic matter drop off location. However you accomplish it, composting is a better option for yard debris as compared to sending it to a landfill.
Spring cleanup is a time honored tradition among gardeners, as an excuse to get outside after the long winter months and to freshen up the landscape as it springs to life. With Xeriscape, spring maintenance is even more important. Since Xeriscape requires less regular maintenance through the summer, it is critical to get the summer started off right so that your Xeriscape will require very little maintenance throughout the year.
Regardless of the landscape style that you are maintaining, if you start off on the right foot in the spring you will be well on your way a beautiful landscape to enjoy throughout the coming year.
This is the official blog of Outdoor Design Group, Colorado Landscape Architects. For more information about our business and our services, click here.
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.
