Look Out For These Invasive Plants in Colorado

Be on the lookout for Russian olive or purple loosestrife on your Colorado property—these invasive plants could pose some issues for your home.
Written by Macy Fouse
Reviewed by Pat Roache
Between the oxeye daisy and myrtle spurge, these are 15 of Colorado’s most invasive plant species to look out for. 
Colorful Colorado has some of the most beautiful nature in the country, but not all of it is meant to be there. If you’re a homeowner in the Centennial State—or you’re considering making a move—learning about the state’s most invasive plants is an important part of taking care of your property. 
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The top 10 invasive plant species in Colorado

1. Cypress spurge

Scientific name: Euphorbia cyparissias
What it looks like: Yellow or green bunches of flowers with bluish-green stems and leaves
Why it’s a problem: Cypress spurge is not only invasive, but its milky-white sap and seeds are toxic and prone to causing skin irritation for humans—or, much worse, for livestock. Covered under “List A” of the
Colorado Noxious Weed Act
, cypress spurge is required to be eliminated anywhere it's found. 
What to do: For younger plants, use protective gloves to physically remove the cypress spurge—the earlier in the year you do it, the easier it will be. The root system can be extensive, so if you’re going the herbicide route, apply the chemicals during the flowering stage for the best results. 

2. Dalmatian toadflax

Scientific name: Linaria dalmatica or L. genistifolia
What it looks like: Tall, vibrant yellow flowers with a tinge of orange, resembling snapdragons with thick, waxy leaves wrapped around the stem
Why it’s a problem: These flowers are undeniably beautiful—but that beauty contains a toxin thought to be harmful to livestock. On top of that, their roots can reach down over six feet and extend more than 10 feet horizontally, crowding out valuable foliage as it grows. 
What to do: Single plants can be manually removed—but there’s a good chance that the roots will break and sprout new shoots, so keep an eye on the spot you removed them from. Herbicides are less effective for dalmatian toadflax due to their waxy leaves. If you attempt it, be sure to do it in the late spring or early fall. 

3. Russian olive 

Scientific name: Eleagnus angustifolia
What it looks like: Silvery white leaves, yellow-red fruits on mature trees, one- to two-inch thorns on branches
Why it’s a problem:  Russian olive trees were once believed to be helpful for windbreaking, but they’re now known to be harmful to the local ecosystem. They crowd out native species, impede natural nutrient cycling, and deplete local water reserves. 
What to do: The best way to eliminate Russian olives depends on how big the plant is. Small trees and shrubs can be dug up and cut down, but larger trees will require a bit more work. 

4. Meadow knapweed

Scientific name: Centaurea pratensis
What it looks like: Tall stems with single thistle-like flowers, often magenta, growing from a woody crown
Why it’s a problem: As a hybrid of black and brown knapweeds, meadow knapweed can grow in virtually any environment, from parks and farms to railroads and industrial sites. Once meadow knapweed is present, it crowds out native species and disrupts the habitat.  
What to do: When you see one (or more!), you can remove them by hand or kill them with herbicide. Populations can also be safely grazed by livestock, especially in early spring when it’s in the early phases of growth. 

5. Myrtle spurge

Scientific name: Euphorbia myrsinites
What it looks like: Low-growing waxy leaves and petals in a rose-like shape with trailing stems
Why it’s a problem: As another invasive ornamental, myrtle spurge could easily be mistaken for a cute succulent in your rock garden—but not so fast. This plant’s milky sap is toxic and can cause serious skin irritations or stomach-related issues if ingested. Colorado deems it a “List A” species, so removal is mandatory. 
What to do: Myrtle spurge is best managed by a combination of mechanical removal and spot treatments with herbicides. Be sure to grab some gloves, long sleeves, close-toed shoes, and eye protection before you dig these babies up. These plants are hardy, so it’s important to be persistent—even for years!

6. Hairy willow-herb 

Scientific name: Epilobium hirsutum
What it looks like: Tall, hairy stems with purple flowers often grown in bushy clusters 
Why it’s a problem: A semi-aquatic plant, the hairy willow herb can produce up to 70,000 wind-dispersible seeds—creating vast infestations that displace native foliage and exacerbate erosion. 
What to do: As a “List A” species, getting rid of hairy willow-herb is crucial. Small infestations can be removed mechanically, but be careful not to leave behind roots or rhizomes. Dispose of these plants in a trash bag. 

7. Orange hawkweed

Scientific name: Hieracium aurantiacum
What it looks like: Clusters of bright orange flowers with notch-tipped petals and hairy leaves and stems
Why it’s a problem: Orange hawkweeds are versatile, and they can infest an area rapidly—displacing the native vegetation and upsetting the ecosystem along with it. 
What to do: As another “List A’ species, orange hawkweed has to be eradicated wherever found. Since populations tend to be pretty extensive, herbicides are usually the go-to treatment for orange hawkweed. 

8. Dame’s rocket

Scientific name: Hesperis matronalis
What it looks like: Tall-stemmed green rosette for the first year, then purple or white bunches of flowers 
Why it’s a problem: Don’t let these beauties fool you—they may look like a good addition to your garden, but they’re a bad idea. Dame’s rocket is considered an escaped ornamental, and they bloom multiple times throughout a season…releasing 20,000 seeds. These seeds result in quick displacement of native flora and mass disruption of the local ecosystem. 
What to do: Luckily, dame’s rocket is pretty easy to pull by hand—but make sure you’re grabbing as close to the root as possible so you don’t leave any behind. Herbicide and goat grazing are also options since dame’s rocket isn’t toxic for animals. Dispose of them in a plastic bag sealed tightly so they don’t reproduce in the compost or dump!

9. Oxeye daisy

Scientific name: Leucanthemum vulgare
What it looks like: Delicate white flower petals surrounding a bright yellow center
Why it’s a problem: These may look like the wildflower version of your favorite gerbera daisy, but these garden ornamentals are more nefarious. Oxeye daisies not only decrease the native diversity but, due to their shallow root systems, they reduce nutrient cycling and increase soil erosion. 
What to do: Avoid wildflower seed packets that list oxeye daisies. As a “List B” species, it’s required to be either eliminated, contained, or suppressed. This can be done with traditional removal methods like pulling, digging, goat grazing, and herbicides. 

10. Purple loosestrife

Scientific name: Lythrum salicaria
What it looks like: Vertically blooming magenta flowers with long stems and lance-shaped leaves
Why it’s a problem: Another escaped ornamental plant, purple loosestrife can produce up to three million seeds in a single year. Purple loosestrife grows in wet areas like ditches and riverbanks, replacing native flora and monopolizing the area. This leads to reduced water flow in the ecosystem. 
What to do: Like most of these species, early detection is key. Small populations can be managed by hand-pulling and cutting in tandem with herbicides. It is a “List A” population in Colorado, so eradication is required. 

11. Saltcedar/Tamarisk 

Scientific name: Tamarix ramosissima
What it looks like: Evergreen shrub with small and scaly leaves and tiny pink or white flowers
Why it’s a problem: Like most of these invasive species, saltcedar or tamarisk replaces native vegetation by making the surrounding soil inhospitable. They also require more water, which contributes to droughts. 
What to do: Keep an eye out in areas that are most susceptible to saltcedar, like floodplains, riverbanks, and marshes. Chainsaws—followed by herbicide—are effective for removing smaller infestations, but larger ones may require a bulldozer or prescribed fire. 

12. Yellow toadflax

Scientific name: Linaria vulgaris
What it looks like: Stems with wood bases topped with yellow and white snapdragon-like flowers that grow vertically 
Why it’s a problem: Yellow toadflax root systems are extensive, posing an issue for surrounding native vegetation. Not only does yellow toadflax crowd out the good guys, but it’s also mildly toxic for cattle, so grazing is not an effective option. 
What to do: Hand-pulling doesn’t work as well for yellow toadflax because roots usually get left behind. A combination of herbicides, physical removal, and biological interventions is the best route. 

13. Mediterranean sage

Scientific name: Salvia aethiopis
What it looks like: Bushes with white flowers and wooly white hairs covering leaves and stems 
Why it’s a problem: Mediterranean sage primarily displaces native plant life, but it’s also prone to taking over as a monoculture. Most grazing animals don’t find it appealing either.
What to do: Flowering plants can be removed by hand-pulling—just bag them up before disposal. Herbicides are also an effective option. 

14. Hoary cress

Scientific name: Lepidium draba
What it looks like: Tall stems with clusters of tiny white flowers
Why it’s a problem: Also known as whitetop, this creeping perennial may look cute…but they absorb more than their fair share of water and nutrients, choking out the natives. 
What to do: Prevention is the best weapon against this invasive species, so be sure you don’t mow over hoary cress—it will spread the seeds. For existing plants, persistent herbicide use is the best defense. 

15. Eurasian watermilfoil

Scientific name: Myriophyllum spicatum
What it looks like: Submersed aquatic weed with whorled leaves and a feathery appearance, growing up to 30 feet or more 
Why it’s a problem: Eurasian watermilfoil grows quickly and forms dense mats, getting in the way of all sorts of water recreation. On top of that, these beds of weeds can disrupt the ecosystem for waterfowl and fish. If left untreated, Eurasian watermilfoil can slow or stop the flow of water in an environment. 
What to do: Raking the plant to remove it can be effective, but any roots or fragments left behind will proliferate again. Producing algal blooms will prevent the plant from rooting again. 
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How to save on homeowners’ insurance in Colorado

These invasive species pose a risk to Colorado’s natural beauty, but they may also be a threat to homeowners. If you spot a Russian olive tree or saltcedar on your property, it may be a good idea to look over your homeowners insurance to make sure you have the best protection. 
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