Purple loosestrife. Photo Credit: Marccophoto (iStock).

Purple loosestrife. Photo Credit: Marccophoto (iStock).

Following the final frost of the season, Colorado's landscape gets green fast. There's one big problem – not all pretty plants are actually good for Colorado. 'Noxious weeds' are classified by the government as being aggressively invasive or detrimental to economic crops and native plant communities.

Here are a few noxious weeds you should watch out for in your garden:

'Class A' Noxious Weeds: These weeds should be eliminated quickly and effectively.

1. Myrtle spurge

Green flowers of Myrtle Euphorbia "Euphorbia myrsinites", the myrtle spurge, blue spurge or broad-leaved glaucous-spurge. Selective focus

Myrtle Spurge. Photo Credit: Salomatin (iStock)

This problematic perennial was introduced in the 1970s as a great addition to xeriscaped yards. Unfortunately, it loves Colorado’s climate and sandy soils, plus it is very difficult to remove once it goes to seed. Myrtle Spurge is extremely poisonous when ingested by animals and the entire plant contains a white, milky sap that emanates whenever a stem or leaf breaks. This sap can be very harmful to the eyes and skin.

2. Cypress spurge

Cyprus Spurge (Wikimedia Commons)

Cyprus Spurge (Wikimedia Commons)

Very similar to Myrtle Spurge, the Cypress Spurge is another dangerous weed to watch out for. Make sure to avoid its milky sap when removing.

3. Orange hawkweed

Photo Credit: Michael Shephard; USDA Forest Service, Bugwood.org.

Photo Credit: Michael Shephard; USDA Forest Service, Bugwood.org.

The Orange hawkweed is a perennial plant originates from Europe. It reproduces from runners, rhizomes, sporadic root buds, and seeds. Its decaying leaves secrete a chemical that inhibits seeds and regeneration of other plants, causing severe issues in landscapes.

4. Purple loosestrife

Purple loosestrife in a Summer Sunset at the Wetland

Purple loosestrife. Photo Credit: GrabillCreative (iStock).

Another perennial native to Europe, the Purple loosestrife was intentionally brought to the States as an ornamental plant. Since then, it has spread to stream banks and ponds, causing environmental damage. A single, mature plant can produce up to three million seeds, all of which are viable in soil for up to 20 years. The most harrowing aspect of Purple loosestrife is that it has an alkaloid within it that inhibits oxygen absorption in the blood system.

'Class B' Noxious Weeds: These plants should be actively prevented from spreading (but not necessarily eliminated)

1. Leafy spurge

Leafy spurge. Photo Credit: Ivar Leidus (Wikimedia Commons).

Leafy spurge. Photo Credit: Ivar Leidus (Wikimedia Commons).

The Leafy spurge is a perennial that's native to Western Asia. It can spread by seed and extensive, creeping roots. Similar to Myrtle & Cypress Spurge, Leafy Spurge contains a white, milky sap that exudes readily upon a stem or leaf breakage. This sap can seriously irritate the skin of people and animals and can cause human blindness upon eye contact. Leafy spurge is one of the earliest plants to emerge in the spring. Leafy spurge contains the alkaloid euphorbon, which is toxic to humans and animals and is a known co-carcinogen. The root sap gives off a substance that inhibits the growth of grasses and surrounding plants.

2. Scentless chamomile

Scentless chamomile. Photo Credit: Wikimedia Commons.

Scentless chamomile. Photo Credit: Wikimedia Commons.

Scentless chamomile is an annual, biennial, or perennial plant that is native to Europe. Each plant can produce 300,000 seeds, which can be a factor in its rapid spread. Its seedlings start sprouting early and tend to create dense patches that do not allow for competing plants to grow.

3. Canada thistle

Canada thistle. Photo Credit: williamhc (iStock).

Canada thistle. Photo Credit: williamhc (iStock).

This perennial is native to – you guessed it – Canada. It spreads by seeds and aggressive creeping, horizontal roots called rhizomes. Canada thistle can grow 2 to 4 feet in height. Unlike other thistles that have one primary flower, Canada thistle flowers occur in small clusters.

'Class C' Noxious Weeds: The government has developed plans to increase education and biological control options for this class of weeds. Active eradication or preventing the spread is not necessary at this stage in Colorado.

1. Field bindweed

Field bindweed. Photo Credit: sajoiner (iStock).

Field bindweed, may also be light pink. Photo Credit: sajoiner (iStock).

Perhaps every gardener’s most hated perennial (or mine, at the very least), the field bindweed reproduces from seeds and creeping, horizontal roots (called rhizomes). Field bindweed flowers are cute little bells that tend to be white or pink. Their seeds remain viable for up to 40 years.

2. Downy brome

Downy brome. PHoto Credit: Marina Denisenko (iStock).

Downy brome. PHoto Credit: Marina Denisenko (iStock).

Also known as Cheatgrass, Downy brome is an annual native to the Mediterranean region, now found in Colorado. Cheatgrass reproduces solely by seed, but has still managed to infiltrate much of the Colorado Springs area and beyond.

Watch list:

1. Tree of Heaven

A large Tree of Heaven specimen. Photo Credit: Darkone (Wikimedia Commons)

A large Tree of Heaven specimen. Photo Credit: Darkone (Wikimedia Commons)

Tree of Heaven is a deciduous tree native to China. It is extremely invasive due to its effective seeds and tendency to form dense thickets. Additionally, it is detrimental to native plants because of a chemical secreted by the roots that inhibits other plant growth. This one is a big one – able to grow up to 70 feet tall in Colorado with a six-foot trunk diameter.

This list may seem daunting, but thankfully, there are various ways to keep these noxious weeds in check. The first step is being able to effectively identify them. By working to educate ourselves, in addition to promoting native foliage as much as possible, we can reduce the dangers to our community and our landscape for years to come.

About the author: Amalie Fellini is a virtual assistant with clients up and down the Rocky Mountains. After watching Colorado's landscape change over her lifetime, Amalie has developed a passion for native plants, noxious weeds, an water-wise gardening. You can often find her rock climbing, enjoying live music, or strolling with her pets, Kenai and Ozark.

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(1) comment

jim remley

ADD: Mullein! Mullein! Mullein! Mullein!

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