CO-Horts

CO-Horts Blog

Thursday, May 26, 2022

The Great Escape! Noxious Weeds Edition

Denyse Schrenker, CSU Extension in Eagle County 

Did you know that the majority of plants used in agriculture, the landscape industry, and forestry in North America are not native to the continent? We introduce non-native plants for a variety of useful reasons and most of the plants introduced do not cause ecological harm. However, there are some that escape our care and cultivation and go on to wreak havoc on our landscapes. 

Many of these introduced species that become invasive were brought here as ornamental plants. Several plants on the Colorado Department of Agriculture noxious weeds list were originally brought to the United States to decorate our landscapes but have since escaped our yards and become pests of our natural areas. These plants are quite attractive and you might think, “oh but I love that flower!” Despite their beauty, they are listed as noxious weeds for a reason and can disrupt native ecosystems. 

Dalmatian toadflax (Linaria dalmatica) is one of these notorious plants. Native to the Mediterranean region, Dalmatian toadflax was introduced as an ornamental in the western U.S. in 1874. Dalmatian toadflax is able to monopolize landscapes because it spreads by creeping roots known as rhizomes, and produces vast amounts of seed. A single plant can produce 500,000 seeds which remain viable for up to 10 years! Early identification is key to preventing a Dalmatian toadflax invasion. This perennial plant has thick, waxy, often bluish tinted, heart-shaped leaves that wrap the stem and bright yellow, snapdragon-like flowers. Dalmatian toadflax’s sibling, yellow toadflax (Linaria vulgaris) is also a listed noxious weed in Colorado. Yellow toadflax is native to south-central Eurasia and was brought to North America in the late 1600s for fabric dyes, medicinal purposes, and ornamental purposes. It is still occasionally sold in nurseries in states where it is not listed as noxious under the name “butter and eggs” or “wild snapdragon”. Like its relative, yellow toadflax develops an extensive root system and can reproduce through creeping roots. Yellow toadflax is distinguished from Dalmatian toadflax by its very narrow, linear leaves, and its flower is lighter yellow with a dark yellow to orange center (hence the nick-name butter and eggs!). Although these perennial toadflax are in the Snapdragon family, the ornamental snapdragons found in planters and landscaped areas throughout the county are not invasive. Ornamental snapdragons are annuals and not well equipped to survive without the help of human cultivation which sets them apart from toadflax.
Dalmatian Toadflax (CSU Factsheet 3.114)
Yellow Toadflax: CSU Factsheet 3.114
Oxeye daisy (Leucanthemum vulgare) was introduced as an ornamental to North America by Pilgrim settlers in the 1700s. Oxeye daisy is also commonly included in wildflower mixes so it is important to read the label, check the scientific names, and make sure you know what you are planting. Oxeye daisy spreads by seed and creeping roots. The seed can remain viable for up to 40 years! This weedy daisy is a fierce competitor and can form dense stands that choke out native plant diversity. Management of oxeye daisy is often thwarted by its good looks. However, do not be deceived, oxeye daisy is no friend to our landscapes. Wildlife and livestock alike do not feed on the lousy tasting daisy and even avoid walking through oxeye daisy infested fields because it irritates their faces and legs so this weed directly reduces wildlife habitat. Oxeye daisy is detrimental to soil health because organic matter does not build up like it does under our native plant communities due to its shallow root system. The shallow root system and its formation of dense stands can also lead to areas of bare soil which causes soil erosion. Oxeye daisy carries several plant diseases such as aster yellows and harbors several detrimental nematode species. Shasta daisy, native daisies and asters are good alternatives to oxeye daisy.
Oxeye Daisy

The last ornamental escapee we are going to discuss today is poison hemlock (Conium maculatum). Poison hemlock is a native of Europe, western Asia, and north Africa. It was imported from Europe in the 1800s as a garden plant and is now found in almost every state in the U.S. As its name suggests, poison hemlock is extremely toxic and has been used as a source of poison throughout history. The ancient Greek used it to poison political prisoners. The famous Greek philosopher Socrates suffered this fate in 339 B.C. in Athens. In the past, Indigenous Americans used it to make poisoned arrows. Today, human deaths most often occur due to confusing poison hemlock with edible relatives such as wild carrots or parsnips. Poison hemlock is a biennial meaning that it forms a rosette in its first year and then sends up a large stem which flowers and then dies. Preventing poison hemlock from setting seed is vital to controlling this species. Poison hemlock has lacy, fern-like leaves that resemble parsley and clusters of tiny white flowers. Crucial to identifying poison hemlock are the purple spots found on its stem.
Poison Hemlock: CSU Guide to Poisonous Plants

The key to controlling all noxious weeds is prevention and early identification. Weeds tend to invade bare and disturbed ground so maintaining a healthy landscape full of non-invasive species is the best thing you can do to avoid a weed infestation. Information about controlling these species is available through the Colorado Department of Agriculture at https://ag.colorado.gov/conservation/noxious-weeds/species-id.

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