Canada thistle (Cirsium arvense)
A deep-rooted perennial that reproduces by seed or underground rootstalks. Seed can remain viable in soil for up to 20 years. Canada thistle has become the most common weed problem in the Western United States.
The most effective herbicides for control of Canada thistle are Milestone, Curtail, Redeem, Transline, Tordon, and Telar. Fall applications, prior to a hard frost, are rated best for control but any of these listed herbicides applied in the spring or summer provides good control. Because of the extensive underground root system, Canada thistle is very resilient and eradication requires several seasons of management.
Shallow tillage (disk, sweep) on a deep-rooted perennial can be counter-productive, creating a denser, more uniform stand. Deeper tillage such as moldboard plowing, if feasible, can provide 1-2 years control.
Mowing effectively suppresses Canada thistle, and in combination with a fall herbicide application provides best control.
Dalmatian toadflax (Linaria dalmatica, Linaria genistifolia)
Dalmatian toadflax is termed an 'escaped ornamental', a former garden species that has become an invasive weed problem in rangeland and natural areas.
Dalmatian toadflax is particularly abundant on the shallow, rocky soils along Colorado's front-range. This perennial plant reproduces from seed and vegetative root buds. Seeds can remain viable in the soil for at least 10 years. Plants grow to 3 feet tall and produce yellow flowers throughout the summer.
The most effective herbicides for control of Dalmatian toadflax are Telar, Tordon and Plateau. Fall applications provide best control.
Mowing, hand pulling, or digging can reduce seed production and stress plants, but this perennial will readily grow back.
Yellow toadflax (Linaria vulgaris)
Better known as butter-and-eggs, this close relative to dalmatian toadflax has become a significant problem in mountain communities. Yellow toadflax is an escaped ornamental plant that is generally more common than dalmatian toadflax at higher elevations and cooler temperatures.
perennial plant reproduces from seed and vegetative root buds. Seeds can remain viable in the soil for at least 10 years. Plants grow 1-2 feet tall and produce yellow flowers with an orange center. Prior to flowering, the narrow leaves and overall size of yellow toadflax is similar to leafy spurge.
If in doubt, remember yellow toadflax lacks the milky latex present on leafy spurge stems and leaves found upon breaking or tearing these plant parts.
The most effective herbicides for control of yellow toadflax are Telar, Tordon, a tank mix of Telar & Tordon, or a tank mix of Tordon & Overdrive.
Mowing, hand pulling, or digging can reduce seed production and stress plants, but this perennial will readily grow back.
Common Mullein (Verbascum thapsus)
A biennial plant commonly found in disturbed areas of range and pasture.
Mullein seldom appears to have invasive characteristics except on heavily disturbed sites such as intensively grazed pastures, prairie dog colonies, or sites severely impacted by wildfire. Mullein seed germinates in the late summer or fall producing a rosette with soft, velvet-like leaves. Flowers grow from a tall, solitary stalk that appears in mid-summer.
The most effective herbicides for controlling common mullein are Escort, Milestone, and Telar.
Hand pulling or digging this biennial is an effective method of control if the top 2-3 inches of tap root is removed.
Hoary Alyssum (Berteroa incana)
An annual, biennial or short-lived perennial plant that grows to 2 feet tall and produces white flowers. This member of the mustard family is commonly found along roadsides and disturbed areas of range and pasture. Hoary alyssum is a common weed in the Estes Valley of Larimer County and is toxic to horses.
The most effective herbicides for control of hoary alyssum are Escort and Telar.
Digging and hand pulling can provide effective control if persistent.Â
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