Yellow toadflax, Linaria vulgaris
Also known as butter and eggs or wild snapdragon.
Weed identification:
This weed has been found throughout North America, with a considerable number of cases reported in Colorado, Canada, and the states along the northern border. Dalmatian toadflax (Linaria dalmatica), and Linaria vulgaris are able to spontaneously hybridize, resulting in risk of offspring infesting a novel range. Linaria vulgaris is currently sold as an ornamental under the common names: Yellow Toadflax and Butter and Eggs. Linaria vulgaris is designated as a “List B” species under the Colorado Noxious Weed Act. Linaria vulgaris quickly takes over disturbed areas and readily outcompetes native plants with its rapid method of vegetative reproduction.
- Growth form is herbaceous perennial, mature plants grow to 1-3 feet tall with extensive root systems with taproot reaching up to 3 feet in depth and creeping lateral roots. Linaria vulgaris can grow in very dense patches, with young plants sprouting from the root system.
- Leaves are simple with smooth entire margins, sessile, and lanceolate with alternate leaf arrangement. The leaves are soft, mostly glabrous, and slightly drooped; they can be bright green to blue-gray green.
- Stems are woody at the base but non-lignified towards the distal end, mostly glabrous but can be hairy near the top of the plant. Bilateral showy flowers resemble snapdragons, are yellow to cream colored with 5 fused petals (3 lower lobes and 2 upper lobes) and an orange bearded throat with a nectar collecting yellow colored spur. Blooms appear early to mid-May in Colorado.
- Flowers are about 1 inch long in dense racemes of 15-20 flowers found in the axils of the upper portion of the stems. Fruit capsules are brown and 0.25-0.5 in long. Ovate fruit capsules contain disc shaped, flat, deep brown to black seeds encircled by a brown papery wing.
Yellow toadflax flowers
Image credit: Michael Shephard, USDA Forest Service, Bugwood.org
Yellow toadflax flowers
Image credit: Maja Dumat — Creative Commons License BY 2.0
Yellow toadflax field plant in bloom
Image credit: Linda Wilson, University of Idaho, Bugwood.org
Biology and life cycle:
- Growth habit: Linaria vulgaris is a flowering perennial, found in open areas throughout ponderosa pine, sagebrush, and plant communities within woodlands or parklands at higher elevation (5,000–10,000 feet) . Establishment linked to degraded areas such as: riparian habitats, roadsides, overgrazed rangelands, waste sites, burn areas, and recently disturbed soils.
- Emergence dynamics: Linaria vulgaris prefers full sunlight with well-draining moist sandy soil, tolerates dry soil and high pH; and can subsist in freezing environments. Blooms emerge early to mid-May. Mature plants grow to 1-3 feet tall with extensive root systems with taproot and creeping lateral roots.
- Seed production: Mature plants can produce from 15,000 to 30,000 seeds per plant. Linaria vulgaris has a low viability in seed production with only about a 10% germination rate under field conditions. Seeds can remain dormant for periods of 8 to 10 years and require stratification, with maximum germination achieved after 8-20 weeks of wet stratification at 5°C. Seeds usually germinate in the top 2 cm of soil. Reproduces readily through underground adventitious buds from the lateral roots, with lateral spread growing up to 3 feet each growing season. Root fragments as small as 1cm can establish and initiate shoot growth. The ramets from vegetative root buds can emerge just 3 weeks after cotyledon appearance.
Resistance
- Total cases: 43,895 in North America
- Colorado Cases: 22,044
- Most common mode-of-action resistance: Group 4 Auxin Mimic
- Linaria vulgaris is resistant to phenoxyacetic herbicides: 2,4-D, MCPA, 2,4-DB or MCPB, and mecoprop.
Yellow toadflax stem, pre-bloom
Image credit: New York State Integrated Pest Management Program
Yellow toadflax
Image credit: Wikimedia Commons. File:Linaria vulgaris 2018-09-01 7588.jpg – Wikimedia Commons
Integrated weed management strategies based on Colorado cropping systems:
Prevention
- Prevent over-grazing to reduce bare field areas where yellow toadflax would be likely to establish.
- Prevent ornamental use from escaping to uncontaminated areas by checking ornamental seed mixes for Linaria vulgaris listed and restrict use to prevent escape.
- Use seed or feed free of contamination by L. vulagaris seeds.
- Moving livestock from contaminated areas to uninfected areas requires livestock to be corralled, six days for cattle and 11 days for sheep, to allow any viable seeds to pass through the digestive tract.
- Clean equipment and vehicles to ensure seeds and roots pieces are removed.
Cultural:
- Early detection and monitoring to prevent establishment.
- Utilize competitive native grasses and forbs to prevent bare areas.
- Rotate livestock to reduce over-grazing.
- Encourage healthy desirable native plants to outcompete L. vulgaris throughout all seasons and soil profiles.
Mechanical:
- Physical methods to control yellow toadflax should focus on destroying the root system, cutting/ mowing is not recommended as it leaves roots intact.
- Hand-pulling and removing all portions of the root system is imperative due to vegetative reproduction from roots.
- Intensive cultivation repeated over years (up to 10) to suppress population.
- Proper disposal of root pieces, seeds, and contaminated debris must be destroyed or bagged and removed.
Biological:
- Grazing is not recommended as L. vulgaris contains glucosides, which are mildly poisonous to livestock in large quantities. Five insect species have been approved by the USDA-APHIS-PPQ for release as biological control agents for L. vulgaris.
- Brachypterolus pulicarius: shoot and flower-feeding beetle, adults feed on shoot tips and axillary buds; lay eggs in buds; larvae feed on immature seeds. Can reduce seed set by 74 percent.
- Calophasia lunula: defoliating moth, effective in larval stage; defoliates leaves, thereby reducing seed production and root carbohydrate levels.
- Eteobalea serratella and Eteobalea intermediella: Root-boring moth, adults lay eggs in lower leaf axils at base of yellow toadflax; larvae bore into stem or root.
- Rhinusa antirrhini (formerly Gymnaetron): Weevil, adults eat leaf buds, young leaves, and young shoot tips. After blooming, adults eat floral tissue and lay eggs in floral ovaries; larvae eat seeds.
- Additional biocontrol agents of interest:
- Rhinusa netum: Weevil, impacts seed production.
- Rhinusa linariae: Weevil, adults feed on stem tissue and sap, lay eggs in root crown near soil surface; larvae form galls and feed on root.
Chemical:
|
Active Ingredient |
Product Example* |
Product Example Rate per Acre |
Backpack Sprayer Treatment Using Product Example** |
Time of Application |
Remarks |
|
Picloram
|
Tordon 22k | 1-2 quarts | 0.5-1% | Late flower to post bloom stage in the fall. | Persistent, selective herbicide. Re-treatment for several years may be required. Labeled for rangeland use. (Picloram is a restricted use pesticide.) |
| Dicamba | Banvel, Clarity, Vanquish | 1-2 quarts | 3-5% | Late flower to post bloom stage in the fall. | Persistent, selective herbicide. Re-treatment for several years may be required. Labeled for rangeland use. |
| Chlorsulfuron | Telar XP | 2-2.6 ounces | Consult label | Late flower to post bloom stage in the fall. | Apply as a high-volume foliar spray using a minimum of 24 gallons of water per acre. |
| Aminocyclopyrachlor + chlorsulfuron | Perspective | 7.5-8 ounces |
Add 5–9 grams of dry flowable powder to 1 gallon of water. Consult label for directions. |
Apply to fall rosettes for best control. | Persistent; selective; may cause temporary injury to some grass species. Labeled for no crop use. |
| Imazapic | Plateau | 8-12 fluid ounces + 1-quart methylated seed oil (MSO) | 0.25-1.5% | Apply to fall rosettes for best control. | Persistent, selective herbicide. Re-treatment for several years may be required. Use lower rate when cool season grasses are present. |
| *Trade names for products are provided for example purposes only, and other products with the same active ingredient(s) may be available. Individual product labels should be examined for specific information and appropriate use with toadflax. | |||||
| ** Herbicide/water ratio – As an example, a gallon of spray water with a 3 percent mixture is made by adding a sufficient volume of water to 4 ounces of liquid herbicide until a volume of 1 gallon is reached (4 oz ÷ 128 oz/gal = 0.03 or 3 percent). For dry formulations, particulates should be added to sufficient water as specified by the label until the required concentration or volume of spray water is reached. | |||||
References
References and resources for more information:
Resources:
Colorado Department of Agriculture- Yellow and hybrid toadflax
USDA (NISIC)- Yellow Toadflax | National Invasive Species Information Center
USDA Weed Field Guide- Managing Yellow Toadflax
EPA Herbicide online search- Pesticide Product and Label System | US EPA
References:
Alaska Natural Heritage Program. (2006). Environment and Natural Resources Institute, University of Alaska Anchorage.
Crocker, W. L. (1977). Wildly successful plants: a handbook of North American weeds. New York, NY: Collier Books.
Early Detection & Distribution Mapping System (EDDMapS). (2025). Early Detection & Distribution Mapping System. The University of Georgia – Center for Invasive Species and Ecosystem Health. Retrieved February 4, 2025, from yellow toadflax (Linaria vulgaris P. Mill.) – EDDMapS
Ņečajeva, J., & Probert, R. (2011). Effect of cold stratification and germination temperature on seed germination of two ecologically distinct species, Linaria loeselii and L. vulgaris (Scrophulariaceae). Polish Botanical Journal, 56.
Ogden, J. A. O., & Renz, M. J. (2005, November 6). Yellow toadflax (Linaria vulgaris). Weed Fact Sheet. New Mexico State University.
Ontario Ministry of Agriculture and Food. (1993). Guide to weed control. Queens Printer, ON. Publication 75, 216p.
Royer, F., & Dickinson, R. (1999). Weeds of the Northern U.S. and Canada. Edmonton, AB: The University of Alberta Press.
Warren, R. (1957). Toadflax. Oregon Agricultural Extension Service, Extension Bulletin 763. Oregon State College, Corvallis, OR.
Zouhar, K. (2003). Fire Effects Information System (FEIS). US Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. Retrieved February 4, 2025, from http://www.feis-crs.org/beta/