Quick Facts...
- Yellow toadflax (Linaria vulgaris) and the Dalmatian toadflaxes
(Linaria dalmatica and Linaria genistifolia) are invasive, perennial
weeds that are noxious in Colorado and other western states.
- Seeds of yellow toadflax germinate and emerge in early to mid-May while
Dalmatian toadflax seeds may germinate and emerge earlier, especially on
south or southeast facing slopes.
- Dalmatian toadflax may be controlled with Tordon 22K at 2 pt/A sprayed
at flowering or in fall. Yellow toadflax (Linaria vulgaris) and
the Dalmatian toadflaxes (Linaria dalmatica and Linaria genistifolia)
are invasive, perennial weeds that are noxious in Colorado and many
other western states. Toadflax invasion is favored by disturbance and
they invade degraded areas such as roadsides, abandoned lots and fields,
gravel pits, clearings, and overgrazed rangeland. In Colorado, these
weed species are found at elevations from 5,000 feet to over 10,000
feet. Yellow toadflax in particular has spread into high mountain valleys
and parks. Yellow toadflax infests 40,800 acres in Colorado and Dalmatian
toadflax infests 34,200 acres.
Origin and History
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| Figure 1. YELLOW TOADFLAX, butter-and-eggs, wild snapdragon
(Linaria vulgaris Mill.). Noxious. Figwort family. This introduced
creeping perennial is an escaped ornamental that reproduces by seed
and extensive horizontal roots. Stems are smooth, erect, leafy, often
in clumps, and 1 to 2 1/2 feet tall. Numerous pale green leaves are
alternate, narrow, pointed at both ends, and 2 1/2 or more inches
long. Flowers resemble those of cultivated snapdragon; each has a
spur extending below from the lower lip of the corolla. They are about
1 inch long, bright yellow with a bearded, orange throat and occur
in terminal, somewhat elongated clusters with the youngest flowers
at the tip. The fruit is a brown, globe-shaped, two-celled capsule,
1/4 inch in diameter containing many seeds. Seeds are small, round,
rough, flattened, with a papery, notched circular wing, dark brown,
and about 1/12 inch in diameter. It is a persistent, aggressive invader
in Colorado from 6,000 to 8,500 feet mostly on the Western Slope.
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The toadflaxes have a storied past and a long relationship with humans.
Dalmatian toadflax is native to the Mediterranean region. Broad-leaved
Dalmatian toadflax (L. dalmatica) has been cultivated as an ornamental
for at least 400 years. It was introduced into the western U.S. as an
ornamental in 1874. The majority of Dalmatian toadflax infestations in
the west are broad-leaved Dalmatian toadflax; however in its native Eurasian
habitat, narrow-leaved Dalmatian toadflax (L. genistifolia) is
more widespread, indicating its potential to also invade and become widely
dispersed in the U.S. Narrow-leaved Dalmatian toadflax currently infests
several areas in western Oregon, northwestern Washington, British Columbia,
and possibly in Colorado.
Yellow toadflax is native to south-central Eurasia where it was used for
fabric dyes and for medicinal purposes. It was imported into North America
in the late 1600s as an ornamental and for folk remedies. It was widely
distributed in North America by the mid 20th century. Unfortunately in
states where yellow toadflax is not noxious, it still is sold by some
nurseries as “butter and eggs” or as “wild snapdragons.”
BIOLOGY
Germination and Emergence
Seeds of yellow toadflax germinate and emerge in early to mid-May while
Dalmatian toadflax seeds may germinate and emerge earlier especially on
south or southeast facing slopes. In Washington, Dalmatian toadflax seedlings
on south facing slopes usually emerge in early to mid-March. First year
Dalmatian toadflax plants often produce prostrate shoots in fall that
survive into the following spring. Mature Dalmatian toadflax may produce
prostrate shoots, but to a lesser extent, and these typically die before
winter and shoots emerge the following spring from roots. Yellow and Dalmatian
toadflax shoots that grow from roots emerge as early as mid-March along
the Front Range in Colorado, but vegetative shoot emergence may not begin
until mid- to late June at 9,000 feet to 10,000 feet. In Canada, vegetative
shoots begin to emerge when soil temperatures range from 42 to 50 F.
Root growth
Seedling root develop is slow and represents a life stage vulnerable
to control attempts and plant competition. Disturbance promotes toadflax
invasion and may be necessary for establishment to occur. However once
established, toadflaxes readily spread into adjacent non-disturbed areas.
Much of this spread is by vegetative means, reflecting a vigorously-growing
root system. Dalmatian toadflax roots may grow 20 inches deep or more
nine weeks after seedlings have emergedand have vegetative buds that give
rise to new shoots. Patch expansion can be dramatic. In Colorado, Dalmatian
toadflax shoot density increased over 1,200 percent in six years at one
location and 190 percent over three years at another. Yellow toadflax
seedlings produce vegetative shoots from root buds two to three weeks
after germination. Mature toadflax have well-developed and extensive root
systems. Dalmatian toadflax roots may penetrate the soil 4 feet to 10
feet and lateral roots may extend 10 feet from the parent plant; while
yellow toadflax roots grow 3 feet deep or more with lateral roots that
may extend several yards.
Flowering
Dalmatian toadflax typically flowers begining in late May or June in
Colorado and may continue until fall, particularly if moisture is not
limiting. Yellow toadflax begins to flower when shoots are from 16 to
24 inches tall, mid- to late May along the Front Range in Colorado, although
at higher elevations (9,000 feet or more), flowering may not begin until
late July. Yellow toadflax may not flower until fall under drought conditions.
Yellow toadflax shoot phenology in any given patch may range from vegetative
to flowering to seed set, depending on the time of season and environmental
conditions (particularly moisture). This contributes to management difficulties.
MANAGEMENT
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| Figure 2: DALMATION TOADFLAX [Linaria dalmatica (L.) Mill.
= Linaria genistifolia ssp. dalmatica (L.) Maire &
Petitmengin]. Noxious. This differs from yellow toadflax principally
in being larger and having differently shaped leaves. Clumps of stems
are 3 to 4 feet tall. Waxy leaves are broad, ovate, sometimes heart
shaped and upper leaves clasp the stem (an important difference).
Seeds are irregular in shape, angular, somewhat flattened, thin-edged,
strongly netted, tan-gray and 1/24 to 1/16 inch across. It is not
as common, but is more aggressive than yellow toadflax. It is reported
in various parts of the state from 5,000 to 6,500 feet. |
All toadflax species are very difficult to control and management plans
should integrate as many strategies as possible to increase potential
for success. Assess the condition and composition of the existing plant
community in an infested area then determine the approximate composition
of the desired plant community needed to achieve land management goals
and objectives. Create a management plan that combines various control
strategies to foster development of the desired plant community.
Chemical and Cultural Management of Dalmatian Toadflax
Dalmatian toadflax may be controlled with Tordon 22K at 2 pt/A sprayed
at flowering or in fall. In Colorado, rates of 2, 4, and 8 pt/A of Tordon
were compared and control longevity was greatest from the 2 pt rate, apparently
because competition from crested wheatgrass was maintained. Researchers
in Wyoming treated Dalmatian toadflax in early September, 1994, with Tordon
at 2 pt/A, then seeded the following year in April or August with ‘Hycrest’
crested wheatgrass, ‘Luna’ pubescent wheatgrass, ‘Critana’
thickspike wheatgrass, ‘Bozoisky’ Russian wildrye, or ‘Sodar’
streambank wheatgrass. The combination of spraying and seeding competitive
grasses controlled Dalmatian toadflax better than spraying alone. Three
years after treatments were started, control of Dalmatian toadflax ranged
from 61 percent to 86 percent where grasses were seeded in April and from
76 percent to 95 percent from the August seeding, compared to no control
from spraying alone.
Chemical Control of Yellow Toadflax
Yellow toadflax appears to be more difficult to manage than Dalmatian
toadflax. In Colorado, control from Tordon applied at flowering has been
most consistent and typically, 4 pt/A is recommended. Yellow toadflax
usually recovers from a single application. For example, Tordon applied
at 4 or 8 pt/A controlled 13 percent and 69 percent of yellow toadflax
three years after treatments were applied. Other research conducted in
Colorado suggests that yellow toadflax control may be improved if Tordon
is applied over three consecutive years, but control varied with location.
In one experiment conducted at high altitude (Camp Hale; elevation approximately
10,000 feet), 4 pt/A of Tordon applied at flowering for three consecutive
years decreased shoot density to zero. However, the same treatment applied
for three years at two other locations (White River drainage, elevation
approximately 8,500 feet) controlled 69 percent and 35 percent of yellow
toadflax. Telar also may be used to control yellow toadflax in non-crop
areas. In an experiment conducted in Middle Park near Parshall, Telar
at 1.25 ounce/A applied during flowering or in fall controlled 84 percent
of yellow toadflax one year later.
Escort, 2,4-D amine, Banvel, and Paramount controlled from 5 percent
to 24 percent of yellow toadflax one year after single treatments were
applied at flowering. Plateau showed some potential to control yellow
toadflax in another Colorado experiment where 8 fluid ounces per acre
applied once in fall controlled 59 percent of yellow toadflax one year
later. While this level of control is unsatisfactory, sequential treatments
may increase control but experiments must be conducted to test this hypothesis.
Mechanical and Chemical Control of Yellow Toadflax
Mowing combined with spraying Tordon did not improve control in an experiment
conducted near Hesperus, Colorado. Yellow toadflax was mowed three times
per year then treated with Tordon at 4 pt/A in fall for two consecutive
years and compared to Tordon applied at 4 pt/A at flowering also for two
consecutive years. Yellow toadflax control was the same (85 percent) whether
Tordon treatments were combined with mowing or not.
Biological Control of Toadflaxes
Several classical biocontrol agents are available to use against toadflaxes.
However, the success of these agents remains largely unknown. A defoliating
moth (Calophasia lunula), an ovary-feeding beetle (Brachypterolus
pulicarius), and two-seed capsule-feeding weevils (Gymnaetron antirrhini
and G.netum) have been released in the U.S. and Canada to control
all toadflax species. Particularly the flowering and seed feeding insects
should help decrease seed production.
A stem-boring weevil (Mecinus janthinus) and a root-boring moth
(Eteobalea intermediella) also were released in Canada and the
U.S. to control all species of toadflax. These species may help to control
shoots and seed production as well as decrease root vigor, but data are
unavailable to document their effects. Several of these classical biocontrol
agents are available from the Colorado Department of Agriculture Insectary
in Palisade. Very few published studies are available to determine whether
grazing by livestock will effect any control of Dalmatian or yellow toadflax.
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