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Toadflax Control Methods in the Methow Valley
by Aileen Jeffries
Dalmatian toadflax (Linaria dalmatica) is an aggressive invasive
plant from the Mediterranean. It spread across the US and Canada at the
turn of the century, reached Spokane in the 20's and the Methow Valley
before 1975. I first noticed it along the West Chewuch road in 1975. It
spread across my property when I had to leave to work in Seattle for a
number of years. Since it is able to displace native vegetation, I began
to consider it a real problem. A massive kill of Ponderosa Pine trees along
the road from herbicides motivated me to explore other ways of controlling
this weed. This document summarizes the observations and control methods
I have explored.
General Observations
Dalmatian toadflax will totally displace almost all other vegetation
including knapweed. My observation is that it spreads primarily from seed
although a mature plant will send out runners to a distance of a foot or
two. The central plant often dies after a few years apparently from root
rot of some kind. I have often found gophers tunneling around plant clusters
and the roots completely eaten away to the point some of these clusters
die out. Unfortunately if also appears that the plant colonizes the newly
disturbed earth mounds gophers produce. I have seen several fungal growths
that appear to attack the roots. I have also seen a yellow caterpillar
(Calophasia lunula), that has completely defoliated plants. This is one
of the recognized biological controls and appears to be a good one to encourage.
In general, seeds do not germinate well unless there is quite a lot of
moisture. I have never seen regeneration from plants that have been pulled.
Maps of the plant distribution in the Methow reveal clustered
growth with satellite populations initiating along the roadways or along
wildlife paths. The weather has a major effect on the spread of toadflax.
In years with little spring precipitation, such as 1999, the plant develops
a limited number of shoots with few flowers. In extremely rainy springs
like 1997, it vigorously develops many shoots, many flowers and sends out
runners.
Reference Information
Studies have shown that the plants average maximum age was 4.2 years
(Robocker, 1974. P13). The seeds are viable for up to 10 years (Robocker,
1997. P1). A stand may persist for 13 years (Lajeunesse, 1993. P6). Because
seedlings are poor competitors for soil moisture they have a difficult
time becoming established (Lajeunesse, 1993. P7). They do not have significant
if any allelopathic effects (Robocker, 1974. p.13). One of the most significant
findings was that if established crested wheatgrass and bromes were fertilized
they quickly used up soil moisture and prevented establishment of toadflax
seedlings.
Controlled Studies
Hand-pulled Plots
Before initiating a massive hand pulling effort, I wanted to determine
the effectiveness of hand-pulling. In May 1997, I selected an area of relatively
uniform toadflax population in the center of a field. I marked off an area
30 x 30 feet. I subdivided it into nine 10 x 10 foot plots. I counted the
number of toadflax plants in each plot, hand-pulled two of the 10 x 10
foot plots and planted grass in others. In July, I checked how many toadflax
plants and seedlings were in the plots I had initially pulled. I pulled
the plot again at this time. In the spring of the next year, May 1998,
I again counted the number of plants and seedlings. The table below shows
the results.
| Date |
Plot 2 - Plants |
Plot 8 - Plants |
| May, 1997 |
65 |
110 |
| May, 1998 |
2 (97% reduction) |
4 (96% reduction) |
There was a reduction of better than 96 % in both the populations
and very little tendency to re-sprout from seed, or root fragments. I had
stacked the plants that were pulled and found absolutely no regeneration
from these plants.
This effort was so successful that we hired a team to pull large
areas that same year. In general we found the same degree of success. The
places where pulling was least successful was in rocky soil where the main
root would break or in places with other vegetation - particularly grass.
Mowing
Mowing was initially tested in 1997 in a large field to stop seed
production. The plant was mowed just as it started to bloom but before
the blooms were developed enough to go to seed if cut. This occurs in June
or early July. We had planned to remow it when it came back but found that
it did not recover enough for that to be necessary. In 1998, we observed
that the plants had lost quite a lot of vigor. We mowed it once that summer.
We will continue to mow it and observe the effect on the plants. Perhaps
the continuing mowing will reduce the toadflax population, since it does
quite obviously sap the plants resources. Irregardless, mowing has been
a means to stop seed production until other methods can be employed.
The mowing does successfully stop seed production in the landscape
described above: dry sandy field that is not watered and has no sub-irrigation.
There are locations where I would not mow. A moist field or a lawn that
is mowed constantly and watered would probably encourage rhizome development.
Cultivation
Cultivation techniques included plowing and disking. To initially
test the effectiveness of cultivation we disked a field in the spring of
1998 and planted spring rye. This reduced the toadflax population by about
75 to 85% depending on soil condition and type, which varied across the
field. This was adequately successful that we plowed two additional fields
and disked a forth field that fall and planted rye. In the spring of 1999
we found that plowing had eliminated the toadflax in the plowed area. The
disked areas showed a reduction of about 80%.
The field we originally
disked in 1998 was disked again but we ran into a problem that we could
not find labor in time to remove rocks. The field was not planted with
rye as intended and knapweed was a problem and some toadflax had re-seeded.
I cannot demonstrate the following observation but I think the seeding
of rye was an important element. That the planted rye competed with weeds
and effected a reduction in the weed seedlings that could survive. I will
try to quantify this observation in the next season.
Another cultivation that I will try and I think will be successful
is to disk a field several times in a season. The disking disturbs the
toadflax and if it is done in the summer the plants vulnerability to low
moisture will be exploited.
Cultivation appears to be a very successful way of eliminating
toadflax. My initial experience is that plowing is more successful than
disking and that the effort must be followed up for a number of years.
One of the most important elements of this is to establish a weed resistant
ground cover before the effort is discontinued. My plan is to plant perennial
grasses and legumes with the rye after several years.
Sheep
In 1999 we tried sheep for toadflax control. We had a 2 acre pasture
that was almost all pasture grass and had a couple of small patches of
toadflax. The sheep spent the summer in the field and we found that not
only did they selectively eat the toadflax, they preferred it to the pasture
grass. As the photo on the left shows the pasture grass was not eaten down
significantly. The second picture of the sheep shows the patch of toadflax
eaten to the ground.
Sheep
are a particularly interesting control for landscapes where the toadflax
has invaded native vegetation. The shrub steppe cannot be cultivated or
mowed. If sheep that are accustomed to eating toadflax will selectively
graze it in a shrub steppe, this might be a most useful alternative for
weed control.
Burning
Generally toadflax does not massively re-seed. In a few areas
we did observe such reseeding. We used a propane torch to heat the seedlings
to the degree that they changed color and we burned others. The torch was
quite successful for seedlings that were less than two inches tall. As
the seedlings got taller their root systems were more developed and the
plant was not killed by the heat or burning. In order for a torch to be
successful it must be used before the seedlings are more than a few inches
tall.
References:
Harris, P.,
Control of Toadflax , Canadian Entomologist, 11/61.p
977
Lajeunesse, Sherry E., et al.
Dalmatian and Yellow Toadflax: Weeds
of Pasture and Rangeland, Montana State University Extension Service,
1993.
McClay, A. S., Effects of Brachypterolus Pulicarius (L.)(Coleoptera:Nitidulidae)
on Flowering and Seed Production of Common Toadflax,
Canadian Entomologist,
July/Aug 1992 p 632
Nowierski, R. M.,
Biological Control of Weeds of the West, November
1995.
Robocker, W. C.,
Life History, Ecology and Control of Dalmatian Toadflax,
Washington Agricultural Experiment Station, WSU, 1974
Other Methow Resources
Methow Conservancy - 509-996-2870 The Methow Conservancy is dedicated
to the conservation and stewardship of the Methow Valley's rich natural,
historic and scenic resources. The Conservancy has a bulletin board and
library on weeds and weed control. They will be putting out a handbook
for residents which will have a section on weed identification and control.
Methow Natives - A source for plants native to the Methow Valley 509-996-3562
Methow Natives is a local horticultural business which grows native plants
for land restoration. Much of their stock is propagated from native stock.
Pacific Watershed Institute - This group has been active in restoration
of riparian areas. They put on a workshop in November, 1999, where experts
shared their knowledge and experiences about restoration with native plants,
plant propagation, plant succession, biology of invasive weeds, practices
for long term removal of invasive weeds, escaped ornamental plants.
Rendezvous Reclamation - Alternative Weed Control and Land Restoration
509-996-3562 Rendezvous Reclamation specializes in restoring agricultural
land that has been fallow and acquired weed infestations.
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