Conservation Tillage Handbook Series No. 16
Chapter 5 - Weed Control Strategies, July 1995
Thistle Management under Conservation Systems in Pacific Northwest Crop-Fallow
Research Agronomist, USDA-Agricultural Research Service, Pullman, WA
Veseth, Extension Conservation Tillage Specialist, Washington State
University and University of Idaho, Moscow, ID
Thill, Weed Scientist, University of Idaho, Moscow
Schillinger, Area Extension Agronomist, Washington State University,
Ball, Weed Scientist, Oregon State University, Pendleton, OR.
- Russian thistle
(Salsola iberica) is a summer-annual broadleaf weed commonly
found in many of the low precipitation cropland areas of the Pacific
Northwest. This weed causes serious production problems in crop, following
harvest, and during summer fallow. Control of Russian thistle is an
important management focus for many growers as they adapt their farming
systems to meet changing environmental and economic demands.
- Growers strive
to continually develop farming systems that improve cropland productivity,
protect the environment and increase profitability. Tillage to control
Russian thistle after harvest and during the summer fallow season can
reduce crop residue on the soil surface and decrease surface roughness,
which increases the potential for soil erosion and soil water loss by
evaporation and runoff. Use of herbicides in the sulfonylurea family
(such as Glean and Finesse) provided effective Russian thistle control
in the 1980's, but widespread Russian thistle resistance to these crop
protection chemicals has resulted in the need to develop other management
- Fortunately, PNW
research and the experiences of an increasing number of growers show
that good Russian thistle control, effective soil conservation, and
profitable farming operations are not mutually exclusive. Management
strategies with improved cultural and herbicide options for Russian
thistle control are providing effective weed control in profitable conservation
- This publication
reviews aspects of Russian thistle biology as it relates to management
and outlines some key considerations for management strategies. The
four primary goals for Russian thistle management strategies are to:
- Reduce Russian
thistle seed production and seedbank in the soil
- Reduce wind
and water erosion potential
- Increase soil
water storage and crop yield potential
- Increase profitability...the
- More detailed
information on the biology and identification of Russian thistle is
available in "Russian Thistle" Pacific Northwest Extension
Weed Series bulletin PNW 461. It is available through local extension
offices in the Northwest.
Thistle Biology Related to Management Considerations
and Longevity in Soil
- Most newly-produced
Russian thistle seeds are dormant for a short period of time after maturity
in the fall. Seed dormancy decreases over winter and is almost nonexistent
by spring, allowing germination to occur over a wide range of temperature
and moisture conditions.
- Research under
irrigated conditions indicates that Russian thistle seed viability in
the soil declines greatly within 2 years. Under irrigated conditions
at Prosser, WA, about 99% of the seeds germinated or did not survive
in the soil past the first year. In dryland wheat-fallow areas, seed
longevity in the soil may be longer, and research has been initiated
to determine this time period. Russian thistle seed is soft and porous,
characteristics that contributes to its lack of longevity and ability
to germinate rapidly. Management strategies that focus on preventing
seed production are effective because Russian thistle has both limited
seed dormancy and longevity in the soil.
- Russian thistle
has a unique mechanism for increasing the area of infestation -- mature
plants break at the ground level and they tumble with the wind to disperse
seeds. An experiment was conducted in eastern Washington in 1991 and
1992 to measure movement of Russian thistle and seed dispersal in the
wind. Average estimated seed number per plant at the start of the experiment
was 61,700. Plant movement was highly correlated with wind direction.
Some plants moved up to 2.5 miles in 6 weeks, while other plants moved
only 200 feet because of variable winds and being compressed with snow
or frozen into wheat stubble. Average percent seed loss for the tumbling
plants was 57 percent.
- A special layer
of cells where the plant is connected to its roots enables plants to
break away with the wind during winter months after seeds are mature.
Cutting off mature Russian thistle plants with tillage implements in
the fall also facilitates spreading of seeds by releasing the tumbling
plants -- a factor to consider when selecting tillage implements and
timing of operations. Because undisturbed Russian thistle can produce
150,000 to 200,000 seeds and seed distribution is rapid and widespread,
there is a high potential for future infestations.
- Russian thistle
seeds require only a short moist period to permit rapid germination
and emergence from the soil. As growers know, Russian thistle can emergence
in significant numbers in crop and fallow after very light rains (about
0.1 inch) on dry soil (Fig. 1). Germinating seeds also can withstand
several wetting and drying cycles until there is sufficient moisture
for emergence and establishment. The primary reason for this unique
survival trait is that Russian thistle seeds consist of fully differentiated,
coiled "seedling" in the form of a spiral helix, ready to
take quick advantage of short periods of favorable environmental conditions.
1. Russian thistle emergence with increasing amounts of rainfall
(Source: Dwyer, D. D. And K. Wolde-Yohannis. 1972. Germination,
emergence, water use and production of Russian thistle. Agron.
- Optimum temperature
for Russian thistle germination ranges between 45 and 95 degrees F.
Seeds can germinate under cooler conditions when night temperatures
are below freezing, if daytime temperatures are above freezing. However,
young seedlings are very susceptible to frost. Emergence typically begins
in late March-early April, extending through the summer when sufficient
- One factor limiting
Russian thistle establishment is seed depth in soil (Fig. 2). Emergence
is optimum at depths less than 0.5 to 1.0 inch, although some seedlings
can emerge from 2-3 inches under favorable conditions.
- Fig. 2.
Russian thistle emergence from seeds buried at increasing depths
in sandy loam soil compacted to a bulk density similar to a planted
field. (Source of greenhouse data: Evans, R.A. and J.A. Young. 1972.
Germination and establishment of Salsola in relation to seedbed
environment - II. Seed distribution, germination, and seedling growth
of Salsola and microenvironmental monitoring of the seedbed.
Agronomy Journal. 64:219-224; Source of field data from Lind, WA:
Young, F.L. 1982. Unpublished data.)
- Russian thistle
establishment also can be limited by compacted soils. The roots cannot
effectively penetrate compacted soil as the coiled embryo unwinds during
germination. In addition, shoot emergence can be restricted by crusted
surface soil, even if seeds are buried shallowly. However, seedlings
can emerge through cracks in the soil surface.
Growth After Establishment
- Flowering commonly
begins around mid-June. To effectively prevent seed production and reduce
weed competition, Russian thistle should be controlled within 4 weeks
after emergence. Flowering increases greatly after crop harvest when
about 90% of the Russian thistle growth and the majority of seed set
commonly occur. Russian thistle usually remain small in a competitive
winter wheat crop but grow rapidly immediately after harvest. Russian
thistles grow larger in a less competitive crop, such as spring wheat,
and even though the top portion of the Russian thistle plants are cut
by the combine, they can regrow quickly after harvest. This rapid growth
after harvest is why Russian thistle not controlled in crop should be
controlled within about 10 to 14 days after harvest to effectively reduce
seed and biomass production, and soil water use. Russian thistle is
indeterminate, therefore it continues to flower and produce seed as
long as conditions allow, typically until a killing frost at around
25 degrees F or less, or until several successive frosts just below
- Studies indicate
that Russian thistle is one of the most efficient plants in the world
at producing plant dry matter per unit of water used. Russian thistle
roots extract water from the soil very efficiently and can extend to
a depth of 5 feet with lateral spread of 6 feet. Winter wheat grown
on silt loam soils in the PNW commonly extracts water down to 4.5 percent
(by volume) by harvest time. Russian thistle roots will continue to
extract soil water when it is no longer available to the wheat plants.
It is important to control Russian thistle postharvest to prevent excessive
soil water loss.
- Growing a competitive
crop is a very important management tool to reduce Russian thistle growth
and seed production. Growth of Russian thistle is suppressed greatly
when the crop establishes first, over-tops the weed, and has adequate
moisture and nutrients. Conversely, Russian thistle causes the greatest
yield losses in crops during drought conditions, with poor stands, and
- Russian thistle
usually reduces crop yield more in spring wheat than in winter wheat.
A study of Russian thistle growth and development in summer fallow,
spring wheat, and winter wheat was conducted at Lind, WA in 1982 and
1983. Russian thistles were established in early April and allowed to
grow until killed by frost in October. Dry weight of Russian thistle
grown in winter wheat was about 75% less than those grown in spring
wheat and 98% less than under fallow with no weed or crop competition
(Fig. 3). After harvest, Russian thistle that had grown in spring wheat
used about four times more water than plants grown in winter wheat.
Results from these and additional experiments at Lind show that winter
wheat reduced Russian thistle emergence 44%, seedling survival 42% and
seed production 74% compared to spring wheat. When Russian thistle was
not controlled through the growing season or after harvest, single plants
produced 150,000 seeds in undisturbed fallow, 17,400 in spring wheat,
and 4,600 in winter wheat.
- Fig. 3.
Russian thistle growth in fallow and when competing with spring
and winter wheat at Lind, WA. Russian thistle emerged in early April.
Winter and spring wheats were harvested at points marked ww
and sw, respectively. (Source: Young, F.L. 1986. Russian
thistle (Salsola iberica) growth and development in wheat.
Weed Sci. 34:901-905).
- Even though spring
wheat provides less competition against Russian thistle than does winter
wheat, management practices which increase crop competitiveness can
help suppress Russian thistle. The same is true with the competitiveness
of winter wheat and other crops. Research data on weed/crop competition
with spring wheat in 1983-85 at Lind reveal the importance of early
spring wheat establishment to improve crop competitiveness (Table 1).
Although Russian thistle density was highest in 1984, yield loss was
much lower than in 1983 or 1985. The low effect of Russian thistle in
1984 was attributed partially to seeding wheat one week earlier than
the other years and the corresponding crop emergence two weeks ahead
of Russian thistle, compared to one week ahead in the other two years.
Although weed densities were similar in 1983 and 1985, Russian thistle
was much more competitive in 1985 when rainfall was low.
- Table 1.
Spring wheat yield losses from Russian thistle competition, 1983-85,
WSU Dryland Research Unit, Lind, WA.
F.L. 1988. Effect of Russian thistle (Salsola iberica) interference
on spring wheat. Weed Sci. 36:594-598).
Wheat and Russian thistle Residue
- Maintaining enough
residue on the soil surface to control wind erosion is a major concern
in the low-rainfall dryland wheat regions. Growers should consider dead
Russian thistle plants or "skeletons" as a residue source
in postharvest residue management decisions. The erosion control challenge
in the very dry areas is three-fold: 1) not enough crop residue is produced;
2) soils are generally coarse textured and seldom retain adequate soil
cloddiness; and 3) traditional soil management techniques often reduce
soil roughness and bury most of the crop residue. By the end of the
fallow period the surface soil mulch is often powdery and deficient
of surface residue. In these dry environments, Russian thistle skeletons
can provide an important source of residue for water conservation and
- A residue management
experiment was initiated at Lind, WA during the 1993-94 fallow cycle
to determine how much tillage could be reduced and still maintain an
agronomically feasible production system. Traditional, minimum, and
delayed minimum tillage systems were evaluated (Table 2).
- Preliminary results
of this study indicate that significantly more winter wheat and Russian
thistle residue can be retained through of the fallow cycle in the minimum
tillage systems and that Russian thistle skeletons can be an important
contributor to the total surface residue (Fig. 4).
2. Field operations conducted in the tillage management experiment
at WSU Dryland Research Unit at Lind, WA during the 1993-94 fallow
cycle (W. Schillinger, WSU, Ritzville).
@ 48 oz
@ 48 oz
@ 12 oz
@ 12 oz
@ 12 oz
@ 40 lb
N @ 40 lb
N @ 40 lb
- Where Russian
thistle roots were severed by postharvest sweeping under traditional
tillage, most of the Russian thistle skeletons had blown away by the
November 7 measurement of Russian thistle residue. In the two minimum
tillage treatments, a postharvest herbicide application for Russian
thistle control and fall chiseling with 72-inch shank spacing left most
of the Russian thistle skeletons anchored overwinter and resulted in
a higher percentage of overwinter precipitation stored in the soil.
In general, leaving more residue on the soil surface overwinter results
in increased soil water storage.
- In the early 1980's,
the registration of sulfonylurea herbicides provided a great advancement
in Russian thistle control. However, the development of extensive Russian
thistle resistance to this herbicide family has set back control efforts.
About 70% of the sites infested with Russian thistle in eastern Washington
now contain plants that are resistant to sulfonylurea herbicides. Resistance
has also been confirmed throughout the Columbia Basin in Oregon and
in Idaho. Sulfonylurea herbicides used in wheat and/or fallow cropland
have included Glean, Ally, Finesse, Express, Harmony-Extra and Amber.
- A key reason for
rapid development of Russian thistle herbicide resistance are the "same"
- Same herbicide
or herbicide family used once or more each year for successive years.
- Same crop
(eg. in the wheat-fallow-wheat rotation).
- Same field.
- Same target
- Also, the tumbling
action of the Russian thistle plants contributed to the rapid spread
of the problem within and between fields. For more information on preventing
and controlling herbicide resistant weeds, see Pacific Northwest
Extension publication PNW 437 "Herbicide-Resistant Weeds and Their
Management." Copies are available through local county extension
offices in Idaho, Oregon and Washington.
- Fig. 4.
Wheat residue (top) and Russian thistle skeletons (bottom) as affected
by tillage method on three sampling dates during the 1993-94 fallow
cycle at WSU Dryland Agricultural Research Unit at Lind , WA (W.
Schillinger, WSU, Ritzville). Averages followed by the same letter
are not significantly different at the 5% probability level.
of Opportunity for Russian Thistle Control
- There are several
opportunities to reduce Russian thistle competitiveness and seed production
during the crop-fallow rotation or other crop rotations in the low rainfall
zones. Making the best use of each available management option throughout
the rotation will provide the best overall control. The following are
a number of "windows of opportunity" for Russian thistle control:
- In Crop - Several
herbicides are available for control of Russian thistle with varying
degrees of effectiveness and costs. See the current year Pacific
Northwest Weed Control Handbook for specific herbicide recommendations.
Most recommended herbicides control Russian thistle best when applied
to 2-inch tall or smaller plants. Remember that production management
practices that increase crop competitiveness also reduce Russian thistle
growth and seed production. It is important to minimize the potential
for increasing problems with herbicide resistance through rotation of
herbicides with different modes of action (see PNW 437).
- Appropriate nonselective herbicides registered for preharvest
application can accelerate dry-down of Russian thistle, improve harvest
efficiency and effectively control Russian thistle for about 60 days
- Control Russian thistle with nonselective or broadleaf herbicides
or with tillage by 10 to 14 days after wheat harvest. It is important
to compare herbicide applications and tillage relative to their effects
on residue retention and soil water storage over winter, during the
fallow period and next winter wheat crop, as well as the cost, effectiveness
of Russian thistle control, and seed production. Sweeping kills most
Russian thistles but will likely result in reduced surface residue levels
and overwinter water storage compared to control with herbicides.
- Summer Fallow
- Control Russian thistle before seed set with herbicides and/or
tillage, but avoid excessive tillage, which reduces surface residue
and roughness. Delay initial tillage and subsequent rod weedings for
Russian thistle control as long as possible after rain during the fallow
period. Rodweeding too soon after a heavy rain may also form a tillage
pan which grain drill openers may have difficulty penetrating at planting
time. Research has shown that operating rodweeders at deeper depths
(4 inches) causes less pulverization of soil clods than when operated
at shallow depths (2 inches). This may be an important management factor
on soils prone to wind erosion.
- Field Borders
and Roadways - Control Russian thistle along field borders, roadways
and other noncropped areas upwind from fields to prevent introduction
or reinfestation of fields with Russian thistle. Because of the high
mobility of Russian thistle skeletons in the wind and extensive seed
distribution potential, an area-wide Russian thistle control strategy
is needed to achieve and maintain effective control. Cooperation of
neighboring upwind producers is needed for effective control of Russian
thistle on a community basis if possible. Unless Russian thistle dispersal
is controlled, it is difficult to manage this weed.
Strategy Considerations for Russian Thistle Infestations
- Management strategies
for Russian thistle, which focus on preventing seed production throughout
the crop rotation, can reduce a serious Russian thistle infestation
to a manageable level. There are a number of important biological traits
and crop production options described in previous sections that need
to be considered in developing Russian thistle management strategies.
- Short Russian
thistle seed dormancy and seed longevity in the soil
period of potential Russian thistle germination from early spring
through late summer
period of Russian thistle flowering and seed formation from early
summer until a killing frost
Russian thistle use of soil water in crop, after harvest, and in
- Lower Russian
thistle populations, growth, and seed production in a uniform stand
of winter wheat than in spring wheat
practices that increase crop competitiveness reduce Russian thistle
- Wind dispersal
of Russian thistle seed by blowing plants can be increased with
fall tillage practices that sever the plant from the roots
- Tillage options
for retention of wheat and Russian thistle residue to increase soil
water storage potential and reduces erosion hazard
Russian thistle resistance to sulfonylurea herbicides
is a series of chronological management strategy considerations for
a heavy Russian thistle infestation in a crop-fallow rotation, beginning
in the crop year. It is assumed that Russian thistle is controlled
on neighboring fields so reinfestations do not occur from seed sources
outside the field.
- Plant winter
wheat rather than spring wheat if possible, because it is more competitive
and will help reduce Russian thistle emergence, survival, growth,
and seed production. Practices that further increase winter wheat
competitiveness also would be important Russian thistle management
tools. At least spot apply herbicides in areas where Russian thistle
may concentrate, such as drill skips, winter-killed areas, and draws
where wind-blown Russian thistle skeletons collect.
- If spring
wheat is planted, due to winterkill or other production problems,
use management practices which optimize crop competitiveness with
Russian thistle. These include: seed early (late February, early
March) so wheat emerges ideally 2-3 weeks or more before Russian
thistle; seed shallow as possible to encourage rapid emergence;
place fertilizer below and near seed rows for early wheat root access
and vigorous crop growth; use conservation tillage systems to minimize
water evaporation which optimizes water availability to the crop.
If possible, use narrow row spacings, such as 6-7 inches, to increase
crop competition with Russian thistle.
- Use broadleaf
herbicides to control Russian thistle in crop. A tank mix of herbicides
with different modes of action can reduce populations and the development
of herbicide resistance, but only if all herbicide mix partners
equally control the weed. The goal is to kill or suppress Russian
thistle in order to minimize competition with the crop, improve
harvest efficiency, and reduce the potential for seed production
later in the season. Herbicides should be applied before the Russian
thistles exceed 2 inches in height. Remember that the first Russian
thistle plants that emerge will be the most competitive. Complete
control is not required. Lower cost treatments may be fine as long
as they provide a reasonable level of control. However, in a severe
Russian thistle infestation it may be more effective to strive for
optimum control rather than suppression. It may be necessary to
consider a preharvest herbicide application to control any escapes
and late-germinating weeds.
- If Russian
thistles were not controlled effectively early in the growing season,
consider a preharvest nonselective herbicide. Russian thistle already
has competed with the crop by that point, but seed production can
be reduced greatly and soil water conserved for the following crop.
A preharvest application of a nonselective herbicide often controls
Russian thistle better than postharvest applications. Preharvest
treatment can provide several advantages:
time at harvest and improve harvest efficiency.
Russian thistle size, seed production, and soil water use.
- May eliminate
the need for postharvest tillage for Russian thistle control
or at least the need for a disc operation to chop Russian thistle
Year 1...Beginning After Harvest
- The Russian thistle
management goal during fallow is to stop seed production for a 2nd year,
while optimizing seed zone soil water and retention of surface residue
- Consider postharvest
herbicide or tillage operations if in-crop and preharvest applications
were not effective. Control practices should be applied 10-14 days after
harvest to prevent prolific growth and seed production of Russian thistle.
An important goal in postharvest management is to save crop residue
on the soil surface for water and soil conservation during the fallow
season and the next winter wheat crop, but prevent seed production as
much as possible. Tillage practices should focus on water conservation
and residue management as well as on Russian thistle control. If tillage
is used after harvest, a sweep will provide greater residue retention
and water conservation overwinter.
can influence the subsequent management of Russian thistle residue.
Non-selective herbicides applied preharvest or postharvest generally
result in dry, brittle Russian thistle skeletons which help reduce
weed residue problems at harvest and during fall tillage operations.
Other postharvest herbicides, such as 2,4-D, can leave the Russian
thistle plants tough, "leathery," and more difficult to
sweep or wide-blade undercutter implements can kill Russian thistle
without excessive loss of surface residue. However, these tillage
operations should be done within about 2 weeks after harvest to minimize
water use, seed set and spreading of infestations due to wind distribution
of severed plants. Effective control in-crop and preharvest also can
help avoid the use of more intensive tillage operations, such as discing,
to control Russian thistle after excessive growth occurs following
harvest. Discing can reduce surface residue and roughness significantly,
and consequently reduces erosion protection and water storage potential
during the fallow season, and in the following winter wheat crop.
Retention of Russian thistle skeletons as residue for soil and water
conservation with herbicide control rather than tillage after harvest
is also a consideration, particularly when crop residue production
In areas where
overwinter runoff on frozen soils occurs commonly, growers should
consider chiseling, subsoiling or other non-inversion tillage operations
to increase water infiltration. Wider shank spacings of 4-6 feet can
achieve this goal with minimal disturbance of anchored Russian thistle
plants and standing stubble, thus optimizing residue benefits for
trapping snow and reducing evaporation.
or first spring tillage as late as possible. Consider using herbicide
treatments for Russian thistle and other broadleaf/grass weeds to
delay spring tillage. On fields known to have Russian thistle infestations,
delay herbicide application and tillage until after the first heavy
flush of emerging Russian thistle in the spring.
Wheat Crop 2
- The field should
have a reduced Russian thistle population after 2 years of seed bank
depletion if Russian thistle control efforts have been effective in
Crop Year 1 and Fallow Year 1, as long as Russian thistle plants have
not moved in from neighboring fields, field borders, and non-field areas.
Strive for good control again in Crop Year 2. Spot spray areas of the
field infested with dense populations of Russian thistle. Russian thistle
should not be a harvest problem in the 2nd crop year, but a nonselective
preharvest herbicide could be used if warranted. If Russian thistle
problems require postharvest attention to prevent Russian thistle seed
production and soil water loss, consider a nonselective herbicide or
tillage soon after crop harvest, at least as spot treatments of small
Russian thistle areas and along field borders.
- Management considerations
are basically the same as in Fallow 1, but there is a potential for
reduced frequency of rodweedings for Russian thistle control since the
Russian thistle seedbank in the soil should be reduced significantly.
Another year of Russian thistle seed production should be prevented
through the summer fallow. Maintain control of Russian thistle in fence
rows and field margins.
Wheat Crop 3
- At this phase
of the crop rotation, a Russian thistle maintenance control program
should effectively keep the problem in check. Continue a good general
broadleaf weed control program in crop. Russian thistle suppression
may be all that is required. Postharvest spot treat small Russian thistle
areas in draws and along field borders, and continue to monitor adjacent
fields and non-cropped areas to reduce the potential of reinfestations.
Copies and Information
- Additional copies
of this PNW Conservation Tillage Handbook Series publication
are available through county extension and conservation district offices
in applicable dryland areas of the Pacific Northwest. Another recent
publication on weed management in the crop-fallow rotation available
at these county offices is "Managing Downy Brome under Conservation
Tillage Systems in the Inland Northwest Crop-Fallow Region," Handbook
Series 15 in Chapter 5, published in June 1994. Copies of these
and other Handbook Series can also be requested from Roger Veseth,
WSU/UI Extension Conservation Tillage Specialist at (208) 885-6386,
or Don Wysocki, OSU Extension Soil Scientist at 503-278-4186. The entire
Handbook can be purchased through county extension offices for
$20, including postage, handling and new Handbook Series publications.
control recommendations, refer to product labels and the Pacific Northwest
Weed Control Handbook, an annually revised extension publication available
from the extension offices of the University of Idaho, Oregon State
University and Washington State University. To simplify information,
trade names have been used. Neither endorsement of named products
is intended, nor criticism implied of similar products not mentioned.
Conservation Tillage Handbook Series publications are jointly produced
by University of Idaho Cooperative Extension System, Oregon State
University Extension Service and Washington State University Cooperative
Extension. Similar crops, climate, and topography create a natural
geographic unit that crosses state lines in this region. Joint writing,
editing, and production prevent duplication of effort, broaden the
availability of faculty, and substantially reduce costs for the participating
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