Learn about kinds of weeds, their characteristics, what happens when they become invasive, and techniques for weed control

1. WEEDS
Linda R McMahan
Faculty for Community Horticulture
Oregon State University Extension
Service
Yamhill County
linda.mcmahan@oregonstate.edu
Photos by the author
Identification and Control for Master
Gardeners

2. Weeds
 Characteristics
 Identification
 Some Examples
 When just plain weeds become Invasive
 Control Strategies

3. So you have a weed
What next. . . . In this case, our weed is really quite lovely, so what then?
This particular ―weed‖ is Jimson weed, Datura stramonium, a toxic plant
that is considered to be an agricultural pest. However, some people grow
it for its ornamental value.

4. This one looks so pretty and it is growing in a
wetland along a nature trail in Beaverton, Oregon
Is it a weed or native plant? This is a good question to ask. The plant is purple
loosestrife, Lythrum salicaria, which is considered to be highly invasive
throughout most of the United States.

5. Removing some weeds can be very hard
work . .
English ivy for example. How do we go about doing such a task? Can the
removed ivy be composted? What are the best methods of removal and
control?

6. First Let’s Look at Weed
Characteristics
A Weed is  Colonizers
sometimes
defined as ―a  Reproduce successfully— some
plant out of
place,‖ but
have numerous seeds, sometimes
perhaps that genetically identical, for rapid
is too simple
establishment
 Others quickly reproduce by
vegetative reproduction
 Have become successful
hitchhikers in bird seed, garden
seeds, plant pots, and on anything
that moves including

7. First Let’s Look at Weed
Characteristics
Weeds can  May engage in chemical warfare by
indeed be
―crafty‖ and suppressing the growth of
it is surrounding plants
sometimes
quite difficult  Many survive in nutrient poor soil—
to ―outsmart‖
them some are even nitrogen fixers to
gain a competitive advantage in
poor soils
 On the following few slides, I have
referred to helpful web resources on
the left panel of each page

8. http://www.nps.gov/plants/ALIEN/fact/alpe
mustard, visit this site maintained by the
For more information on garlic
National Park Service
1.htm
Now here’s a nasty weed known as garlic
Some mustard, Alliaria petiolata. It is introduced from Asia
Examples
and Europe.
This weed is spreading rapidly throughout Oregon. It is a biennial and
difficult to hand-pull. It helps maintain its weed status through chemical
warfare by suppressing growth of other plants or interfering with their
ability to form beneficial relationships with soil fungi.

9. For more photos and information about this
http://www.missouriplants.com/Whitealt/Ca
tiny weed, visit an information site at The
rdamine_hirsuta_page.html
Missouri Botanical Garden
Some This plant, Cardamine hirsuta, is sometimes called snapweed or
Examples shotweed for its habit of explosive release of its seeds when touched.
As gardeners, you are perhaps more familiar with this plant. It
is a winter annual, sometimes blooming and producing seeds
before we emerge in late winter to check on our gardens.

10. http://plants.usda.gov/java/profile?symbol=
profiles many plants, both native and not-
To find out more, to to a USDA site that
PLMA2
native
Some
Examples
Common plantain, Plantago major
Here’s another familiar lawn weed. Each flower stalk reproduces over a
long period of time, producing hundreds of seeds.

11. Visit this helpful website at The University
http://www.ipm.ucdavis.edu/PMG/PESTN
of California, Davis to learn more about
annual bluegrass characteristics and
OTES/pn7464.html
control
Some Yet another familiar weed is annual
Examples bluegrass, Poa annua, a common lawn pest.
Annual bluegrass is extremely difficult to control because it seems to
thrive with our common garden practices. Even though these plants have
been sprayed with herbicide, they do not seem to be responding and even
these small plants have already produced seeds.

12. Weed Categories
Different  Winter Annuals – some of our most
groups
require challenging garden pests
different
control
 Summer Annuals
methods
 Biennial weeds
 Perennials
 Some have deep and persistent
taproots
 The ―nastiest‖ create underground
runners or storage systems
 Shrubs, Vines and Trees

13. Another Distinction
These two  Some weeds are
major
groups of Monocotyledons, including most
flowering notably weedy grasses and sedges
plants can
require  Other weeds, often referred to as
different
kinds of broadleaf, are Dicotyledons
control
 If you are controlling broadleaf
weeds such as dandelions in lawns
using herbicides, be careful to
choose appropriate products so as
not to kill the grass as well

14. Native Plants as Weeds?
 You may be confused to find native
plants listed as weeds in some of the
resources
 Examples are yarrow (shown left), wild
cucumber, poison oak, native
irises, and horsetail
 This is because some of these plants
are considered to be agricultural
pests, garden pests, or cause human
or animal health issues-so control
methods are available and may be
appropriate in some cases

15. Weed Examples and Controls
 The following slides show examples of
various categories of weeds
 In each case, control methods are
mentioned
 Please note that we will cover controls
more thoroughly near the end of this
presentation

16. Winter Annuals
Because  Typically germinate in the fall and
they bloom
and produce flower in the winter or early spring
seed in the
winter, these
 Examples include snapweed, some
weeds tend mustards, chickweed, and some
to catch us
by surprise if weedy geraniums
we are not
watching  They spread through seed
production

17. Winter Herb Robert, Geranium robertianum, sometimes called ―stinky
Annuals bob‖ is a newly listed invasive species in Oregon.
The best control for this small winter annual is persistent hand weeding. It
has a relatively short seed life of approximately 3 years so persistent
control can effectively reduce and even eliminate the population. You
have to be fast, however, this one can bloom in mid-winter! Pre-emergent
treatments can also be effective.

18. Summer Annuals
At  Seeds of summer annuals
least, summ
er annuals germinate in the spring, then bloom
are more and set seed before fall frosts
visible, but
that does not  Examples include
mean they
are always lambsquarters, pigweed, mallows, r
easy to agweed, and spurge
control

19. Summer
Annuals
Common mallow, Hibiscus trionum
This weed often appears as a contaminant in bird seed. It has small but
attractive flowers and a distinctive post-flower appearance shown here. it is a
summer annual and can be controlled through hand weeding.

20. Biennials
Since they  Biennials typically have a 2-year life
form
rosettes the cycle. The first year, the seed
first germinates and the plants produce
year, they
may escape a rosette, a round ―circle‖ of leaves
our attention
until the
that remains flat to the ground. The
second year following year, the stem ―bolts‖ to
flowering
stem produce flowers and seeds
suddenly
bolts  Examples include bull
thistle, foxglove, and common
mullein

21. Biennial Bull thistle, Cirsium vulgare, is a biennial forming a prickly
Weed rosette the first year, than a tall blooming stalk
One control for bull thistle is removal of all flowers before they set seed.
Hoeing while plants are small is also effective. Seed are prolific.
Repeated tilling or mowing and control with contact herbicides can also be
effective.

22. Biennial
Weed
Mullein, Verbascum thapsus
This biennial produces hairy felt-like leaves in a rosette (shown above) the first
year. The second year, it sends up a tall stalk of attractive yellow flowers. A
single plant can produce more than 100,000 seeds. Control is through
competition with other plants (it likes bare ground), hand removal when the soil
is loose, and deadheading to remove flowers and seeds before dispersal.

23. Biennial
Weed
Teasel, Dipsacus fullonum
This common roadside weed seems to be increasing in Oregon.
Recommendations for control include deadheading flowers or using
herbicide at the rosette stage because some are not effective when the
plant begins to bolt. Some say the ―hooks‖ on the seed stage of the
flowering head may have been the inspiration for Velcro.

24. Biennial Wild carrot or Queen Anne’s lace, Daucus
Weed carota
Another ―pretty weed‖. This one has a tap root—it is a wild carrot after all.
Its habit is usually biennial, but plants sometimes persist for additional
years. This common roadside weed is resistant to many herbicides.
When young, they often can be hand-pulled from soft or moist soil.

25. Perennial Weeds
 Perennials are herbaceous plants
that die back and regrow from
underground roots or stems each
year
 Dandelions are simple perennials
spreading by seed
 Canada thistle, quackgrass, field
bindweed, yellow nutsedge and
others create underground
networks as part of their
reproductive strategy

26. Simple Ahh. Our old favorite the dandelion, Taraxacum
Perennial officianale
Each plant can produce hundreds of seeds which take off on the wind to
infest the neighbor’s lawn as well as yours. Can be controlled through
persistent hand-weeding with a weeding tool or selective broadleaf
herbicide applications. Young leaves of dandelion are often available for
use as salad greens.

27. Spreading Canada thistle, Cirsium arvense, is difficult to
Perennial control –it is a perennial and spreads underground
Unlike the biennial bull thistle, even persistent hand weeding or
deadheading makes control difficult because underground pieces readily
regenerate into new plants. Can sometimes be effectively controlled with
weed barriers and with repeated 2-4 D or other contact herbicides
beginning in September.

28. Spreading Wild morning glory aka field bindweed, Convolvulus
Perennial arvensis, is another perennial weed requiring persistence
The PNW Weed Handbook recommends 2,4 D or glyphosate applied in
the fall while plants are still actively growing. Most of the other chemicals
listed are not available to homeowners. Persistent clipping and covering
with landscape cloth are sometimes effective controls in smaller
landscapes.

29. Weedy Woody Plants
 Many woody plants can become
pests, even invasive ones
 Examples of invasive woody plants are
Himalayan blackberry, honey
locust, Norway maple, English ivy, and
Scotch broom—all are problems in Oregon
 Less invasive but common ones include the
nightshades and poison oak

30. http://cru.cahe.wsu.edu/CEPublications/PN
See the following publication for more
information about nightshades in the
W0588/PNW0588.pdf
Pacific Northwest
Nightshades, Solanum nigrum and others
These vine-like woody plants with poisonous black or red berries are of
concern both as pests and for their toxicity. Seeds readily germinate
throughout the year and they are tolerant to many herbicides, making control
more difficult. Manual removal is often effective. If you choose herbicides, use
during strong summer growth or early fall when plant resources are being sent

31. Scotch broom, Cytisus scoparius
Scot’s broom is a shrub first introduced into Astoria, Oregon as an
ornamental. Now it ―ornaments‖ our landscape, displacing native species
and causing allergic reactions for many allergy sufferers. Biological
controls have been introduced and are sometimes available. Other
methods of control include pulling with a ―weed wrench,‖ burning, and

32. The issue of weed control is
tough!
Let’s  Identification is important
begin to  ID helps us learn if the plant is a
find weed or not
some  ID will tell us how important the
answer weed is to control
s
 ID will give us the information we
need to recommend appropriate
weed control

33. Weeds of the West
Publishe  A picture-based
d by OSU book, with
Press entries grouped
and by plant
available
in the families
master  Most of the
gardener common weeds
library
of Oregon are
in this book
 This is your first
place to look!

34. Weeds of the Northern US and
Canada
Publishe  Includes many
d by weeds and
Lone relatives
Pine
Press –  Also arranged
also by family
available  Includes many
in the
master
weeds of
gardener wetlands and
library waterways

35. Plant Identification Tools
To find out  http://weeds.ippc.orst.edu/pnw/wee
how it
works, go to ds?weeds/id/index.html
the site and
look at the
 This is the picture part of the Pacific
picture of Northwest Weed Handbook
bull thistle.
Make sure  Unfortunately, it does not include a
you look at
both the key, but it can be used to help verify
mature plant names and identities you find
and seedling
to get idea of elsewhere
this resource
 This source includes photographs
of seedlings which can be very
useful

36. Oregon Small Farms Website
 The OSU Small Farms website offers another resource
to learn about weeds
 It includes information on pasture weeds, management
practices, weed toxicity, and guides to controlling some
of the weeds (see field bindweed, for example)
 Remember that toxicity questions need the involvement
of an OSU faculty member
 Pasture and commercial questions also need to involve
an OSU faculty member
http://smallfarms.oregonstate.edu/weedsplants%2526pest
s

37. Some examples of common
weeds and recommended
treatments
 Once you have an identification, solutions for control
may be found in the PNW Weed Control
Handbook, which is also available online
 This resource was developed primarily for commerical
users and has limited information in available on weed
control for home gardeners
 You may need to use approved web resources to find
solutions
The online version of the PNW Handbook
is at
http://weeds.ippc.orst.edu/pnw/weeds

38. When do ―Just Plain Weeds‖
Become Invasive?
 Some weeds are so aggressive that
they have the ability to take over
natural ecosystems, competing
successfully with native plants
 These have crossed the line into
―invasive plants‖ and are costing
billions of dollars annually to control
 Examples already considered are herb
Robert (shown left), purple
loosestrife, English ivy (shown
left), and garlic mustard
 More are shown in the next few
slides, many were introduced
intentionally to the US as herbs or

39. Shining geranium, Geranium lucidum, is a newly listed noxious weed in
Oregon. Usually grows as a winter annual.
This species is spreading rapidly throughout the state. Oregon Department of
Agriculture notes that no biological control methods are available
http://www.oregon.gov/ODA/PLANT/WEEDS/profile_shinygeranium.shtml The
Western Invasives Network recommends hand weeding, weeding with a flame
torch, or selected broadleaf herbicide treatments.
http://www.westerninvasivesnetwork.org/phpBB3/viewtopic.php?f=11&p=178

40. Himalayan or Armenian blackberry, Rubus
armeniacus
A vigorous grower introduced as a berry crop which subsequently
escaped to the wild. Control is usually mechanical (mowing, cutting) or
chemical (typical applications in the fall).

41. The evergreen blackberry, Rubus laciniatus
This introduced species is also considered to be invasive. Control
methods are generally the same as for Himalayan blackberry

42. Yellow flag iris, Iris pseudacorus
Despite the symbolism—this species was inspiration for the fleur-de-lis
symbol, yellow flag iris is invasive in many parts of the world. It grows in
streams and is spread through waterways. Control is difficult and
expensive.

43. Weed Control – checking the
arsenal
Fast action  Mechanical and Cultural–preferred
can save a methods
lot of effort. If  Deadheading or hand weeding
you can keep  Weed whackers, mowers, flame
a weed from torches
flowering or  Mulching or covering – most weeds
spreading, yo need sunlight to germinate
u have
eliminated  Biological control if available
untold hours  Tolerance of minor weeds
of future  Chemical control if it is effective –
control! the last choice – always read the
label

44. Become Familiar with the Home
Garden Section of the PNW Weed
Handbook
http://uspest.org/pnw/weeds
 Go to this site and look at the
guidelines
 Chemical tools are limited to only a
?30W_INTG01.dat
few – glyphosate, triclopyr, 2,4
D, and dichlobenil are the most
common
 Check the table on page 2 to see
which practices and chemicals are
effective on certain weeds and
which are not

45. Pre-emergent Herbicides
 Check out this site made available through Washington
State University Extension on the various kinds of
herbicides available to the homeowner
 If using materials from other states, the most beneficial
and relevant information can usually be found in
neighboring states like Washington, Idaho, and California
http://cru.cahe.wsu.edu/CEPublications/eb1214/eb1214
.html

46. Corn Gluten Meal to control
weeds?
 Some answers just seem too good to be true –
Corn Gluten, unfortunately, fits into this category
 Corn gluten meal appears to work in other parts
of the United States, but not in most of
Oregon—instead it may make your weeds grow
even more quickly. In our climates, it kind of
acts like a fertilizer.

47. Vinegar as Weed Control
 Acetic Acid was first introduced as an herbicide in
Oregon around 2002
 People began using it because ―concentrated
vinegar‖ seemed like a good idea – Vinegar is 5%
acetic acid
 The 7% solutions generally found in these products
are not concentrated enough to be effective
 More concentrated products are possibly hazardous
and may not be legal for use as herbicides in
Oregon
 See this factsheet for more information:
http://oregonstate.edu/dept/nursery-

48. Thank You for Viewing this
Presentation
 You may use this presentation and the
photographs freely for educational
purposes without express permission.
 If you would like to use photographs for
commercial purposes, please contact the
author at
linda.mcmahan@oregonstate.edu