Invasive Plants and Noxious Weeds:
Identification and Management
Sasha Shaw
King County Noxious Weed Control Program
www.kingcounty.gov/weeds
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Agenda
• Brief review of definitions, lists and impacts
• Biology and control of common invasive plants
and noxious weeds
• Early detection training for less common
noxious weeds
• IPM for managing noxious weeds in natural
areas
– Methods for staff and volunteers
• Field trip to Kiwanis Ravine (two stops)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
What is an Invasive Plant?
• Successful invader
– Introduced from
elsewhere, non-native
– Escapes into natural
areas
– Persists and spreads
– Generally lacks predators
and natural controls
• Causes harm
– Disrupts ecosystems
– Out-grows, out-spreads
and out-competes native
plants
Invasive plants such as English ivy
displace native plants and wildlife and
can transform entire ecosystems
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
How bad is the problem in our cities?



Seattle has ~8,000
acres of public land
Invasive non-native
plant species are
present in 94% of these
urban natural areas
20% of the city’s
forested areas are
highly invaded by a
suite of invasive
species
Percent cover of invasive species present in Seattle's
urban forests
Trace (0-10%)
Very High (>100%)
11%
20%
High (81-100%)
Low (11-30%)
9%
25%
18%
17%
Moderately High
(51-80%)
Medium (31-50%)
In 47% of Seattle’s forests, the
majority of the plant cover consists of
invasive species
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Sources of Introduction
• Intentional (accidentally on
purpose)
– Ornamental planting
– Crops
– Erosion control
• Accidental
Ornamental
escapee
Ship ballast and
ornamental escapee
– Ballast material
– Contaminant
• Seeds
• Hay
• Live plants
Erosion control
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Seed and hay
contaminant
Dispersal Methods
• Natural Processes
– Wind
– Water/flooding
– Birds and other animals
• Human Causes
– Hitchhiking on vehicles,
boots, boats, etc
– Construction/Grading
– Mowing/Equipment
– Yard waste dumping
– Being planted
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
What is a Noxious Weed?
• Legal term, defined and regulated by Washington
State law (RCW 17.10)
• Non-native plant that impacts agriculture,
wildlife, human health, land values or natural
resources
• Private and public property owners are required
to control selected weeds
– Goal of law is to prevent spread of new invaders to uninfested areas
– Weeds are regulated only where they are not
widespread
– Regulated at the county level; lists and priorities vary
by county
• Not all invasive weeds are on the noxious weed
list and not all noxious weeds impact natural
areas
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Invasive
Plants
Ecologically-based concept
Noxious
Weeds
Legal, policy-based concept
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
One Law, Two Lists
(both covered under RCW 17.10)
• Noxious Weed List (WAC 16-750)
– Dictates which weeds need to be controlled statewide or in
particular counties or regions
• Weeds are regulated where they are beginning to invade but not yet
widespread or according to county priorities
– Updated annually by the State Weed Board, administered by county
noxious weed boards
– Each county has its own weed list set by the county weed board
• County lists can include more but not fewer noxious weeds than required
by the state
• State Prohibited Plants List (WAC 16-752)
– Determines what can’t be sold (quarantine list)
– Administered by WSDA Nursery Inspection Program
– Goal is to prevent introduction of new weeds
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
What are the Categories on the King
County Weed List?
• Class A Weeds – new invaders, control required statewide,
still a chance to eradicate
– Examples: garlic mustard, giant hogweed
• Class B and C Regulated Weeds – control required in King
County, still have a chance to stop them from getting
established
– Examples: garden loosestrife, policeman’s helmet
• Non-Regulated Class B and C Noxious Weeds and Weeds of
Concern – widespread noxious and invasive weeds in King
County, control not required but definitely a good idea
whenever possible!
– Examples: English ivy, holly, butterfly bush, Scotch broom, etc.
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Most Abundant Regulated Noxious Weeds in King
County
Giant
Hogweed
Garlic
Mustard
Tansy
Ragwort
Purple
Loosestrife
Orange
Hawkweed
Policeman’s
Helmet
Garden
Loosestrife
Sulfur
Cinquefoil
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Spotted
Knapweed
Dalmatian
Toadflax
Examples of Non-Regulated Weeds
www.mobot.org
English Holly
Butterfly Bush
Canada Thistle
Yellow Archangel
Bittersweet Nightshade
Blackberry
Scotch Broom
English Ivy
Poison-hemlock
Knotweed
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Yellow Flag Iris
Bull Thistle
Some Local Invaders and How to
Control Them
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Himalayan Blackberry (Rubus armeniacus)
Evergreen Blackberry (Rubus laciniatus)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Blackberry Control
• Base control plan on ability to
maintain
– Least infested areas first, then more
heavily infested areas
– Preserve native vegetation
• Clear mature blackberry with
loppers, brush mowers, machetes,
or goats before seed set
• Dig up root crowns or spot spray resprouting canes at about 2 ft tall in
late summer or fall (foliar spray or
wiped onto fresh cut stems)
• Or cut several times a year for
several years to exhaust the roots
• Mulch and re-vegetate as needed to
reduce soil erosion and weed
seedlings
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Chemical Control of Blackberry
•
Triclopyr amine (2-3% solution) apply anytime during active
growth period – spring to fall
–
•
Or triclopyr ester (4% solution) is effective in the winter,
when many native plants are dormant
(leave 60 foot buffer from water)
–
•
e.g. Garlon 4, Crossbow
Or glyphosate (2-3%) fall application
–
•
e.g. Garlon 3A, Brush B Gon, Lilly/Miller Blackberry and Brush Killer
e.g. Roundup, Aquamaster
Cut stem – apply any of above per label to fresh cut stems,
need to apply to both sides if rooted at nodes
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
English Holly (Ilex aquifolium)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
English Holly Control
• Prevent further spread by
removing berries or berryproducing trees
• Re-sprouts from cut stumps
and root crowns so digging
is best non-chemical
method
• Cut stump or girdling with
herbicide (glyphosate or
triclopyr) is more effective
than foliar spray
http://web.reed.edu/canyon/natu/invasives
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Scotch Broom (Cytisus scoparius)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Scotch Broom Control
• Plants under 3 feet can be hand
pulled when soil is moist
• Remove larger plants with weed
wrenches
• Can cut older plants near ground
level in the dry season (JulySeptember)
• Spray plants spring to early
summer (before leaves fall)
• For less dense sites, use selective
treatment – cut stump (late
spring), stem injection, basal/stem
spray (in fall)
• Monitor site for at least ten years
for new seedlings
Broom removal with weed wrenches
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
English Ivy (Hedera helix, H. hibernica)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Ivy Control: “Lifesaver Method”
Kill upper growth by cutting the vines all around the tree at
about shoulder height and clearing the trunk. Pull up ivy
about 6 ft from base of tree and maintain ivy-free area.
Use a hand saw or clippers to cut
vines and then pull vines off trunk
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Ivy Control: Hand Pulling/Digging
Hand-pulling ivy is hard work but very
effective – and gets done quickly with
large groups of young volunteers!
Pulled ivy can be rolled up like
a carpet – easiest to do from
the top of the hill down!
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
English Ivy Control with Chemicals
• Waxy coating on leaves reduces herbicide uptake, especially during the
summer and fall growing season
• Young ivy leaves absorb more herbicide than older leaves
– In the early spring or regrowth from being mowed
• Applying pelargonic acid before or with herbicide may increase absorption
• Or can create “wounds” with a weed trimmer before spraying
• Either glyphosate or triclopyr (2-5% solution) sprayed on a sunny winter or
early spring day is more effective than summer or fall
• Can also cut stems and apply 25% solution glyphosate to freshly cut
surfaces
• Ivy may respond slowly to herbicide, so wait to follow up for at least a few
months
• Can combine methods: pulling where roots are accessible and chemical
control for rocky areas or steep slopes
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Old Man’s Beard
(Clematis vitalba)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Old Man’s Beard Control
• Combination of cutting and
chemical treatment
• Cut climbing vines in the winter at
waist height (leave the top stems to
wither)
• Spray the foliage in the spring to
summer with triclopyr or
glyphosate, avoid natives
• Mature plants – use cut stump
method
• Stem and root fragments should be
collected and burned or disposed
of; do not compost
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Bittersweet Nightshade (Solanum dulcamara)
This aggressive woody vine can form dense patches
and blanket trees and shrubs along creeks
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Bittersweet Nightshade
Control
• Manual removal of roots can be effective
 Hand pull the stem close to the ground and pull or dig up the roots,
taking care not to break the slender roots
 This method is most effective with young plants and new infestations
– Be thorough because stem and root fragments can re-sprout
– Along streams, need to prevent erosion, contain sediment
• Cutting may improve access to roots but won’t control plants
• Chemical treatment: use care to avoid drift, spot spray or
wipe on herbicides
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Invasive Knotweed (Polygonum spp.)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Knotweed has hollow, upright, bamboo like stems, often reddish
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Knotweed Control – Non-Chemical
• Dig up individual plants growing in soft
soil
– Dispose of roots and rhizomes in the garbage
Seattle Public Utilities
• Cut down twice a month between April
and August, once a month after that until
the first frost
– Don’t let stems exceed 6 inches
– Cut stems can be dried, crushed and
composted
• Cover with heavy-duty erosion control
fabric or sturdy plastic
– Weigh down with rocks or cement blocks (no
stakes)
– Monitor and maintain (stomp down regrowth, pull or spray re-sprouts along edges,
fix holes)
– Leave in place for five years
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Loose fabric allows
growth without
breaking through;
rocks hold it down
without creating
holes.
Chemical Control – Spraying or Injecting
• Spray the leaves and stems
– Imazapyr (most effective) or glyphosate
(next best)
– Usually mid-June to September
– Continue to monitor sites for at least
three years after knotweed appears to
be gone
• Stem injection with concentrated
glyphosate
–
–
–
–
Spraying knotweed re-growth
Need to inject every stem
July to September
Highly effective and reduces drift
Some limitations
• Labor-intensive and uses more herbicide
• Limited to about 2500 canes per acre per
year per glyphosate label
• Some stems will be too small to inject
• Will need to spray re-growth the next year
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Injecting knotweed
Yellow Archangel
Lamiastrum galeobdolon (a.k.a. Lamium)
Small yellow mint-type
flowers in leaf axils
Silvery markings on leaves of this popular garden
plant make it easy to spot invading into shady forests
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Yellow Archangel Infestation
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Yellow Archangel Control – Persistence Required!
• Hand pulling – collect all roots
and stems or they will regrow
• Covering – needs to be very
well-covered, no holes and
extending beyond plants
• Spraying – combinations and
repeat treatment
– Glyphosate (Roundup) mixed
with either triclopyr (Garlon),
imazapyr (Habitat) or
aminopyralid (Milestone)
– Add surfactant for best results
– Concentrated vinegar products –
short term control is good, need
to re-apply regularly
Regrowth
after
spraying
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Seedlings emerging
in treated area
Herb Robert (a.k.a. Stinky Bob)
(Geranium robertianum)
Weak roots make this plant easy to pull but plants seed prolifically and
germinates multiple times in a season so repeat visits to the same location are
needed for complete control.
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Some Threatening Noxious
Weeds
• Prevention of spread in the county and state is
highest priority
• Call your County Noxious Weed Program if you find
these or any other regulated noxious weeds
– King County 206-296-0290
– Pierce County 253-798-7263
– Snohomish County 360-435-7830
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Garlic Mustard
(Alliaria petiolata)
Infestation along Longfellow Creek
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Class A Noxious Weed
Class A Noxious Weed
Garlic Mustard
(Alliaria petiolata)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Class A Noxious Weed
Garlic Mustard
In undisturbed forests, garlic
mustard spreads up to 120 feet in
one year. It inhibits tree growth
through negative impacts on
beneficial fungi and has no natural
enemies in North America.
Garlic mustard covering a hillside
near Carkeek Park in Seattle.
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Garlic Mustard in King County in 2000
2000
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Garlic Mustard in King County 2011
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Lots of Look-A-Likes
Garlic Mustard
Nipplewort
(Lapsana communis)
Nettles
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
White-Flowered
Money Plant
Prevention of Spread

Don’t take it with you
Clean your boots and equipment
Don’t move mulch from infested sites
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Giant Hogweed
(Heracleum mantegazzianum)
15 feet tall with a stout,
purple-blotched stem, large
white umbrella-shaped
flower clusters, and giant,
sharply toothed leaves
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Class A
Noxious Weed
ashtabula.osu.edu
Class A Noxious Weed
Giant Hogweed – Use Caution When Handling
Sap from
hogweed
causes painful
burns
Wear gloves and long sleeves when
cutting off flower heads, then dig up roots
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Giant Hogweed (Heracleum mantegazzianum)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Purple Loosestrife
(Lythrum salicaria)
Garden Loosestrife
(Lysimachia vulgaris)
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Class B Noxious Weeds
Class B Noxious Weed
Policeman’s Helmet (Impatiens glandulifera)
Annual with fleshy, reddish stems, 3-10 ft tall, flowers resemble English policeman’s
helmet, vary in color from white to dark pink-purple
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Class B Noxious Weed
Policeman’s Helmet (Impatiens glandulifera)
Policeman’s helmet spreads along creeks
and out-competes and crowds out other
plants
Can grow to 10 feet tall in one season
Courtesy of Sasha Shaw, King County Noxious
Weed Control Program
Managing Invasive Weeds in
Urban Forests
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
FOOD FOR THOUGHT
Some people say that invasive plants are just taking
advantage of disturbed or poor conditions and aren’t
actually causing any problems.
Is this true and should this impact how we do urban
restoration?
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Ecology-Based Weed Management
• Old approach – see the weed, kill the weed
• New approach – focus on prevention and establishing invasionresistant plant communities
– minimize disturbance like tillage, trampling
– use competition from established vegetation to help hold off weeds
– different strategies work at different stages of weed invasion
Weed Population Growth
Restore
Control
Prevent
Time
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
First Step: Decide on Goals
New and small infestations
•Prevention of spread is first priority
•Eradication is still feasible
Established, large infestations
•Decide on level of control needed to
reduce impact
Remove entire plant for
small infestations or high
priority weeds like garlic
mustard
Consider regular cutting or covering to
contain large infestations if resources are
limited to allow natives to get established
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Now, Consider Your Options
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Manual Control
•Pull seedlings and young plants when
small, before root systems fully develop
•Remove as much of the root as possible
•Limit disturbance and be sure to follow-up
Digging giant
hogweed
Pulling gorse with a
weed wrench
Pulling and bagging garlic mustard
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Mechanical Control
• Useful to reduce size, seed production and to starve roots
• Weeds will resprout after mowing and can be spread on
equipment or by breaking up roots and rhizomes
• Best results when combined with manual control
Himalayan Blackberry can be
mowed to remove brambles
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Reduces seeding, but be careful
not to move weeds on equipment
A Few Weeds Spread by Mowing
Garlic
Mustard
Knotweed
Tansy
Ragwort
Knapweed
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Chemical Control
Safety:
•Use with caution, protect skin and eyes
•Always follow the directions on the label
•Prevent drift (in water, other plants, etc.)
Applications:
•Spot treatments, target the weed and
avoid injury to desirable plants
•Incorporate other treatment methods
•Choose the least harmful herbicide that
is appropriate for the weed and the site
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Cultural Control
• Changes to environment to
suppress weeds
• Shade/Cover
– Thick mulch
– Cardboard plus woodchips
– Weed fabric
• Competition
– Sterile grass
– Dense plantings of quick
growing shrubs and trees
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Finally, Follow up and Dispose
Follow Up:
• Check site for several years
• Mulch to suppress weeds & improve soil
• Fill in holes with low-maintenance natives
Disposal:
• Best: control before flowering!
• Noxious weeds: discard in garbage
• Widespread weeds: city yard waste or leave on site
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Making the Best Use of Volunteers
• Large work parties
– Require oversight but can cover large areas quickly
– Best for dense patches of one or two target species (e.g. ivy
and blackberry) in easy terrain
– Usually not as good at distinguishing weeds from natives
– Not so good for weeds requiring herbicide or careful
weeding (e.g yellow archangel or knotweed)
• Dedicated forest stewards or “friends of” groups
– Better trained to detect and remove scattered infestations;
don’t waste them on large infestations
– Monitoring and mop up after large work parties
– Early detection and removal of new invaders (like garlic
mustard)
– Marking invasive trees or weeds needing chemical control
for removal by city crews
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Making the Best Use of Staff
• Earthcorps, WCC, VCC, WA State Corrections, etc.
– Best for large, dense infestations and/or challenging terrain
(steep slopes, dense blackberry)
– Better for targeting a limited number of species since ID skills
can vary
– Clearing or grubbing out large areas and installing erosion fabric
or plants
– Can do some herbicide applications if trained (e.g. knotweed
injection, cut and wipe)
• City staff
– Coordination with stewards and work party supervisions
– Best for ongoing maintenance and mop up after large work
crews
– Can be trained for early detection and control of a wide range of
invasive species
– Most herbicide applications and power tools
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Case Study: Happy Creek Park
1. Read the case study
2. Review the questions
3. Discuss as a group
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
The Problem: Happy Creek Park
• The Mayor has asked the Parks Department to restore Happy Creek Park, a 40
acre weed-infested natural area that has been left to itself since it was
purchased for open space 50 years ago. The park has a pretty healthy stand of
large Douglas firs and cedars and some native plants like sword fern and salal
in the understory, mixed in with lots of herb Robert, English ivy, scattered
English holly saplings, a few large English holly trees, and a few small patches
of garlic mustard and yellow archangel. The maples and alders along the creek
and several of the firs are covered with English ivy and Old Man’s Beard. The
streambanks are draped with dense vines of bittersweet nightshade. In
addition, one side of the upper 500 feet of creek is bordered with invasive
knotweed and lower down there are patches of policeman’s helmet and purple
loosestrife starting to spread from nearby private properties. And, of course,
Himalayan blackberry is growing throughout the park wherever it can get
enough sunlight.
• The Mayor wants the park to be weed-free in two years, but wants to keep the
cost down by relying mostly on volunteers with small amounts of paid crew
time.
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Questions for Discussion
1. What control method and timing will work best on each weed species?
2. Where would you use volunteers or staff? Large work parties vs. forest
stewards vs. park staff?
3. Which weeds are going to need repeat work in the same year? The following
year?
4. Where are you likely to need to plant, mulch, control erosion? Where can you
leave it to nature?
5. Are there any weeds you are unlikely to eradicate in 2 years? What should you
tell the Mayor about what is realistic given the resources and timeline?
6. Do you think herbicides are needed to accomplish the goals of this project? If
herbicides were not allowed, what, if anything, should change in terms of
resources and/or expectations?
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
For More Info on Noxious Weeds in King
County:
http://www.kingcounty.gov/weeds
Weed Photo Page:
Search by Weed Name
Click thumbnail picture to get
more information and photos
Courtesy of Sasha Shaw, King County Noxious Weed Control Program
Sasha Shaw
King County Noxious Weed Program
206-296-0290
sasha.shaw@kingcounty.gov
http://www.kingcounty.gov/weeds
Courtesy of Sasha Shaw, King County Noxious Weed Control Program