Every year more weeds gain the status of "Noxious Weed". These five are our choice for being the
worst of the bunch, although all of the weeds have the potential
to seriously damage Idaho's economy, landscape and wildlife.
This perennial from Eurasia can grow four feet tall, with
a taproot twice that length! In the early summer it produces
yellow flowers. Mature plants can produce 1,500 flower
This plant is built to reproduce. One plant can produce
20,000 seeds, which have a parachute of fine hairs which
allow them to travel along wind currents up to twenty
miles. Its leaves are inconspicuous, which gives this
plant its skeleton-like appearance. This also gives it
a leg-up in drought conditions, as does its mammoth taproot.
This noxious weed was discovered in Idaho in the 1960’s
and has been spreading rapidly throughout Idaho. In places
around Garden Valley, Idaho, this weed has already taken
over the hillsides!
It displaces native species grazed by wildlife and livestock.
It spreads from rangeland to croplands by seed. Its incredibly
large root system makes it virtually impossible to pull
by hand. In fact, attempting to pull it or cut it actually
spreads the weed, since shoots can form from the lateral
roots and from root fragments buried four feet deep.
It is difficult to kill this deep-rooted, rhizomatous
perennial, since it seems to be tolerant to herbicides.
For example, field trials suggest that a late fall application
of Transline has a 95% control rate, although plants did
show up three to five years later!
Perhaps the best way to control rush skeletonweed is
with biological control agents, like the gall mite (Eriophyes
chondrillae), which affects the flowering buds, preventing
seed production. It also affects the roots. But the likelihood
of ever eradicating this noxious weed is a long ways off!
The best we can probably do is slow its progress.
What you can do:
Learn to identify rush skeletonweed . Prevent the plant
from going to seed by cutting or isolating plants before
they flower. If flowering has occurred, bag and remove
plants for burning. Use selective herbicides on already
established plants. Control small infestations immediately.
Clean equipment, tools vehicles and footwear before leaving
This plant is Idaho’s first aquatic weed. It was
discovered in Idaho in the late 1990’s, but has been spreading
rapidly since then. It lives in lakes, ponds, and slow
moving rivers. It is a slender, tan-stemmed plant with
finely dissected feather-like leaves, arranged in whorls
of four around the stem at each node. It doesn’t mind
cold water and can reproduce in many different ways. It
produces flowers and seeds that appear above the water,
and it can spread by roots or runners in the ground.
It forms very dense mats of vegetation on the surface
of the water. Needless to say, swimming, boating and water
skiing are a problem when this fast-growing weed takes
This aggressive weed creates a dense canopy that shades
out native plants. What is left is a poor substitute for
waterfowl, fish and other wildlife. When the plant’s
leaves decompose at the end of the growing season, they
increase phosphorus and nitrogen in the water. The plant
also raises pH of the water, decreasing oxygen and increasing
This extremely adaptable plant thrives in a variety of
conditions and can easily be spread through boating activity
and by anglers. It doesn’t take much to create a big,
expensive problem in a short amount of time! This weed
is already in McCall’s Payette Lake, as well as in some
of the northern Idaho lakes.
Some places have pulled or cut the weed with underwater
roto-tillers, but it almost always comes right back. Herbicides,
like fluridone, have been used successfully in some places,
although the results have been spotty in other places.
In reservoirs, draw downs have sometimes helped keep the
weed under control.
A natural predator, the milfoil weevil, has also been
effective in controlling the growth of this weed. The
weevils lay eggs in the stems of the milfoil; when they
hatch, the larvae eat the milfoil.
What you can do:
Learn to identify the plant. If you spot it, notify your
local weed manager. Remove all plant material from your
shoes, your boat, your trailer before you leave the boat
access area. Make sure your bait bucket doesn’t have plant
material in it; dump it on the land. Wash down your boat
when you get home, as well as your trailer, your tackle
and anything that could transport this plant to another
body of water.
This member of the sunflower family emigrated from Europe.
Its long taproot allows it to continue growing long after
other annual plants have died. It has stiff, upright stems
that branch from the base. The flower is yellow.
A horse that eats this plant develops “chewing disease,”
whose first signs are an inability to eat or drink. Permanent
brain damage results, and horses can die of thirst and
starvation. Strangely, cows and sheep are not affected,
and can eat the plant until the sharp spines on the flower
head make it inedible.
A single plant can produce thousands of seeds, which
can last ten years or more in the ground. They act like
dandelion seeds, floating in the weed. This weed will
take over land from Idaho fescue, blue bunch wheat grass,
and cheat grass, but not from sagebrush.
This noxious weed is a threat to all western rangeland,
because it can spread rapidly on disturbed soils. Its
root system allows it to grow faster and deeper than annual
grasses, so its infestations can smother a large area
and displace other vegetation.
This is not a weed that you can pull out, so don’t try!
Herbicides, like Tordon 22K, work, but are expensive.
An insect, called the Yellow star hairy weevil, is making
a sizable dent in the flowering heads of this weed. The
weevil comes from yellow star thistle’s home land, near
the Mediterranean, and after much research, have been
released in areas like Hells Canyon.
Goats have also been used successfully in Hells Canyon,
to keep the weed population under control. The goats actually
love to eat the seed heads off the thistle, and thus control
the weeds’ numbers.
What you can do:
Learn to identify the weed. Diligence can go a long way
here! If you see this weed, notify your local weed manager.
Once yellow starthistle colonizes an area, it is very
difficult and expensive to eradicate.
This deep-rooted, aggressive perennial has a lot of weed
managers concerned! It came to America more than 100 years
ago as an ornamental that some folks grew in their yards.
It can grow more than three feet tall; but its roots can
grow to more than twenty feet! The ability to maintain
high root reserves means that this plant can recover quickly
from chemical and physical damage.
The small flowers are green and inconspicuous, but are
surrounded by a pair of yellow-green, heart-shaped leaves
that are often mistaken for flowers. When dry, the seed
capsules shatter, ejecting seeds fifteen feet away from
the plant. The seeds can live up to eight years in the
Leafy spurge produces a milky latex that can cause blistering
and death to some animals. It can cause blindness in humans
if the milky substance gets in one’s eyes.
This perennial noxious weed readily adapts to most environments;
and has doubled in acreage every ten years since the early
1900’s. This plant is an economic and environmental catastrophe;
it reduces rangeland productivity and plant diversity
wherever it gets a foothold.
It reproduces from spreading roots and by the production
of enormous amounts of seeds, making it almost impossible
This is a difficult plant to control. Chemicals are only
partially successful, because of the plant’s extensive
root system. Goats and sheep have been used to “keep the
yellow out” and to retard its spread. Several flea beetles
are also being tested for leafy spurge control.
What you can do:
Notify your local weed manager immediately, if you identify
this weed. Once it’s been spotted in your area, persistence
is imperative in gaining control of this weed.
The pinkish-purple flower on this noxious weed is sometimes
confused with a beautiful wildflower, but don’t be deceived;
this plant can dominate an area if left to its own devices!
The weed is a prolific seed producer, which releases a
toxin that hinders growth of plants around it.
The plant grows two to four feet tall, with leaves that
are one to three inches long. The purplish flowers are
surrounded by stiff black-tipped bracts; thus the spotted
This Eurasian transplant tends to dominate an area, reducing
plant diversity. A knapweed infestation can also increase
surface run-off and sedimentation. This weed spreads readily
in hay and on undercarriages of cars.
Spotted knapweed reproduces solely by seed. The trouble
is, each plant can release a thousand seeds, and they
can live in the ground for seven years. This is one weed
that can be removed by digging or pulling, when the soil
is moist. Chemicals can also be effective, but the chemicals
needed to control this weed are quite potent. Some experimentation
has been done with root mining moths, a flower moth, and
a root mining beetle.
What you can do:
Notify your local weed manager. All visible spotted knapweed
plants should be removed and destroyed. Treating the area
with a herbicide to prevent reinfestation from seedlings