Invasive Plants

Narrow purple petaled flowers with even narrower green leaves surrounding them are clustered together.
Bulbous Bluegrass; an invasive plant at Grand Canyon.

NPS Photo

Invasive plants are often referred to as weeds, which might be slightly misleading. Many people think weeds are useless or toxic or ugly. While this can be true, many invasives plants are far from that. In fact, many invasives arrived on the continent due to their usefulness or beauty. Many are cultivated for food or other important uses, especially because they are tough and grow easily. The reason invasive plants at Grand Canyon are classified that way is not a judgement on the plant—it is simply because these plants came from different environments and threaten the natural habitat of native species that we aim to preserve for future generations at the park.

Common Invasive Plants at Grand Canyon

Clusters of small yellow flowers make up large bundles, and many of these are on green stalks on the plant.
Foeniculum vulgare – Apiaceae 

NPS Photo / Will Elder


Foeniculum vulgare – Apiaceae

  • Best known as a prominent spice in Mediterranean cuisine. While the wild varieties have not been specifically bred for flavor, the differences with cultivated fennel are very minor.
  • Originates from the Mediterranean region, and in the US often thrives in areas with a “Mediterranean climate” such as parts of California.
  • Large forb/herb—can grow up to 10 feet tall.
  • Umbrella shaped clusters of yellow flowers.
  • Feathery, needle-shaped, aromatic green-yellow leaves
  • Flowers between mid and late summer
  • Highly competitive and difficult to control, reproducing from both root and seed.
Small white clustered 5 petaled flowers are on the end of bright green stalks with scattered pointed leaves behind.
Poison hemlock
Conium maculatum – Apiaceae 

USDA NRCS Photo / Doug Goldman

Poison hemlock

Conium maculatum – Apiaceae

  • Biennial (2-year lifespan) native to Europe. Originally brought to the US as a decorative plant due to its attractive flowers.
  • It is a well-known poison, used historically as a method of death penalty—including, famously, for the execution of Socrates.
  • Member of the Apiaceae, or carrot, family and bears a strong resemblance to other members of the family, especially Queen Anne’s Lace.
  • Large forb/herb—can grow up to 10 feet tall. Strong, musty odor.
  • Umbrella shaped clusters of white flowers, purple spotted stems, and shiny, green leaves, which are tripinnately divided—meaning they are divided 3 times as many fern leaves are.
  • All parts of the plant are highly toxic to humans and livestock.
Sagging red flowers stand at the top of a bright green plant with many large oval shaped leaves pointing upward.
Cynoglossum officinale – Boraginaceae 

NPS Photo / Jacob W. Frank


Cynoglossum officinale – Boraginaceae

  • Cynoglossum officinale first appeared in the US in 1893 in Oregon and was likely a product of cereal seed contamination.

  • Medium-sized forb; can grow between 1 and 4 feet tall.

  • Flowers ranging from red to purple

  • Leaves said to resemble a “hounds’ tongue”.

  • Each flower can produce 4 fruits

  • Can grow in dense clusters of up to 400 seedlings per square foot.

  • Shown to be toxic to livestock.

Oval shaped white bulbs burst into light purple flowers, washed out by the sun.
Russian Knapweed
Acroptilon repens – Asteraceae 

NPS Photo

Russian Knapweed

Acroptilon repens – Asteraceae

  • Perennial (plant that lives more than 2 years) in the Aster family
  • Native to Eurasia but has been documented in the US since 1898.
  • Medium-sized forb; can grow up to 3 feet in height
  • Small flowers ranging from pink to lavender
  • Grows in thick stands through expansive root system up to 14 yards across and 23 feet deep.
  • Stands can have up to 27 root-borne shoots per square foot.
  • A single plant can also produce up to 1200 seeds per year
Eight small black berries attached to vines come off of a branch that has large green leaves with saw tooth edges.
Himalayan blackberry
Rubus discolor – Rosaceae 


Himalayan blackberry

Rubus discolor – Rosaceae

  • Perennial (lifespan over 2 years) shrub native to Armenia and thus is also referred to as Rubus armeniacus
  • Introduced to the US as a cultivated crop by Luther Burbank, who also developed the precursor to the Russet potato
  • Escaped its cultivation and became widely invasive but is still intentionally cultivated throughout the US
  • Woody shrub up to 12 meters across and 3 feet tall; can reproduce asexually by seed or vegetatively (regrowth) by either root or stem sucker
  • Dark green, serrated leaves (leaves with saw-like edges) -- generally in clusters of 5 or 3
  • Distinguished from other blackberries by whitish coloring on the back of its leaves
  • Flowers range from white to rosy pink
Green stalk has long green stems projecting in all directions.
Bulbous bluegrass
Poa bulbosa – Poaceae

BLM Photo / Sheri Hagwood

Bulbous bluegrass

Poa bulbosa – Poaceae

  • Invasive grass originating from Southern Asia, Europe, and Northern Africa. Bulbous Bluegrass arrived in the US from what was likely a contaminated grain shipment in Portland, Oregon and has spread across the US from there.
  • Small to medium sized grass—up to 28 inches in height.
  • Produces “bulbils”—tiny bulb-shaped pods between 5 and 15 mm long, which allow it to reproduce asexually.
  • The bulbils cluster closely together, giving it an almost feathery appearance.
  • Poa bulbosa can also be identified by its rounded basal bulbs which range from white to purple.
  • Bulbils can spread easily by wind or by being tracked through mud on tires or shoes.
A purple pom-pom flower is surrounded by 4 dead flowers and on branches with spiny serrated leaves.
Musk thistle
Carduus nutans – Asteraceae

USGS Photo / Larry Allain

Musk thistle

Carduus nutans – Asteraceae

  • a biennial (2-year lifespan) or sometimes winter-annual (1 year lifespan) native to western and central Europe.
  • First appeared in the US in 1852 in Pennsylvania, likely as a product of ship ballast.
  • Large forb—up to 6 feet tall.
  • Dark green leaves with distinctive white margins and midrib.
  • Leaves often continue onto the stem which gives a “winged appearance”.
  • The flowers are light to dark shades of purple or violet.
  • Can grow in dense stands and crowd out more desirable plants.
A small green bud with tiny hairs has write stringy petals that stick in all directions.
Diffuse Knapweed
Centaurea diffusa – Asteraceae 

©2014 Zoya Akulova


Centaurea spp. – Asteraceae

  • There are two priority invasive species of Centaurea (the genus of most knapweeds) in the Grand Canyon—spotted knapweed (Centaurea stoebe) and diffuse knapweed (Centaurea diffusa). Both species originated in Eurasia and infest roadsides and rangelands.
  • Diffuse knapweed tends to be biennial (lifespan of 2 years) while Spotted Knapweed tends to be perennial (lifespan greater than 2 years)—living up to 9 years; Spotted knapweed can regrow from root buds
  • Diffuse and Spotted Knapweed both have similar distinctive deeply-lobed leaves which differentiate them from other species.
  • Flowers range from white to purple for Diffuse Knapweed and pink to purple for Spotted Knapweed.
  • The main way to tell them apart are the dark-tipped “bracts”—the leaves on the underside of the flower—on the spotted knapweed. These dark tips give it a spotted appearance that give it its name.
  • Both can form tumbleweeds as the mature, making distribution of seeds easy.
Small green leaves sit opposite each other on white stalks that have tiny spines protruding outward. A five petal flower sits among them.
Tribulus terrestris – Zygophyllaceae 

NPS Photo / M. Garmoe


Tribulus terrestris – Zygophyllaceae

  • Also known as goathead because of its fruits, which have two spikes, giving it the appearance of a goat’s head.
  • Annual (1-year lifespan) in the caltrop family (Zygophyllaceae) and native to the Mediterranean region.
  • Fruits are burrs which split into 4 sections, each section containing a pair of spines which latch easily onto passing objects such as shoes, animals, or tires. They can be painful to step on and are known to pop bike tires and hurt animals trying to eat them.
  • Grows prostrate—flat against the ground and spread out.
  • Stems have “viny” appearance and grow up to 2 meters; small white hairs cover the plant
  • Green leaves grow in pairs up the stem with 2 apical leaves at the end of each “vine” rather than 1
  • Produces yellow flowers with maturation
Grey green stems with tons of small spines has a flower with a spiny base and fluffy purple petals.
Scotch thistle
Onopordum acanthium – Asteraceae 

NPS Photo / Daniel McConnell

Scotch thistle

Onopordum acanthium – Asteraceae

  • Onopordum acanthium is a biennial (2-year lifespan) or annual (1-year lifespan) forb from the Mediterranean. It was originally brought to the US as an ornamental.
  • Typically takes 2 years to grow, flower, spread seed, and die (although length of lifespan can vary longer or shorter).
  • Leaves are generally large with spined edges.
  • Leaves are covered in white wooly hairs, which can give a greyish appearance to the leaf.
  • Lower leaves that start in the autumn and grow through the winter can reach up to 90 cm.
  • Has spiny leaf continuations or “wings” that run down the stem
  • Flowers are deep pink or purple
Yellow flowers with wide petals and one single petal pointing downward on a diagonal green stem.
Dalmatian toadflax
Linaria dalmatica – Scrophulariaceae

©2010 Steven Thorsted

Dalmatian toadflax

Linaria dalmatica – Scrophulariaceae

  • Forb native to Southeastern Europe. Specifically, it has a history in the Dalmatia region of Croatia, for which it is named.
  • Believed to have been introduced it North America as early as the late 1800s as an ornamental, however the earliest authentic specimen was not collected until 1920. In addition to being an ornamental, it has been used for fabric dye and medicinal purposes.
  • 1-3 feet tall, typically branched at the top; waxy, heart-shaped leaves
  • Yellow flower, with distinctive “tail," the length of the rest of the flower
  • Well-developed root systems which can reach up to 10 feet deep and 12 feet across; can reproduce from buds on the underground root system, which makes it difficult to be removed.
White petaled flowers in a group with green sheaths on their outsides, supported by beige stems.
Mediterranean sage
Salvia aethiopis – Lamiaceae

©2016 John Doyen

Mediterranean sage

Salvia aethiopis – Lamiaceae

  • A member of the mint family--native to northern Africa and the Mediterranean region.
  • Likely came to the US as a result of contaminated alfalfa seeds.
  • Biennial (2-year lifespan) herb in the mint family
  • Strong, distinctive scent
  • Grayish-green wooly leaves.
  • Grows complex branch system in second year with yellowish flowers.
  • Can produce thousands of seeds per plant and spread them easily as it becomes a tumbleweed.
Green joints make up a stalk of grass with long strings coming off of either side.
Jointed goatgrass
Aegilops cylindrica – Poaceae 

©2014 Al Keuter

Jointed goatgrass

Aegilops cylindrica – Poaceae

  • Winter annual (1-year lifespan) native to southern Europe.

  • Known primarily as a major contaminant to winter wheat crops as it is often difficult to distinguish from the crop itself.

  • Between 15-30 inches tall

  • Often produces long awns, which appear to be long, straight hairs moving upwards from the top.

  • Grows “joints” or spikelets, which grow one atop the other through the upper half of the grass

  • Spikelets come in groups of 2-12 and contain single seeds or clusters of seeds.

Last updated: February 8, 2022

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