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
Fennel Foeniculum vulgare – Apiaceae
NPS Photo / Will Elder
Fennel
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.
Highly competitive and difficult to control, reproducing from both root and seed.
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.
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.
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
Himalayan blackberry Rubus discolor – Rosaceae
USDA NRCS Photo
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
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.
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.
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.
Puncturevine Tribulus terrestris – Zygophyllaceae
NPS Photo / M. Garmoe
Puncturevine
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
Scotch thistle Onopordum acanthium – Asteraceae
NPS Photo / Daniel McConnell
Scotch thistle
Onopordumacanthium – 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
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.
