When it comes to the variety of plants at Yellowstone, the only limiting factor is the high altitude—otherwise, the diversity of terrain, weather, and soils permits a fairly wide range of vegetation. Estimates vary, but there are more than 1,500 native plant varieties in the Greater Yellowstone ecosystem. Some species are found living on the dry valley beds in hostile soil, close to other species that predominate in lush meadows and riverbeds. Some thrive in thermal areas, while others do well in alpine areas, near mountain lakes, and in cirques near glaciers.

Examination of plant fossils indicates that life began during the Eocene epoch, approximately 55 million years ago, and continued for 17 million years. The inspection of petrified tree stumps in Yellowstone’s Lamar Valley led to the identification of 27 distinct layers of forests, one atop the other. Climatic conditions during the Eocene period were similar to those in the southeastern and south-central United States. Difficult as it might be to imagine, the area was once a warm, temperate zone in which rainfall might have averaged 50 to 60 inches per year at what was then an elevation of 3,000 feet above sea level.
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These days, the elevation ranges from 5,000 to 13,000 feet, the average low temperature is approximately 30 degrees F (–1 degree C), and hundreds of inches of snow fall each year. Plants have adapted to a growing season that is a mere 60 days in duration. As a consequence, forests once populated with hardwoods, such as maple, magnolia, and sycamore, are now filled with conifers, the most common of which are pine, spruce, and fir. A smattering of cottonwood and aspen also thrive in the cool park temperatures.

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Above 10,000 feet in the alpine zone, plants adapt to wind, snow, and lack of soil by growing close to the ground, flowering soon after the snows melt. You’ll find such flora on trails near Dunraven Pass in Yellowstone.

In Yellowstone, the canyon and subalpine regions, at 7,000 to 10,000 feet, are known for conifer forests and open meadows of wildflowers. As elevation increases, wildflowers are abundant and healthy, while trees are stunted and shrublike.
 
Identifying the plants described below does not require a degree in botany, but Kurt F. Johnson’s A Field Guide to Yellowstone and Grand Teton National Parks (Farcountry Press, 2013) is a handy addition to your trip kit. 
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Trees

Coniferous trees are most common in the parks because of the high altitude and short growing season, but there are some hardy deciduous trees as well, such as cottonwood and aspen. The most common cone-bearing trees in the parks are lodgepole pines, which cover as much as 80% of Yellowstone, and Douglas fir, subalpine fir, Engelmann spruce, blue spruce, and whitebark pine. The key to identification is the trees’ basic shape, the shape of their needles, and various characteristics of their cones.
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Lodgepole Pine: This familiar tree grows tall and slender, with bare trunk at the bottom and needles near the top resulting in dense stands that look like the spears of a closely ranked army. The needles of the lodgepole are clustered in pairs, typically around 3 inches long. You’ll see logs from this tree supporting tepees and forming the walls of cabins.
 
Douglas Fir: “Doug fir” is actually a member of the pine family, with prickly cones and dark, deeply etched bark. This tree has flat, flexible, single needles that grow around the branch, giving the tree the appearance of fullness. Another giveaway is that its cones hang downward and do not disintegrate aloft, but litter the forest floor. These trees like the north-facing side of the mountain.

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Subalpine Fir: You can distinguish firs by their needles, which sprout individually from branches instead of in clusters, like a pine; and by their cones, which grow upright on the branch until they dry up and blow away. Look for the slender, conical crown of this tree. When heavy snows weigh down the lower branches, they often become rooted, forming a circle of smaller trees called a snow mat. You’ll find subalpine fir at high elevations near the timberline.
 
Engelmann Spruce: This tree also likes the higher elevations. Look for it near Kepler Cascades, Spring Creek, and the south entrance of Yellowstone National Park. It is distinguished by single needles that are square and sharp to the touch, and by cones with papery scales that are approximately 1 1/2 inches long.
 
Blue Spruce: The Engelmann spruce’s cousin, this tree is most commonly found along the Snake River near Jackson. True to its name, it is characterized by its bluish appearance; and cones that are twice the size of the Engelmann's.

Other Plants
Here’s a brief listing of some of the most common and interesting plants found in the ecosystem.
 
Glacier Lily: A member of the lily family with a nodding bloom on a 6- to 12-inch stem, this bright yellow spring flower is found in abundance in both parks at elevations of more than 7,500 feet. Also known as the fawn lily, trout lily, and adder's-tongue, it is especially common near Sylvan Pass and on Dunraven Pass.
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Plantastic Photos 

To successfully record your discovery of the parks’ flora, consider using a microlens that will allow you to focus within 6 inches of blossoms. Speeds of 200 ISO or faster will add to the chances of proper exposure, even on cloudy days.

Indian Paintbrush: This is the Wyoming state flower. It has a distinctive narrow, bright scarlet bloom that is most commonly found from mid-June to early September in the Snake River bottomland. Other species are white, yellow, orange, and pink.

Plains Prickly Pear: This member of the cactus family is only one of two such species found in the park, usually in the Mammoth area and near the Snake River. It is distinguished by thick, flat green stems armed with spines and, during midsummer, a conspicuous yellow flower with numerous petals. American Indians, who recognized prickly pear’s medicinal qualities, treated warts by lacerating them and then applying juice from the plant.
 
Fringed Gentian: This member of the gentian family is the official flower of Yellowstone Park, where it is common and blooms throughout the summer. Its purple petals are fused into a 2-inch-long corolla and sit atop 1- to 3-foot-tall stems. 
 
Silky Phacelia: This is one of the most photogenic and easily recognized species in the parks. Growing in purple clumps alongside the road at Dunraven Pass, the flower derives its name from the silvery pubescence that covers its stems and leaves. It’s best photographed in July and August.
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Shooting Star: The shooting star is characterized by pinkish [bf]1/2- to 1-inch-long flowers that dangle earthward like meteorites from a 12-inch stem; they bloom in June. It is commonly found near thermal areas, streambeds, and Yellowstone Lake.
 
Yellow Monkey Flower: This flower exhibits a bright yellow petal that, together with orange spots, attracts insect pollinators near streambeds at elevations of 7,000 to 9,000 feet all summer. It is also found near thermal areas and Yellowstone Lake.

Fairyslipper (also known as the Calypso Orchid): Finding this beautiful orchid might require some serious detective work. It is one of 15 orchid species found in the parks and is considered by many to be the most beautiful and striking. Seen during May and June, it usually has one small, green leaf and a red-pink flower that resembles a small lady’s slipper. It is found in cool, deep-shaded areas and is becoming rare because its habitat is disappearing.
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Bitterroot: The state flower of Montana, the bitterroot makes its first appearance in early June in dry, open, sometimes-stony soil and in grassy meadows. Its fleshy rose and white petals extend up to 1 inch in length. It was a source of food for Indians, who introduced it to Captain Meriwether Lewis of Lewis and Clark fame, hence its botanical name, Lewisia rediviva.
 
Columbia Monkshood: This purple flower has a hood-shape structure with two sepals at its side and two below (all of which make up the calyx, the leafy parts surrounding the flower). It varies in height from 2 to 5 feet. You’ll find these flowers in wet meadows and stream banks from June to August, often near thermal areas and Yellowstone Lake.

Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.