Aspens are incredibly hardy but elegantly beautiful trees that thrive in frigid climates. They are famous for the way their leaves tremble in the wind, an adaptation that has helped them survive in cold, windy environments.
Aspens are all part of the Salicaceae or Willow Family of trees and shrubs, flowering plants of the Angiosperm phylum.
Aspens belong to the Populus genus, with 35 currently accepted species worldwide. Poplar and cottonwood trees also belong to this genus, and they all share some general characteristics.
According to fossil evidence, Populus evolved over 59 million years ago, during a time when worldwide speciation occurred following the mass extinction event that killed the dinosaurs.
Aspens are native to the northern part of the Northern hemisphere. They thrive in the boreal zone and only live south of that in mountainous areas. None are found south of about 20° north in Mexico and subtropical China.
Guide to Aspen Tree Varieties – 5 Types and Their Distinctive Features
1. Quaking Aspen (Populus tremuloides)
Quaking Aspens are medium-sized trees with a pyramidal crown that becomes rounded on a partially bare trunk.
The photosynthetic bark is smooth white, greenish, yellowish-white, or gray, with noticeable scars. It becomes shallowly grooved only on the lower trunks of large trees.
Winter buds are ¼”, shiny reddish-brown, pointy-tipped, and slightly resinous.
Leaves are sometimes dimorphic with flattened leaf stalks about the same length as the leaf.
Leaves are somewhat rounded to egg-shaped, 1 – 3”(-4.7”) long and wide, with rounded, shallowly wedge-shaped to somewhat heart-shaped bases and long-pointed to acute tips. The lower surfaces are hairless, slightly glaucous whitish-green, and the dark green upper surfaces may have sparse hairs. Margins are almost entire to finely crenate-serrate toothed throughout.
Catkins are densely 20 – 130-flowered and ¾ – 2 ¾” long.
Male flowers have 6 – 12 stamens. Female flowers have two thread-like stigmas.
Capsules are 0.08 – 0.28” narrowly egg-shaped.
Other Common Names: Trembling Aspen, American Aspen, Golden Aspen, Mountain Aspen, Trembling Poplar, and in Spanish, Álamo Blanco, Álamo Temblón
Identifying Features: Quaking Aspen has characteristically white, grayish, or greenish photosynthetic bark that is thin and remains mostly smooth its entire life other than black scarring from animals or the “eyes” of branch scars. It also has usually rounded leaves that are finely crenate-serrate toothed throughout, and their twigs and buds are hairless though their leaves may have sparse hairs. They are similar to Bigtooth Aspen, which has much larger teeth on their more often hairy leaves and larger terminal buds that are not as sharp-pointed and are not resinous.
Native Area: Alaska east to Newfoundland, south through the Rocky Mountains and other mountains of western North America to northern Mexico, and east to Missouri, Virginia
USDA Growing Zones: 2 – 8
Average Size at Maturity: 25 – 75 ft (to 115 ft) tall, 10 – 30 ft spread
Some Cultivars Available:
2. Bigtooth Aspen (Populus grandidentata)
Bigtooth Aspen has a narrow rounded crown and a thick trunk to 5 ft wide, with light gray to somewhat bronze or olive-green bark that is smooth and only becomes grayish-brown and grooved on lower trunks of large trees.
Winter buds are reddish, downy-hairy at the base, and not resinous.
Leaves are strongly dimorphic, with flattened leaf stalks ½ – ¾ the leaf length.
Leaves are egg-shaped, 0.8 – 9.8” long, ¾ wide as long, with an obtuse to almost heart-shaped base and acute tip. Lower surfaces are greenish-white, densely appressed silky-hairy but becoming hairless. Upper surfaces are bright shiny dark green. Margins are either coarsely serrated mid-blade towards the tip and entire below, or they are finely crenate-serrated toothed throughout.
Catkins are very densely 30 – 175-flowered and 1.6 – 4” long.
Male flowers have 6 – 12 stamens, and female flowers have two erect thread-like stigmas.
Other Common Names: Common Aspen, Large-tooth Aspen, American Aspen, Canadian Poplar, Canadian Aspen, White Poplar, Long-toothed Aspen
Identifying Features: Bigtooth Aspen has strongly dimorphic, usually egg-shaped leaves that are densely appressed silky-hairy when young, longer than they are wide, and coarsely serrated on the upper half only or finely crenate-serrated toothed throughout, with leaf stalks that are shorter than their length. They are similar to Quaking Aspen, but that has a narrower trunk, leaves that are usually not hairy and have much smaller teeth, and their buds are smaller, sharp-pointed, and not downy-hairy at their base.
Native Area: Eastern North America from Manitoba east to Nova Scotia, Canada, south through Minnesota to northeastern Missouri, east to Delaware
USDA Growing Zones: 3 (2 with protection) to 5
Average Size at Maturity: 50 – 75 ft (to 115 ft) tall, 20 – 40 ft spread
3. European Aspen (Populus tremula)
European Aspens are medium to large-sized trees with open, rounded crowns and smooth grayish-green bark that may become irregularly grooved at the base.
Buds are somewhat rounded to somewhat egg-shaped.
Leaf stalks are flattened and about as long as the leaf.
Leaves are almost rounded, 1.2 – 2.75”, and sometimes pilose-hairy when young but otherwise are hairless. The base is truncated, rounded, or shallowly heart-shaped, and the tip is obtusely rounded. The margin has rounded teeth. Leaves on new growth may be larger and somewhat triangular in shape with a crenate-serrate margin.
Male catkins are 2 – 3.15” long with downy-hairy rachis and have flowers with 5 – 10 stamens.
Female catkins are 1.6 – 2.4” and twice that in fruit.
Capsules are narrowly conical, hairless, and more or less stalk-less.
It is not on any invasive species databases but is introduced in Missouri and Connecticut.
Other Common Names: European Trembling Aspen, European Quaking Aspen
Identifying Features: European Aspen has grayish-green, mostly smooth bark, usually rounded leaves that are sometimes pilose-hairy, and have obtusely rounded tips and rounded teeth on leaf stalks that are about as long as their leaves. Male catkins have downy-hairy rachis, and their capsules are narrowly conical. It can be differentiated from the North American aspens by its leaves that are never acute or long-pointed at the tips and narrowly conical rather than egg-shaped capsules.
Native Area: Europe, Siberia, Middle East, North Africa
USDA Growing Zones: 2 (1 with protection) – 6
Average Size at Maturity: 30 – 70 (to 90 ft) tall, 10 – 30 ft spread
4. Swedish Columnar Aspen (Populus tremula ‘Erecta’)
Swedish Columnar Aspen is a cultivar of the European Aspen that has a much more compact, narrowly columnar habit with strongly ascending branches that are erect and almost vertical.
It has the same fluttering leaves on long flattened petioles like its parent strain that allows them to dance in the wind. Leaves have a rounded base, obtusely pointed tip, and finely-toothed margins. They are green and turn a mix of gold to red in the fall.
This one is a seedless cultivar, so it won’t produce any mess or reproduce sexually by seeds.
It grows fairly quickly, like most members of its genus, and grows best in full sun with medium moisture.
Other Common Names: Swedish Aspen
Identifying Features: Swedish Columnar Aspen is a cultivar of the European Aspen, and as such, it shares most of its general leaf and bark characteristics. It differs, however, in its narrowly columnar habit with branches so strongly ascending they are almost vertical.
Native Area: N/A
USDA Growing Zones: 2 – 6
Average Size at Maturity: 30 – 40 ft tall, 8 – 10 ft spread
5. Japanese Aspen (Populus tremula var sieboldii)
Japanese Aspen used to be its own species but is now recognized by plant authorities as a botanical variety of the European Aspen.
It is a fast-growing medium-sized tree with a broadly columnar to open and rounded habit and light to dark gray bark that is sometimes shallowly grooved when mature, even high above the base.
The leaves are typically egg-shaped to almost rounded in shape with finely serrated margins. They are light green on the upper surface and silver-green on the lower surfaces. They turn a pleasant light yellow color in the fall.
It does best in moist but well-drained soils of most types. It must be grown in sun and will not tolerate shade.
Other Common Names: None
Identifying Features: Japanese Aspen has light to dark gray bark that sometimes becomes shallowly grooved its entire height rather than just the base, and egg-shaped to almost rounded light green leaves that are silver-green below and have finely serrated margins. It could be mistaken for the European Aspen, but that usually has darker green leaves that are not so silvery on the lower surface, and its bark is grayish-green and never becomes shallowly grooved past the base of the trunk.
Native Area: Mountainous areas of East Asia and Japan
USDA Growing Zones: 4 – 8
Average Size at Maturity: 40 – 65 ft tall, 15 – 30 ft spread
Aspen Tree Identification (With Photos)
Identifying Aspen Trees by Their Leaf Shape
Aspen are deciduous trees that lose their leaves in the fall, turning lovely shades of yellow, gold, or sometimes red. This adds pretty fall colors to the landscape that contrasts nicely with their light and often white-colored bark.
Aspens are a small group of trees without much variability in leaf shape or size. They tend to have egg-shaped (ovate), almost rounded (orbiculate), or sometimes somewhat triangular (deltoid) leaves that are between 1 – 4” across.
Occasionally aspen may have heterophyllous or dimorphic leaves, meaning that they have two different shapes and/or sizes of leaves on the tree simultaneously.
Identifying Aspen Trees by Their Leaf Bases
Leaves also have characteristic shapes to their bases. These shapes can be used to help identify the different types of aspen trees.
Aspen leaf bases may be rounded, heart-shaped (cordate), truncate (appears cut off abruptly), wedge-shaped (cuneate), or obtuse (wide-angled, sometimes called broadly cuneate).
Identifying Aspen Trees by Their Leaf Tips
The shape of the leaf tip or apex can also be used to help describe and identify the different types of aspen trees. The tips can be acute (angled, less than 90°), obtuse (wide-angled, greater than 90°), somewhat rounded, or long-pointed (acuminate), where they gradually narrow to a very fine point where the angle is usually less than 30°.
Identifying Aspen Trees by Their Leaf Margins
Leaf margins are another feature that is used to help identify the different types of aspen. They can be almost entire (smooth, without any teeth), or more often, they have finely or coarsely toothed margins that can be rounded (crenate) or serrated (sharp, angular).
Sometimes the teeth are described as crenate-serrate because they are intermediate between rounded and serrated. Finally, sometimes they can be toothed on the upper half of the leaf only and entire on the lower half towards their base.
Identifying Aspen Trees by Their Leaf and Twig Hairs
Occasionally, aspens have hairs on their leaves but mostly only when they are young, becoming hairless as they mature. Some have hairs on their buds, twigs, or the rachis of their catkins. The presence and type of hairs can be used to help identify the different types of aspens.
Some hairs are finely tomentose, a kind of matted woolly-looking hair, but they are never as thickly tomentose as some of their poplar tree cousins. Sometimes the hair can be pilose, which are soft and long, straight hairs, or downy (pubescent), which are soft, short hairs, or they can be appressed silky-hairy which are long soft hairs that are flattened against the surface in one direction.
Identifying Aspen Trees by Their Winter Buds
All deciduous trees, like aspens, develop buds on their branches which contain the primordia of the leaves that will grow the following spring. These buds come in various shapes but are usually more or less egg-shaped, and they may or may not have a pointy tip. Some are resinous while others are not, and sometimes there is some downy hair at their base.
These factors can sometimes help to identify aspen trees, especially in winter after they have lost their leaves. Sometimes you can even use their buds along with characteristics of the bark and twigs, as well as the overall size and shape of the tree to identify it without having leaves or catkins.
Identifying Aspen Trees by Their Catkins
Aspens are always dioecious trees with separate male and female flowers on separate male and female plants.
Aspens produce their flowers in elongated spikes called catkins. Many catkin-producing plants produce only male catkins, while female flowers tend to be solitary. However, the entire Populus genus produces both male and female catkins.
Aspen catkins typically contain numerous tiny flowers, usually more than twenty and sometimes even as many as 175, that are all attached to a central stalk known as a rachis.
In all Populus species, their tiny flowers are significantly reduced compared to most other angiosperms. They all lack petals, sepals, staminodes, nectaries, or vestigial parts of any kind.
Instead, each tiny flower consists only of a certain number of stamens (male flowers) or stigmas in their pistil (female flowers), as well as a usually brownish subtending bract. Bracts are derived from leaf rather than floral tissue, so they are not considered petals or sepals.
Stamens are the male reproductive structures of a flowering plant. They are usually made of a filament (like a stalk) with an anther on top. Anthers are the reproductive structures that produce and release pollen on male trees.
In aspen, there are typically 6 – 12 stamens per flower. Even though there are usually fewer stamens per flower than their cousins, the poplars, they can also be challenging to count. But, like poplar, they tend to be reddish in color, and their appearance can help you identify them as a male aspen tree. The fact that they are less dense will help distinguish them from most poplars (note that the photo below is of a male poplar catkin so that the aspen would be similar but less densely packed with anthers).
A pistil is the entire reproductive structure of a female flower. In aspen trees, it includes a usually green and somewhat egg-shaped ovary containing ovules (the unfertilized seeds) plus a characteristic number of stigmas held above the ovary on a style (a stalk).
Stigmas are receptive structures that receive pollen from male flowers and direct it down into the ovary, which is usually situated immediately beneath it. In aspens, the stigmas tend to be thread-like (filiform) and cannot easily be used to help identify the different species, but it can help determine that it is likely an aspen and not a poplar which tends to have lobed or plate-like stigmas.
In the photo of the fertilized female catkin below, you can see the remnants of the filiform stigmas and the green ovaries that are expanding into larger green capsules.
Identifying Aspen Trees by Their Fruits
Aspen, like all angiosperms, produce fruits formed from their ovaries. All aspens produce a type of fruit known as a capsule.
Capsules appear a lot like large seeds and can be seen hanging from the much-elongated female catkin after the flowers have usually withered and fallen off.
Aspen capsules are green when immature and, like most capsules, turn brown when they mature. Once mature, they split open (dehiscence) along lines known as valves into a characteristic number of pieces still often held together at their base. This process then releases the seeds within.
In aspen, however, capsules are roughly the same shape, and they are all two-valved. So, while this is not a reliable way to identify the species of aspen, it will narrow it down and eliminate all poplars with 3 – 4 valves.
Aspen often, but not always, produce a cottony attachment on their seeds that aid in wind dispersal. This is pretty characteristic among the entire willow family (see the photo in the catkin section above of willow dispersing cottony seeds).
The interesting thing about aspen is that their seeds seldom even germinate. Instead, they spread more clonally by their roots producing clonal plants. Those clonal plants then spread via their roots to produce more clonal plants from them, and so on, until they often produce almost pure stands of the same genetically identical trees.
Identifying Aspen Trees by Tree Habit
Tree habit is the overall shape of a tree as viewed from a distance.
Aspen trees tend to start out with pyramidal or conical crowns when young but then mature to have more rounded crowns that are often up off the ground with a section of bare trunk beneath them. Cultivars sometimes have narrower, more columnar crowns. Aspen rarely develops irregular or flattened crowns because their flat leaf stalks allow free movement in the wind, protecting them from wind shear.
Identifying Aspen Trees by Branch Growth
The arrangement of branches on a tree can occasionally be used to help identify the different types of aspen. However, there is not as much variability in aspen as in some groups of trees. They tend to have either ascending or spreading branches.
Sometimes, in aspen cultivars, their branches are so strongly ascending they are almost vertical. In horticultural terms, these trees are often called fastigiate.
Identifying Aspen Trees by Their Unique Bark
The last feature used to help identify aspen trees is their bark. Aspen bark is unique in the Populus genus, remaining mostly smooth its entire life, unlike cottonwood or poplar, which both tend to have thick gray bark that becomes deeply grooved. Aspen bark only sometimes develops grooves and only at the base of the trunk of large mature trees.
Aspen bark tends to be thin and smooth and may be white, light to dark gray, greenish, grayish-green, or yellowish-white. The Japanese Aspen sometimes does have shallowly grooved bark, which can help to identify it when present.
It is also unique in that the bark of some aspen is photosynthetic, giving them the ability to grow in winter months in the snow when virtually all other deciduous trees in their northern or alpine habitats go dormant. Sometimes their bark has a greenish color to it, especially when the trees are young, due to the high chlorophyll content. Even when white, though, the bark is still capable of photosynthesis.
Aspen scars are the other unique characteristic of aspen bark. Quaking Aspens, in particular, are known for their unique scars that come from a few different sources. Elk and deer rub their antlers on aspens, leaving broad, vertical black scars around chest height to head height on the trunks. During the winter, voles often chew on aspen bark from beneath the snow, leaving black, scaly marks at the trunk’s base.
Aspen also often have branch scars that are eerily very much shaped like an eye. The trees really do have “eyes,” but these are simply from the loss of side branches as they mature, not some spooky tree spirits watching you!
Finally, because aspen is so easy to scar, humans often sadly leave graffiti on them, writing their name, symbols, or other marks that the tree must now carry its entire life. Trees are only truly alive in their inner bark because the wood inside does not grow, conduct nutrients, perform photosynthesis, or any other functions necessary to life. So, when you scar a tree’s thin bark like that of an aspen, you are interfering with its ability to grow. Please resist the urge to write your name on aspen.
How to Tell an Aspen From Poplar and Cottonwood Trees
Aspen, poplar, and cottonwood are all part of the same Populus genus. Being so closely related, they naturally share many morphological characteristics. These similarities between the groups can make it challenging to tell them apart. However, if you learn a few general but significant differences, you should be able to tell them apart.
Aspen, in particular, is much more uniform as a group and has some unique features that will help you determine pretty quickly that it is an aspen and not a poplar or cottonwood.
Aspens tend to be medium-sized trees with more rounded leaves that are typically rounded-toothed or serrated-toothed along the margins. They all have flattened leaf stalks (petioles), which allows them to flutter characteristically in the wind.
They sometimes produce cottony seeds like those of cottonwood, but these seldom germinate. Instead, they mostly regenerate through the clonal spreading of root clones.
Aspen bark is unique in the group because it tends to remain thin and more or less smooth as it matures, often developing unique scars that make it very easy to identify. Unlike poplar or cottonwood, they only develop shallow grooves near the trunk base on mature and large trees, never large grooves, and rarely on the whole trunk.
Cottonwoods are the least variable group. They are typically huge trees with massive trunks covered with thick and very deeply grooved bark when mature. The bark is similar to poplar but generally thicker and more deeply grooved.
Their more triangular or heart-shaped leaves are usually shallowly rounded-toothed or serrated. Their leaf stalks are always rounded and never flattened.
Cottonwoods get their name from the cottony seeds that all of the female cottonwood trees produce.
Poplars are the most variable. They are generally tall trees that are sometimes narrower (columnar or conical) in habit but may also be rounded and open. Their bark gets thick as it matures and tends to develop vertical grooves, some very similar to cottonwood while others are more shallowly grooved, but it never stays smooth like an aspen.
They have highly variable leaves that may be triangular, egg-shaped, diamond-shaped, or slightly lance-shaped. Their margins are sometimes toothed and sometimes entire or almost entire.
Poplar leaf stalks (petioles) are also highly variable. They can be flattened like aspen, rounded like cottonwood, or somewhere between.
Like aspen, some female poplars produce the cottony seeds characteristic of cottonwood, while others do not. Some of their seeds will germinate readily, while others spread more clonally, like an aspen.
Cottonwoods and poplars share the most characteristics, with poplars being the most variable. Many poplars are also called cottonwoods because they are so similar. Fortunately, the similarities between those two groups will help you separate them from the more unique aspens.
Growing Aspen Trees in Your Garden
Growing aspen trees can be very rewarding for their beauty and elegance. Their leaves, with their usually flattened leaf stalks, flutter elegantly in the wind. In the fall, they turn pleasing shades of yellow to red, and in the winter, their bark adds a nice feature to the landscape. If you have decided to grow an aspen tree, as with any tree, it is essential to do some research to ensure success.
In addition to understanding the USDA Planting Zones, you must check your chosen tree’s soil, light, and moisture requirements and compare that to your site. Check out How to Pick A Tree For Your Yard for more information on choosing the right tree for the right spot in your yard.
Most aspens are adapted to cold climates, with a couple of species even being able to tolerate USDA zone 1. The Japanese Aspen and the Quaking Aspen will tolerate warm temperate climates up to zone 8, but none will survive in subtropical or tropical environments.
Aspens are sun-loving trees and should be planted in full sun. They generally will not do well in partial shade.
They typically grow well in most soil types, provided they are well-drained. However, they generally do best in moist soils and do poorly in any soil that is too dry.
Unlike some members of its genus, none of the aspens are currently listed on any invasive species databases. However, the European Aspen has naturalized in some areas of the USA but does not appear to be spreading aggressively. Like some of the poplar trees, they too can hybridize with native Populus species, which still threatens our native species on a genetic level.
Therefore, you should always try to choose species native to your area to enhance biodiversity and wildlife values and minimize the risk of spreading invasive species. Also, trees that are native to your area tend to do better once established than non-native trees because they are already adapted to your local conditions over often millions of years of evolution.
Amazing Aspen Trees
Interesting Facts
The world’s largest living organism, by mass, is a clonal male Quaking Aspen in south-central Utah, USA, known as Pando, which occupies 108 acres and is estimated to weigh 6,000 tonnes.
It spreads by its root system, which is estimated to be several thousand years, which also makes it one of the oldest known living organisms as well. Sadly, Pando has begun to die due to human influence, including habitat alteration, grazing, fire suppression, and climate change.
Some aspen trees continue to grow throughout the winter, unlike most deciduous trees, which go dormant. In these aspens, their trunks are actually capable of photosynthesizing. So, even when the trees are bare, and the ground is covered with snow, these beautiful trees are still growing. This could partly explain why they seem to thrive so well in northern climates.
Human Uses
Aspen tree bark and leaf have long been used to make medicines used in the treatment of joint pain, prostate problems, sciatica, and nerve and bladder problems. The buds were commonly used to treat coughs and colds.
Aspen wood is used for biofuel, paper, and particle board. Because it is so lightweight, it is favored for making oars and paddles, and it also used to be used for surgical splints and wagon bottoms. It does not burn well, however, and is not generally used as firewood.
Wildlife Values
The leaves of the Quaking Aspen are eaten by snowshoe hares, deer, and elk, and the trees are an important supply of food and building materials for beaver. Native Ruffed Grouse are especially dependent on them for food and nesting sites, and they feed on the buds throughout the long cold winters.
Aspens are the primary food source for numerous moth and butterfly species in their larval stages.
Deer and elk often give birth in young aspen stands or mature aspen forests with lots of understory growth that hides their newborns from potential threats.
Now know how to identify these lovely trees with their fluttering leaves, and you can go out and use your skills to identify the beautiful aspens around you. Enjoy!
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