045 Cottonwood: The Fast-Growing Hardwood of Rivers and Floodplains

Know Your Woods: The Softer Eastern Hardwoods Series

Cottonwood is another wood that fits naturally into our Softer Eastern Hardwoods series.

It is a true hardwood, but its lumber is relatively light and soft. It grows rapidly, often becomes a very large tree, and is strongly associated with rivers, streams, floodplains, and other areas where water has shaped the landscape.

And after looking at aspen and willow, cottonwood brings several pieces of our series together.

Aspen and cottonwood are botanical relatives.

Willow and cottonwood are ecological neighbors.

All three can be fast-growing pioneer trees.

And all three can produce lightweight hardwood that behaves very differently from oak, maple, hickory, or other dense eastern hardwoods.

Eastern Cottonwood

For our eastern hardwood series, the species we're primarily interested in is eastern cottonwood (Populus deltoides).

Other common names include:

That word poplar appears again.

But this time it is botanically legitimate.

Cottonwood belongs to the genus Populus, the same genus that contains quaking aspen and bigtooth aspen.

Yellow-poplar, despite its name, does not.

So our wood-name puzzle now looks like this:

Aspen = a true Populus.

Cottonwood = a true Populus.

Yellow-poplar = not a Populus at all.

Common names can certainly keep a woodworker on their toes.

One of the Giants of the Eastern Floodplain

Eastern cottonwood can become an enormous tree.

Mature trees commonly reach impressive heights, and exceptional specimens have been reported at 175 feet or more.

Trunks can also reach several feet in diameter.

What makes that particularly remarkable is how quickly the tree can achieve substantial size.

Eastern cottonwood is among the fastest-growing native hardwoods in North America and is considered the fastest-growing commercially important forest species on the continent.

On favorable sites, young cottonwoods can put on extraordinary amounts of height in only a few years.

That rapid growth has obvious implications when the tree reaches the sawmill.

Large logs can represent relatively young trees compared with slower-growing hardwoods of similar diameter.

A Tree That Follows the Water

Cottonwood and moving water have a close relationship.

Eastern cottonwood thrives particularly well on moist, well-drained sands and silts along rivers and streams.

Floods continually reshape these environments.

They deposit fresh sediment.

They expose bare soil.

They create new sandbars and riverbanks.

And those newly disturbed surfaces create perfect opportunities for cottonwood seedlings.

This makes cottonwood a classic pioneer tree.

Instead of waiting for an established forest, it specializes in getting there early.

The Cotton in Cottonwood

The name cottonwood comes from the extraordinary masses of cottony material associated with the seeds of female trees.

Cottonwoods have separate male and female trees, just as we saw with willow.

Female trees produce tiny seeds attached to fine white hairs.

When enormous numbers of those seeds are released, the ground can look as though it has been covered by snow.

The cottony fibers help the seeds move through the air until they hopefully reach suitable moist ground.

The individual seeds are short-lived, so timing is important. A seed that lands on fresh, damp sediment has a much better chance of becoming the next cottonwood tree.

Identifying the Standing Tree

Eastern cottonwood has broad, generally triangular or somewhat heart-shaped leaves with coarsely toothed margins.

The leaf stalks are flattened, allowing the leaves to move noticeably in the breeze.

That gives us another similarity with aspen.

The bark on young trees is relatively smooth, but older cottonwoods develop deeply furrowed gray-brown bark.

Large mature cottonwoods growing near rivers can develop massive trunks and broad crowns.

When we have the tree, leaves, bark, location, and overall form give us many clues.

At the sawmill, most of those clues disappear.

Identifying Cottonwood Lumber

Eastern cottonwood lumber is generally pale.

The sapwood can be nearly white to pale yellow, while the heartwood tends toward light brown.

The transition between sapwood and heartwood may not be strongly defined.

The grain is usually straight, although it can become slightly irregular or interlocked.

When trying to identify cottonwood lumber, I would look for several clues together:

Cottonwood is diffuse porous, once again putting it into the same broad structural category as aspen, willow, basswood, yellow-poplar, and cucumbertree.

How Hard Is Cottonwood?

Eastern cottonwood has a Janka hardness of approximately 430 pounds-force.

That puts it remarkably close to basswood and bigtooth aspen.

Our comparison now looks like this:

That's a surprisingly tight grouping at the softer end.

Basswood, bigtooth aspen, and eastern cottonwood are separated by only about 20 pounds-force on the Janka scale.

Yet their working personalities can be very different.

Once again, hardness alone doesn't identify the wood.

Cottonwood Is Light... Eventually

Dried eastern cottonwood averages approximately 28 pounds per cubic foot.

That's only slightly heavier than basswood, quaking aspen, and black willow at roughly 26 pounds per cubic foot.

But anyone handling freshly cut cottonwood may wonder how something supposedly lightweight could feel so heavy.

The answer is water.

Living cottonwood can contain a tremendous amount of moisture.

A green cottonwood board can therefore weigh dramatically more than the same board after proper drying.

This is an important sawmill lesson:

The weight of green lumber tells us about both the wood and the water inside it.

From Heavy Green Lumber to Lightweight Dry Lumber

Cottonwood may provide one of the best demonstrations in this series of how dramatically drying changes lumber.

Fresh from the saw, a cottonwood board can contain so much moisture that it feels far heavier than its eventual dry density would suggest.

As the water leaves, the board becomes much lighter.

If you encounter cottonwood regularly at a sawmill, comparing a freshly cut board with an air-dried board of similar dimensions would be an excellent exercise.

Pick up one.

Then pick up the other.

The difference is a physical reminder of just how much water can be stored inside a living tree.

Machining Cottonwood

Cottonwood's softness makes it relatively easy to cut, but that doesn't automatically mean it produces a beautiful surface directly from every machine.

Its fibers can sometimes compress or pull rather than shear cleanly.

This can produce fuzzy surfaces, particularly when moisture content is high, grain direction is unfavorable, or cutting edges aren't sharp.

That should sound familiar.

We saw very similar behavior with aspen.

And that makes sense because cottonwood and aspen are relatives.

Cottonwood Versus Aspen

This may be one of our most useful comparisons so far.

Quaking aspen, bigtooth aspen, and eastern cottonwood all belong to Populus.

The woods share several characteristics:

The exact lumber can vary considerably from tree to tree, so separating an unidentified pale cottonwood board from aspen based on appearance alone may not always be easy.

This is another situation where knowing the log, bark, location, diameter, and other circumstances can be extremely useful.

If the unknown log was enormous and came from a river bottom, cottonwood moves higher on the list.

If the wood came from a smaller upland tree in an aspen stand, aspen becomes more likely.

Those Fuzzy Fibers Again

Our aspen article included an important observation from the sawmill.

Some freshly cut aspen boards coming off the planer while making special pallet pieces can develop masses of loose, fuzzy fibers approaching 1/4 inch thick.

Cottonwood belongs to the same genus and can also produce fuzzy or woolly surfaces when machined, especially when green.

This does not mean every cottonwood board will behave that way.

But when I'm looking at an unidentified pale, soft hardwood that is behaving this way under a planer, the Populus group—cottonwood and aspen—would certainly deserve consideration.

Drying Cottonwood

Drying cottonwood deserves attention because green lumber can begin with such a high moisture content.

Proper stacking, consistent stickers, airflow, and restraint against distortion are important.

Cottonwood can warp during drying, particularly if boards are poorly stacked or the grain is irregular.

The difference between radial and tangential shrinkage is also significant enough that drying stresses and distortion deserve consideration.

Once properly dried, however, we have a lumber that is much lighter and easier to handle than the freshly cut material might have suggested.

Working With Dry Cottonwood

Dry cottonwood is generally easy to saw and machine because of its relatively low density.

Sharp tools are important if a clean surface is desired.

The wood nails well and generally has relatively little tendency to split when nailed.

It also glues successfully.

Because the wood is soft, however, it can dent and bruise relatively easily.

That makes it less suitable for surfaces expected to withstand heavy abrasion or impact.

What About Carving?

At approximately 430 lbf, eastern cottonwood is certainly soft enough to carve.

But our Basswood lesson applies again.

Softness alone does not make a perfect carving wood.

Cottonwood's fibers may not always sever with the same exceptionally clean and predictable behavior that makes basswood so desirable for detailed carving.

Still, cottonwood can be used for carving, particularly when the tools are very sharp and the project doesn't depend upon extremely fine detail.

And there is another form of cottonwood that carvers sometimes find especially interesting.

Cottonwood Bark

Large, mature cottonwoods can develop remarkably thick, deeply furrowed bark.

Pieces of cottonwood bark have become a carving material in their own right.

The thick bark is considerably softer than the wood and can be carved into whimsical buildings, faces, figures, ornaments, and other decorative pieces.

That gives cottonwood an unusual place in carving:

Sometimes the part of the tree being carved isn't the wood at all.

It's the bark.

Oil and Finishing

Cottonwood's natural appearance is generally quiet.

An oil finish will normally warm the pale tan and brown tones and make the grain somewhat more visible, but it usually won't produce the dramatic transformation we might see in highly figured walnut, cherry, or maple.

Because the grain can be rather plain, cottonwood has often been used where appearance isn't the primary consideration or where the wood will be painted, covered, or hidden inside another product.

Staining should be tested first because soft, absorbent areas may not accept color perfectly evenly.

Practical Uses of Cottonwood

Cottonwood has historically been valued as a practical lightweight hardwood rather than a prestige furniture species.

Its uses have included:

Its relatively neutral odor and taste have also made cottonwood useful for certain containers.

Like several woods in this series, cottonwood demonstrates that an enormous amount of useful woodworking happens where nobody is particularly concerned about showing off the grain.

Pallets: A Perfect Use for a Soft Hardwood

Cottonwood's use in pallets makes a lot of sense.

A pallet doesn't need the hardness of sugar maple or the beautiful grain of walnut.

It needs to perform a job at reasonable cost.

Cottonwood is light, relatively easy to nail, available in large dimensions, and grows rapidly.

Those characteristics can make it well suited to pallets, crates, boxes, and similar industrial applications.

This is another important lesson about lumber:

The best wood isn't necessarily the strongest or prettiest wood. It is the wood whose properties fit the job.

A Tree That Doesn't Make Durable Outdoor Lumber

Here's another apparent contradiction.

Cottonwood thrives beside water.

It grows on floodplains.

Its roots may spend their entire lives associated with moist soil.

Yet the lumber itself has poor natural resistance to decay.

Like willow, cottonwood reminds us that a tree's ability to live in a wet environment tells us very little about the decay resistance of its lumber.

Once the tree dies and the wood becomes lumber, the biological defenses and conditions that kept the living tree functioning are gone.

Untreated cottonwood is therefore generally a poor choice for long-term ground contact or exposed wet environments.

A Pioneer Tree

Cottonwood's ecological story may be every bit as interesting as its lumber.

Eastern cottonwood requires abundant sunlight and is highly intolerant of shade.

That means it needs openings.

Floods provide those openings naturally.

Fresh sediment is deposited along a river.

Sunlight reaches the bare ground.

Cottonwood seeds arrive.

If moisture conditions are right, seedlings establish quickly.

The trees then grow extraordinarily fast, creating a young forest where bare ground existed only a short time earlier.

Cottonwood and Willow Together

Our previous article covered black willow.

It isn't unusual for cottonwood and willow to occupy similar environments.

Both are excellent pioneer species on moist, recently disturbed soils.

Both need abundant sunlight.

Both can establish along waterways.

And both grow quickly.

Over time, the environment they create may allow more shade-tolerant hardwoods to establish.

So those fast-growing soft hardwoods can become the first stage in creating conditions for a future forest.

Cottonwood and Wildlife

Cottonwood also provides food and habitat for wildlife.

Its buds, bark, twigs, and foliage can be used by various animals, while large trees provide nesting and roosting opportunities for birds.

Beavers are particularly well known for cutting and using trees in the willow-poplar family.

Large cottonwoods can also develop cavities as they age, increasing their habitat value.

The Family Connection: Salicaceae

There's another relationship worth noticing.

Willow belongs to the genus Salix.

Aspen and cottonwood belong to Populus.

But Salix and Populus themselves belong to the same botanical family: Salicaceae, the willow family.

So our last three woods are more closely connected than they may have initially appeared.

Aspen and cottonwood are close relatives within Populus.

Willow is their somewhat more distant cousin within the same family.

And all three share some interesting ecological characteristics involving rapid growth, abundant light, disturbance, and moisture.

Aspen, Cottonwood, and Willow at the Sawmill

Imagine standing at the mill with three unidentified piles.

All three are relatively light-colored hardwoods.

All are fairly soft.

All can be lightweight when dry.

All may produce somewhat fuzzy machining behavior.

Now identification becomes interesting.

Aspen may be exceptionally pale and lightweight, and we've seen some green aspen produce extraordinary fuzzy fibers when planed.

Cottonwood may arrive as a much larger log from lowland or floodplain areas and can be extraordinarily wet when freshly sawn.

Willow may show somewhat more tan, grayish, or reddish-brown heartwood and irregular grain.

But none of those clues should stand alone.

We keep adding evidence until the identity makes sense.

Our Softer Eastern Hardwoods Comparison

Our identification library is becoming substantial:

That list reveals something I find increasingly interesting.

The physical numbers overlap tremendously.

We cannot say:

"It's around 26 pounds per cubic foot, therefore it must be basswood."

Or:

"It's around 420 pounds on the Janka scale, therefore it must be aspen."

Several woods occupy almost the same territory.

The identification comes from the combination.

Weight.

Hardness.

Color.

Grain.

Pores.

End grain.

Moisture.

Machining behavior.

Bark.

Growing location.

And sometimes simply experience.

Fast Growth Has a Woodworking Personality

Looking at aspen, willow, and cottonwood together raises another interesting idea.

These trees invest heavily in rapid growth.

They quickly occupy disturbed ground and capture sunlight.

They don't build the extremely dense wood we associate with hickory or hard maple.

Instead, they produce lighter structures quickly.

That biological strategy eventually becomes something we can feel at the sawmill.

The lightweight board.

The soft fibers.

The relatively low Janka number.

The enormous moisture content of green wood.

The occasional fuzzy surface coming from the planer.

What happened in the forest is still visible in the lumber.

From a Cottony Seed to a Massive Log

Perhaps the most remarkable thing about cottonwood is the contrast between how it begins and what it can become.

The tree starts with a tiny seed floating through the air on a tuft of cotton.

That seed may land on freshly deposited soil beside a river.

If everything goes right, it germinates quickly.

Then the race begins.

Sunlight, water, and rapid growth can transform that tiny seed into one of the largest hardwood trees in the eastern landscape.

Eventually that enormous tree may arrive at a sawmill.

The log becomes boards.

The boards dry.

And something that once contained an extraordinary amount of water becomes surprisingly lightweight lumber.

That is another reason I enjoy learning individual woods.

Every board is the final chapter of a much larger biological story.

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Brad Zehr | ZehrWoodartistry.com | brad@zehr.net

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