Wood species
Cottonwood wood
Cottonwood is a hardwood; this page covers the cottonwood, led by eastern cottonwood (Populus deltoides). At 12% moisture content it has a Janka hardness of 430 lbf (1,913 N) and it weighs about 28 lb/ft³ (448 kg/m³), 2.3 lb (1.06 kg) per board foot.
Names and species on this page: eastern cottonwood, black cottonwood, balsam poplar, Carolina poplar.
Key numbers
- Janka hardness, 12% MC
- 430 lbf (1,913 N)
- Green: 340 lbf (1,512 N)
- Weight at 12% MC
- 28 lb/ft³ (448 kg/m³)
- 2.3 lb per board foot
- Specific gravity (oven-dry weight)
- 0.40 at 12% MC volume
- 0.37 on green volume
- Shrinkage, green to oven-dry
- 9.2% tangential
- Radial 3.9%, volumetric 13.9%
One source: USDA Forest Products Laboratory, the authority for species properties it measured
Why: Measured by the USDA Forest Products Laboratory; cottonwood's density is worked from its specific gravity with the Handbook's own equation.
How much cottonwood moves
A 12 in wide flatsawn board of eastern cottonwood that goes from 6% to 12% moisture content swells about 0.24 in (about 1/4 in, 6 mm). Quartersawn, the same board moves about 0.1 in (about 3/32 in, 2.4 mm). Flatsawn faces move 2.4 times as much as quartersawn ones, because the wood shrinks 9.2% around the growth rings but only 3.9% across them.
Worked with the Wood Handbook's dimensional change coefficients for eastern cottonwood (Table 13-5: 0.00327 tangential, 0.00133 radial per 1% change in moisture content), which hold between 6% and 14%. Individual boards vary: the Handbook gives shrinkage a coefficient of variation of about 15% (p. 4-11).
Color, grain and working
- Color
- Grayish white to light brown heartwood blending into whitish sapwood.
- Grain and texture
- Fairly uniform texture and generally straight grain; odorless when dry.
- Working and character
- Moderately soft and fairly weak, with fairly high shrinkage. Tension wood can leave a fuzzy surface that is hard to machine clean.
- Uses
- Boxes, crates, baskets and pallets; also veneer and pulpwood.
Adapted in our own words from the Wood Handbook, Chapter 2, p. 2-6.
Strength
Bending strength is the modulus of rupture (MOR); stiffness is the modulus of elasticity (MOE). Clear, straight-grained samples of eastern cottonwood tested by the Forest Products Laboratory (Wood Handbook Table 5-3b). Knots and cross grain make lumber weaker than these clear samples (Handbook Chapter 5).
| Property | Green | 12% MC |
|---|---|---|
| Bending strength (MOR) | 5,300 psi (37 MPa) | 8,500 psi (59 MPa) |
| Stiffness (MOE) | 1,010,000 psi (7 GPa) | 1,370,000 psi (9.4 GPa) |
| Crushing strength (compression parallel to grain) | 2,280 psi (16 MPa) | 4,910 psi (34 MPa) |
| Compression perpendicular to grain | 200 psi (1 MPa) | 380 psi (3 MPa) |
| Shear parallel to grain | 680 psi (5 MPa) | 930 psi (6 MPa) |
| Tension perpendicular to grain | 410 psi (3 MPa) | 580 psi (4 MPa) |
A dash means the Handbook prints no value; none is filled in.
The cottonwood
The Handbook tests each species separately; they are sold together as cottonwood. Across the 2 species measured, Janka hardness at 12% runs from 350 to 430 lbf.
| Species | Janka, 12% MC | Weight, 12% MC | Tangential shrinkage |
|---|---|---|---|
| Eastern cottonwoodPopulus deltoides | 430 lbf | 28 lb/ft³ | 9.2% |
| Black cottonwoodPopulus trichocarpa | 350 lbf | 24 lb/ft³ | 8.6% |
| Balsam poplarPopulus balsamifera | — | 24 lb/ft³ | 7.1% |
Dust and health
- Wood dust is classed as carcinogenic to humans (Group 1) by the International Agency for Research on Cancer, chiefly for cancers of the nose and sinuses.
- NIOSH recommends keeping wood dust exposure to 1 mg/m³ of total dust, as quoted in OSHA's sawmill guidance.
This is what the named authorities publish, not medical advice.
Compare with
Why these numbers
Why these numbers?
The situation
The USDA Forest Products Laboratory measured eastern cottonwood at 430 lbf of side hardness at 12% moisture content, on clear, straight-grained samples (Wood Handbook Table 5-3b).
One source: USDA Forest Products Laboratory, the authority for species properties it measured , sources [1]
What that means
Side hardness (Janka) is the load needed to press a 0.444 in (11.28 mm) steel ball halfway into the wood across the grain; a higher number dents less (Handbook p. 5-5).
One source: USDA Forest Products Laboratory, the authority for species properties it measured , sources [1]
So
Density at 12% is water's density × specific gravity × (1 + 0.12) (Handbook Eq. 4-14): 62.43 × 0.40 × 1.12.
Arithmetic: exact by definition , sources [1]
The number
About 28 lb/ft³, or 2.3 lb per board foot (1/12 ft³), at 12% moisture content.
Arithmetic: exact by definition
Sources