Wednesday, Woody Wednesday – How Does Growth Rate Affect Strength and Density?

I’ve written before about the strength of various different wood species. One shortcoming of any discussion of strength by species is the unavoidable fact that the same species can produce wood with very different strength properties, dependent upon a number of factors. Today, we’ll discuss an important factor affecting wood strength within a species: growth rate.

It is commonly – and incorrectly – assumed that faster growth inherently results in weaker wood. As we’ll see, this is hardly the case. The reason that this myth has become so popular is primarily because it is true for certain species that also happen to be among the most common woods that a layman is likely to use. Let’s start by breaking up some common domestics species into groups based on their anatomical similarities.

The first division we can make is into hardwoods and softwoods. Or more accurately, conifers and broadleafs. Or most accurately, gymnosperms and angiosperms. But I digress. We’re woodworkers, and we tend to refer to gymnosperms (trees that don’t have flowers) as “softwoods” and angiosperms (trees that do have flowers) as “hardwoods”. That’s because common softwoods, like white pine, spruce, fir, and cedar, do indeed have softer wood than common hardwoods, like oak, beech, maple, cherry, and walnut. But there are confusingly some hardwoods, like basswood, willow, and balsa, that have wood much softer than some softwoods, like southern yellow pine and yew.

So we have established that the anatomical difference between hardwoods and softwoods is not hardness. What is the difference? Pores. Hardwoods have them, softwoods don’t. Softwood xylem tissue is composed primarily of narrow, short cells known as tracheids. Tracheids are not open on the ends, so water moves from cell to cell up the tree via small perforations known as pits. Hardwood xylem also contains tracheids, but they evolved a different cell type for transporting water known as pores. Pores are larger in diameter than tracheids, and they are connected end-to-end like a straw. As we will see, the pattern and size of these pores have a profound impact on the strength of hardwood lumber.

Softwoods have only narrow tracheids.

Hardwoods have tracheids plus wide pores.

We can further divide the hardwood group by how the pores are arranged in the xylem. Hardwoods that have an even arrangement of pores throughout the wood, regardless of the time of year that the wood was produced, are known as “diffuse-porous hardwoods”. The other group of hardwoods produce a continuous band of pores around the circumference of the trunk each spring when growth begins anew. We call these “ring-porous hardwoods”. There is also an intermediate group that we call “semi-ring-porous hardwoods” whose pore arrangement is intermediate between the two.

Red oak – ring-porous

Persimmon – Semi-Ring-Porous

Sugar maple – Diffuse-Porous

The softwoods can be divided by seasonal growth characteristics as well. In softwoods, it is the thickness of the cell walls of the tracheids that may vary with the seasons. Woods that have cell walls of consistent thickness throughout the growing season we refer to as “even-grained softwoods”. If the cell walls start out relatively thin in the spring, but become abruptly thicker later in the season, we refer to these as “uneven-grained softwoods”. This is a more subjective measure than the pore arrangement in hardwoods, and there is a wide range of overlap, sometimes even within a species.

Yellow Pine – Uneven-grained

Red Spruce – Intermediate

White Pine – Even-grained

Let’s have a look at some familiar species and see where they are classified:

Now, to the heart of the matter: How does growth rate of a particular tree affect the strength and density of the wood from that tree? I mentioned previously that conventional wisdom states that slower growth = stronger wood. In fact, this is only true for a very small group: the uneven-grained softwoods, a tiny group that includes yellow pine, red pine, and Douglas-fir, as well as a few less common species. So how did this myth become so prevalent? Well, if you were framing a house or a barn somewhere in the U.S. in the last 150 years, chances are very good that the lumber you were using was yellow pine or Douglas-fir.

The reason for the increase in strength with a decrease in growth rate is simple: the cell walls of the tracheids get thicker (and thus, denser and stronger) as the season goes on. The light wood in the two photos below is known as “earlywood” since it is produced in the spring and early summer. The darker wood is “latewood”, produced in the late summer and fall. Notice that the width of the latewood in these two specimens of yellow pine is about the same, despite the fact that the earlywood is much wider in the second sample. Since the majority of wood in the sample on the right is earlywood, the whole piece will be weaker as a result.

Slow-grown yellow pine still has a wide band of dense latewood…

But fast-grown yellow pine has a wide band of earlywood and a comparatively narrow band of latewood.

There is a limit to this correlation, however. When the rings get spaced very tightly, the latewood bands begin to get considerably narrower and the wood actually begins to get somewhat weaker and less dense. This is especially true in Douglas-fir and to a lesser extent in yellow pine. So the next time you’re picking out some yellow pine or Douglas-fir framing lumber, keep two things in mind: you want a slower growth rate, and wide bands of latewood. These two factors will be the two most important in determining the strength of the lumber (aside from knots and defects). As an added bonus, slower-grown wood tends to be more stable and workable than fast-grown wood.

What about the rest of the softwoods? Well, for the vast majority, including white pine, spruces, cedars, hemlocks, baldcypress, and true firs, there is absolutely no correlation between growth rate and density or strength. That’s not to say that density doesn’t vary widely within these species. Some of them – hemlock and baldcypress are two that come immediately to mind – can be as light as white pine or nearly as dense as yellow pine, but the dependent variable is simply not growth rate. It can still be advantageous to use slower-grown wood. It will be more stable and consistent, but it will not be stronger.

White pine density does not vary much throughout the year…

So fast-grown wood is as dense as slow-grown.

And what of the hardwoods? Diffuse-porous hardwoods and semi-ring-porous hardwoods, like even-grained softwoods, display no correlation between growth rate and strength or density. So don’t worry about how fast your maple, cherry, birch, or walnut was growing. It might change the appearance of the wood, but it won’t affect the strength.

The ring-porous hardwoods, on the other hand, pose an interesting conundrum. They actually get stronger as the growth rate increases. The reason for this is intuitive once you understand how ring-porous hardwoods actually grow. The earlywood layer consists mostly of pores – big, hollow tubes that mostly contain air. Unlike conifers, the earlywood in ring-porous hardwoods tends to be about the same width from tree to tree regardless of the growth rate. The strong, dense latewood band that is produced in the summer and fall  tends to get much wider in fast-growing trees. So we can think of ring-porous hardwoods as sort of the opposite of uneven-grained softwoods.

Slow–grown red oak has a wide band of earlywood pores and a narrow band of latewood…

But fast-grown red oak has a much wider band of dense latewood.

So should you look for fast-grown wood if you’re using a ring-porous hardwood? Well, it depends what what you’re using it for. If you’re making a baseball bat from ash, an axe handle from hickory, or a steam-bent oak rail for a Windsor chair, then absolutely, your preference should be for faster-grown wood. But keep in mind that oak and ash and hickory are already quite strong and dense, so if you’re building a normal furniture piece where strength is not a critical issue, the slower-grown wood will be easier to work and still plenty strong (and possibly more attractive, depending on your definition of attractive).

Thanks to The Wood Database for the excellent pictures. Go visit the website for a tremendous collection of pictures and wood property information.