Garden Tips: Softwoods and hardwoods — it’s not all about the wood

Which trees are hardwoods and which are softwoods? Botanically, a particular type of tree is classified as a hardwood or softwood based on how it reproduces. Angiosperms are plants that reproduce via seeds with a covering or seed coat. Trees that are angiosperms are considered hardwoods and most are deciduous.

Softwood trees are gymnosperms that bear uncovered or “naked seeds” in cones. Most gymnosperms are needled evergreens. However, the classification of hardwood and softwood is not an indication of the hardness or softness of a tree’s actual wood.

Xylem cells are produced by the cambium layer of cells located beneath the bark of a tree. Xylem is produced on the inside of the cambium towards the center of the tree. In angiosperm trees, xylem vessel cells are elongated and arranged end-to-end longitudinally. They serve as pipelines for the transport of water and nutrients upward to plant leaves. Ray cells are another type of xylem cell. They are arranged in columns perpendicular to the vessel cells and their function is to transport water and nutrients laterally within the xylem.

When you cut down a hardwood tree, you can often tell its age by counting the “annual rings” in the wood. Annual rings are created because the cambium produces the vessel cells in rings. The diameter of vessel cells created early in the season tend to be larger than those made later. However, not all hardwoods produce rings that are easily discernible because they have vessel cells that are produced throughout their wood.

Softwoods transport water via tracheid cells that differ somewhat from vessel cells. While different, they also tend to be larger in the spring and smaller later in the season, creating annual rings.

The newest xylem cells in trees are referred to as “sap wood” because they are transporting water and nutrients up the tree. As they age, xylem cells stop transporting water and become filled with air. At this point they become part of the “heartwood” in the center of the tree. The purpose of heartwood is the mechanical support of the tree. So why am I providing this brief botany lesson? It is because knowing about xylem in woody plants helps explain some things gardeners should know:

1. A tree that has a great deal of internal rot can still appear fairly healthy because the xylem cells in the sapwood are still working to transport water and nutrients. However, a tree that is rotten on the inside can become a hazard because the tree’s structural integrity has been compromised.

2. There are a few tree borers that feed just beneath the bark of trees, but many go deeper into the heartwood. Systemic insecticide applied to the soil and taken up by the roots will be ineffective against borers that are feeding within the heartwood. This is because heartwood xylem cells are dead and do not transport water. A systemic insecticide can not reach the heartwood tissues on which the borers are feeding.

3. Not all hardwoods (angiosperms) are the same. Some hardwoods trees — such as oak, hickory, some maples, apple, and walnut — have wood with a higher density. Fast growing hardwoods, such as poplar, aspen, and willow, tend to have lower density wood.

It is interesting to note that balsa (a hardwood) has the lowest density wood of all the hardwoods and softwood trees. Ebony, one of the most prized hardwoods, comes from a tree related to persimmons. The highly prized black heartwood of the ebony tree is one of the densest hardwoods available and is very valuable.

This story was originally published November 17, 2017 at 1:20 PM with the headline "Garden Tips: Softwoods and hardwoods — it’s not all about the wood."