Cambium

Cambium is the layer of plant tissue responsible for the secondary growth of roots and stems.

When a plant first starts to grow, all it knows is up and down. But, once it’s basic systems are in place, the plant can start filling out. This secondary growth is why tree trunks, branches, and some roots get thicker as they grow. This secondary growth is all because of cambium.

What is cambium?

Cambium is a thin layer of living tissue, found between the xylem and phloem of vascular plants, that manufactures the new cells used in secondary growth. Cambium cells are parallel to each other and they encircle the stem or trunk. The cambium layers produce secondary xylem and phloem cells.

Where is the cambium layer?

There are actually two different layers of cambium tissue and each is found where it will do the most good. But, before we explore the different types of cambium, let’s review a little plant anatomy. Working our way in from the outside, a tree trunk is made up of various layers:

1. Outer bark, or cork
2. Inner bark, or primary phloem, carries food downward
3. Cork cambium
4. Sapwood holds functional vascular tissue and vascular cambium
5. Heartwood (inactive)

Interior of redwood tree (NPS) Public Domain

Types of cambium

There are two different types of cambium. They are cork cambium and vascular cambium.

    Cork cambium produces new bark on its outer edge and it has a layer of cells containing chlorophyll on its inner surface. If you scrape the outer bark off of a twig, you can usually see a green area under the bark. This is the cork cambium layer. Potato skins are another example of cork cambium tissue. If you are ever lost in the woods and hungry, you can eat the cambium layer of most pines, slippery elm, black birch, yellow birch, red spruce, black spruce, balsam fir, and tamarack trees. [I have no idea how it tastes. At that point, you probably won’t care.] 

          Vascular cambium is where most of the width of dicots and gymnosperms is produced. [Most monocots lack secondary growth.] Vascular cambium is found between the phloem and the xylem. Vascular cambium produces xylem tissue on its inner surface and phloem tissue on the outer surface. You can see the vascular cambium in herbaceous plants as beads on a necklace when a cross-section is taken.

Cambium through the seasons

In spring, when water is more abundant, the ring produced by the vascular cambium is usually wide and light colored, while the portion of that ring produced later in the summer is usually darker and thinner. These two rings together represent one year of growth. The cambium layer is relatively inactive during cold periods, which is why plants don’t do as much growing in winter. Many gardeners take advantage of this fact when grafting new scions onto existing trees and other plants. It is the cambium layers of both plants that must connect for a graft to be successful.

Damage to the cambium

If the cambium layer is killed by fire, freezing, pests, or disease, the plant will die. Many animal pests are attracted to the moisture found in the cambium layer. Gophers, ground squirrels, voles, deer, horses, and rabbits will gnaw through the outer bark, especially during severe drought. This activity can girdle a tree, killing it. Deliberate girdling (removal of the outer bark around the circumference of a tree) is used to increase fruit yield, size, and quality, but care must be taken to not damage the cambium layer. Tree supports, left on for too long or tied too tightly can also damage the cambium layer, killing the tree.

Many insect pests will burrow their way into the cambium layer, using it as food, breeding grounds, and protection. Very often, these insects can kill a mature tree. They include bark beetles, American plum, shot hole, Pacific flathead and other borers. If that weren’t enough, freezing temperatures and fungal cankers, such as phytophthora, armillaria, crown gall, and crown rot, can also kill the cambium layer.

Sapsuckers are a type of woodpecker that drill a series of holes in the bark and cambium of trees to get at the sap and the insects attracted to the sap. 

Who (or what) is attacking your plants' cambium layers?