Modern findings on effective wound treatment after pruning focus on the technical term “callus”. Instead of sealing every cut with wax, prudent home gardeners leave this task to the self-healing powers of shrubs and trees. This guide gets to the heart of the function callus takes on in the regenerative process.
What is callus in woody plants and what function does it have?
Callus is a bulging tissue that arises from undifferentiated cells at the edges of cuts and injuries in woody plants. It forms from the cambium and helps the plant to heal wounds independently by covering the open area and thus protecting against fungal spores and pests.
Callus – botanical definition with explanations
Bulge-like growth of new tissue from undifferentiated cells at the edges of cuts and other injuries to woody plants. As it progresses, the wound expands from the outside to the inside.
It is characteristic of callus that the tissue forms directly from the thin cambium ring, which is located below the bark and bast, as shown in the figure below.
Branch cross section: 1 heartwood, 2 sapwood, 3 cambium, 4 bast, 5 bark/bark.
How callus contributes to wound healing – the process in detail
On woody plants, any injury puts the exposed cambium on alert. Fungal spores and pests lurk in every open wound on bushes and trees as an ideal target. Only rapid wound healing reduces the latent risk of infection. In order for healing to occur on its own, the cambium tissue undergoes a transformation process and becomes callus, which quickly covers the wound. The following overview explains the individual phases of this exciting process:
- First phase: Callus formation in the form of irregularly shaped tissue cells as a small bulge along the edges of the wound
- Second phase: Soft tissue forms bark tissue towards the outside, wood tissue towards the inside
- Third phase: Fresh tissue overflows the wound from all sides
Smaller cuts are quickly covered with callus tissue. However, for larger injuries, the process can take many years. As soon as the edges of the wound meet in the center, the damaged tissue underneath is sealed off from the air supply. Any fungi and pests that may already be established die.
The inward-facing tissue cells close open vessels and form tannins. If rot and mold have already spread, these problem areas are separated from the rest of the tree by the tannin barrier.
Wound closure torpedoes callus function
We have the father of modern tree care, Alex Shigo, to thank for his knowledge of how callus works. This goes hand in hand with the logical conclusion that any wound closure counteracts the self-healing process of woody plants. Since then, the use of impermeable sealants for wound treatment after a tree pruning has been frowned upon - with one exception:
If a wound is caused on the tree or shrub in the middle of winter, the exposed cambium should be protected from frost damage. To do this, smooth the injury with a knife. Then coat the edges of the wound thinly with tree wax (€11.00 at Amazon) to cover the valuable dividing tissue under the bark until next spring. This procedure is recommended for cuts with a diameter of 2 centimeters or more.
Tip
So that cambium can go through the conversion process into callus unhindered, branches are always cut to astring. The bulge between the branch and the trunk contains the valuable cambium tissue in high concentration and must therefore not be injured.
