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THE LAST WORD

Tree healerThe trunk of a small fir tree in my garden broke, and the tree fell over. More in hope than expectation I pulled the top part back up, taped the halves back together, and screwed in metal plates to hold them in place. To my amazement the tree survived and, months later, when I removed the tape and the plates, the break had healed and the trunk was whole again. How do trees repair such trauma? Can all tree species do it?

n Well done. I would have bet against such a successful outcome, especially with a conifer. At such an injury they often produce enough resin to smother the tissues and strangle the tree.

Essentially you connected the cambium, the growing tissue under the bark, at the area of the break. It grew rapidly, joining the edges nicely. This is what one aims for when grafting a bud or stem onto a plant: joining cambium to cambium, keeping it moist, protected, and generally

supported until it has grown enough structural tissue to hold it in place and enough conductive tissue to keep the graft fed.

A word of warning: your tree has grown good supporting tissue around the trunk and, as it grows

larger, that layer of new growth will become stronger; in future years the tree should be as good as new. However, the older wood inside the break will never mend.

For now, your tree is being held up by little more than a skin of new wood. Give it flexible support for a few years in case a minor bump or strong wind breaks it again. Rigid support discourages the growth of strong buttress tissue.Jon RichfieldSomerset West, South Africa

n Your questioner has rediscovered the principle of grafting. Not only can most trees do this but, given the right circumstances, other plants can too. Many fruit trees and ornamental trees and shrubs, including roses, are propagated through grafting; even some vegetables, such as tomatoes, can be grafted.

Grafting is often used to combine desirable qualities from the rootstock, such as dwarfing or disease resistance, with desirable attributes from the scion – the branch or stalk being added – which might include fruit quality or desirable flowers. It can be an effective method of propagation. It also occurs naturally: where stems or roots of the same species grow together they will unite.

Two points are essential for successful grafting: the cambial layers of the scion and rootstock must be brought together; and both halves must be kept alive long enough for the union to

form. Often the scion is removed from its own roots and, in practice, keeping the scion alive means preventing it from drying out. In the case of the questioner’s tree it sounds as if the trunk didn’t break completely, so putting the top back in place would satisfy the first requirement and probably

enough of the vascular system remained functioning for the natural supply of water to the top to continue.

The process depends on the fact that, generally, mature plant cells in vegetative tissue have a much wider ability to grow and differentiate than do animal cells. So once the grafter has put the two halves in place, new tissues form that bind the parts together. That said, where large grafts are concerned the vascular tissue will often be more effective in regeneration than the internal wood, so a graft can remain a weak point.

Grafting has other uses too. In my career as a plant pathologist I have used several versions to transmit plant pathogens that are difficult or inconvenient to spread in other ways.

Grafting is not always successful, but the reasons may not necessarily be the obvious ones of the scion being the wrong size or drying out. In walnuts, blackline disease, which causes

some trees to die several years after apparently successful propagation, is due to a virus in one part causing a necrotic reaction in the other part as it moves across the graft union.

The bottom line for gardeners is that when a stem or branch breaks in a tree or shrub you would rather keep, you don’t need to immediately dig it out and rush off to buy a replacement.

Do as the questioner did and consider trying to repair it by self-grafting.

If you want to know more about grafting, any edition of The Grafter’s Handbook by R. J. Garner, first published in 1947, contains a wealth of information.D. J. BarbaraWellesbourne, Warwickshire, UK

This week’s questions

STrange TidingSSailors in coastal waters say that when the tide changes, more often than not the wind direction changes too. What links these two events, one of which is astronomical and the other meteorological?John FranklinLondon, UK

Brrrrrrrrrrrr!Why do we have an instinctive tendency to hunch our shoulders when walking outside in cold and rainy conditions? Stephen BarkerUK

“The older wood inside the tree’s break will never mend. For now it is held up only by a skin of new wood”

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“grafting can be used to transmit plant pathogens which are difficult to spread in other ways”