What causes root callusing? Why is it that some in some varieties some of the cuttings root readily while others develop a large amount of callus which prohibits rooting.

Incitement for root production is initiated as soon as a stem is severed from its parent. This is called wound shock.

The first reaction is the formation of callus tissue which protects the wound. Next, roots are produced.

Both the callus tissue and the roots arise from the cambium (layer of actively dividing cells found beneath the bark).

The formation of callus is a necessary preliminary to rooting, although roots do not arise in the callus, but from the cambium immediately behind it.

Rooting is encouraged by moisture, warmth (but the air temperature should be below the that of the soil), good aeration.

Hormones (especially synthetic auxins like IAA, IBA and NAA) assist in the formation of roots, especially when the soil temperature is at least 15ºC.

We guess that the initiation of roots in different cuttings is hormonally controlled and experiments with different hormones and/or different concentrations would be a suitable topic for investigation.

John Hewitson and Barry Meatyard

Charles Hill comments:-

Some brief research indicates that root formation from callus depends upon the amount of both auxin and cytokinin (e.g. kinetin, benzyl adeneine) present.

It may also depend upon the auxin being used. Use of a rooting powder is not always the answer because it also depends upon the amount of endogenous growth regulator present. I suspect that the variation in response is due to fluctuations in growth regulator levels and endogenous auxin levels will probably be related to the amount of damage done when preparing the cutting.

From personal experience I prefer no rooting powder and just take more cuttings accepting some will fail, I have had reasonable success in Pyracantha, Lavandula and of course roses, blackcurrants and pelargoniums with no rooting powder.