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Eucalypts by Cuttings

Christian Narkowicz

When discussing the propagation of Australian plants by cutting, there appeared to be a law which stated "thou shall not propagate eucalypts by cutting", and I presumed this to be true However, a few years ago I saw an article on the propagation of eucalypts in India, and they showed a photograph of a nursery with the caption slating that the juvenile plants were from cuttings. Surely this must have been a mistake! More recently I came across a letter in Australian Plants online from a resident of the Netherlands requesting information on the possible propagation of eucalypts from cuttings. Why would a Dutchman be interested in such a thing? Hadn't he heard the law of eucalypt propagation?

Intrigued, I did a search on the Internet and found a reference to Dr Ruad of the CSIRO, who is working on vegetative (tissue culture) eucalypt propagation. Dr Roud provided some references and I subsequently found a book entitled Eucalypt Domestication and Breeding, Eldridge K et al (eds), Clarendon Press, Oxford (1993), from which most of the following information has been extracted.

It is believed that the first successful, deliberate, rooted cuttings of eucalypts were achieved in Canberra in 1948 using shoots from seedlings of various species, in greenhouses with bottom heat and mist sprays Vegetative propagation was also discovered by accident in Morocco, with discarded prunings of Eucalyptus camaldulensis seedlings fallen on the ground striking roots, under favourable warm, moist conditions, after a few days. Following these observations more formal trials were conducted. It was discovered that many eucalypt species could be propagated by cuttings of seedling material, up to about node 15.

The observation of aerial roots in mature E.deglupta and E.robusta suggested that these species may readily strike from cuttings. Work by J Davidson in Canberra revealed that E.deglupta cuttings would strike from crowns taken from trees five years old and older, from new growth of scions taken from mature trees grafted onto seedlings, and from coppiced mature trees.

The success of plantation-grown eucalypts, often commercially superior hybrids, propagated by cutting, ensured continuing research into vegetative propagation in Africa, South America, Europe, Asia, New Guinea and Australia. Propagation by cuttings is increasingly the method of choice for plantation eucalypts.

So, why the belief that eucalypts cannot be propagated by cutting? Well, there are many difficulties with the procedure. The main problem seems to be getting the right cutting material. New growth from young plants is essential for most species that will strike. Growth from older parts of a tree may be difficult or impossible to strike. It was found that in most species adult trees, over five years old, produce a rooting inhibitor which prevents cuttings from striking. The position in the crown where the shoot is taken is also important and influences vigour and habit.

Most eucalypts will strike from material taken from seedlings, however it is often desirable to select characteristics from a mature tree. It is possible to bring about juvenile growth by several different methods. Cutting down the tree leaving a stump of about 20cm is one way. New coppiced growth is of a juvenile nature and suitable for cutting. Another technique is to rejuvenate the growth by repeated cuttings, with the striking rate increasing with each generation. A further technique is repeated grafting. Material from a mature tree is grafted onto a seedling (preferably from the same parent), with new growth repeatedly regrafted until juvenile growth occurs, from which cuttings can be taken.

In tropical climates cuttings root and grow quickly, with plants ready for planting out two to three months. In temperate climates (eg. France) it takes longer. In France cuttings are taken throughout the year, other than in mid-winter. A typical cutting may be about 30 cm long, with 2-4 nodes (4-8 leaves). Cuttings are treated by immersion in a fungicide solution (such as Benlate, 6 gm/10 litres water) and drained. Hormone treatment is typically a light dusting of the extreme basal end with 1% IBA. Potting medium may be any sterile, free-draining, moisture retaining medium. Sand, vermiculite, aged sawdust, milled pinebark and pumice have all been used successfully. Cuttings, individually potted, are kept under polythene covers in a plastic greenhouse. The temperature of the substrate is kept at 20oC. Lower temperatures will retard root growth and higher temperatures may encourage rot. Medium is treated with fungicide on a weekly basis. Slow release Osmocote would be a suitable fertiliser. Ambient temperature does not require strict regulation but should be kept below 30oC. A minimum of 200 hours of light per monthis desirable, and 400 hours per month is better. This may be achieved with artificial lighting, if necessary.

So why would a gardener want to grow eucalypts from cuttings? Perhaps there is a magnificent flowering gum or an unusual colour form of some other eucalypt you would like to (try to) propagate, with an assurance of identical flower colour. Perhaps you would like to do it just to see if you can.

And why did the Dutchman want to grow eucalypts from cuttings? Perhaps he was crazy but he wanted to breed cultivars, subject them to freezing Dutch winters (as low as -30oC) and then propagate the survivors vegetatively. I sent him some seeds of E.coccifera, which may be a good start.

Further information is available from this article in the June 1997 issue of Australian Plums online (originally published in the Eucalyptus Study Group newsletter).

From Eucryphia, the newsletter of the Australian Plants Society (Tasmania), September 1997.

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