After seeing the interest on GardenDrum about my earlier post on grafted flowering gums, I feel there needs to be some follow up on the subject. A lot of comments generated by that blog suggested to me that many gardeners have had very mixed results with these plants.
The process of grafting is an age old technique that goes back thousands of years in Asia and Europe and can provide a variety of advantages to a plant grower. For instance, in grape growing the use of Phylloxera- (a root aphid that devastates vines) resistant rootstocks has restored viability to the commercial production of grapes in many parts of the world. In other cases such as citrus and roses, tough rootstocks provide resistance to root rotting fungal diseases that can kill or severely impair plants.
Grafting also makes possible the propagation of many woody plants that may be difficult to propagate otherwise, such as by cuttings. Many of the variegated forms of northern hemisphere deciduous trees such as claret ash fall into this category. Another function for grafting is the production of weeping standard plants where a prostrate form of a species such as weeping flowering cherry is grafted onto a tall rootstock to create a cascading plant.
In the case of flowering gums the purpose of grafting is twofold. First, it is about making it possible to propagate them successfully on a commercial basis. They are almost impossible to propagate routinely by cuttings and so grafting makes it possible to routinely multiply a superior selection. Secondly, the Western Australian flowering gum (Corymbia ficifolia) does not perform well in the hotter, more humid climates of places like Sydney and Brisbane and grafting them onto species such as spotted gum (Corymbia maculata) theoretically should make them more vigorous in these areas.
The problem for me is that I am seeing a lot of problems with grafted flowering gums in gardens and parklands and the supposed advantages of grafting do not always meet with the realities of real life situations. I would love to get feedback from you if you have had either a good or bad experience with grafted flowering gums so we can build knowledge base on the subject. My hunch is that there are several contributing factors so if any of these ring a bell with you then it would be fantastic if you could send in a photo or a comment outlining your experience.
The first thing to look for when there are problems with the graft union of a flowering gum is abundant suckering of the rootstock (see photo above). Other signs are an overall lack of vigour in the tree as well as dieback in the shoot tips. These problems can manifest in the first months after a graft is done but they can also take years to show up such that a tree may grow quite well and then all of a sudden the graft union deteriorates and the tree shows symptoms such as those listed above.
My suggestion to avoid problems is to look for young trees that are not showing any signs whatsoever of suckering from the rootstock and also ones that have a graft union that is evenly matched in diameter between the rootstock and scion and has signs of vigorous growth on top of the tree. Also ensure that there is no sign of the tree being root bound by asking a sales assistant to remove the tree from its pot so you can check the root system.
[Postscript – GardenDrum reader Jeff Howes in Sydney has kindly sent in a photo of his red-flowering marrii from Western Australia, Eucalyptus calophylla var ficifolia – see Jeff’s comments below about growing this as an alternative to a grafted form.]
[You can now learn all about pruning your flowering gum in a new post by Angus, How to prune a flowering gum]