Planting trends in our streets and parks
by John Fitzgibbon (Metropolitan Tree Growers) and Peter May (May Horticulture Services)
Most of the native trees being sold into the urban landscape market in Australia are still seedling grown. This means that plant-to-plant genetic variation is part of the deal. While genetic variability is important in some applications, there is a strong argument that some aspects of urban tree planning would be enhanced by having access to clonally propagated stock, allowing selection of desirable characteristics. Over the past few decades, some selection of clonal material has occurred, especially in species that are readily propagated by cuttings (e.g. Callistemon).
In the past few years we have seen an increase in the availability of clonally propagated native trees, especially trees that have been grafted. The most obvious of these has been the range of cultivars of Corymbia ficifolia and C. ficfolia X C. ptychocarpa hybrids that have appeared.
There seems to be two basic approaches to development of cultivars of some of these species. One of these is to identify trees that have particularly outstanding attributes that warrant their selection and propagation. This is typical of much cultivar selection in Australia until fairly recently.
The other approach is to evaluate potential cultivars for more functional criteria. While these cultivars may not necessarily be particularly showy, they may meet the needs of the arborist or street tree manager by meeting their requirements for more consistent performance or reduced tendency to structural faults. An analogous situation exists with the development of cultivars of Pyrus calleryana where over a period of time a number of cultivars with useful functional attributes (e.g. Aristocrat and Chanticleer with better branch attachment than older cultivars or with the columnar habit of Capital).
A search of nursery catalogs shows that grafted cultivars of Corymbia ficifolia Corymbia citriodora, Eucalyptus cladocalyx and Eucalyptus leucoxylon are available in the market place. These cultivars are generally showing predictable flower colour, flowering at a younger age, dwarf stature or coloured foliage or some combination of these attributes.
Nurseries have been experimenting with, and using, grafting as a propagation method for several decades and a few cultivars like the variegated form of Lophostemon confertus have been in the market place for many years. What the increased apparent success with grafting means is that clonal selection is going to be aided by having access to a method that allows material to be collected from adult trees and put into cultivation for evaluation. While preferable protocols such as the use of cuttings or micropropagation may not yet exist for these plants, having cultivars in use and being trialled will build a base that will encourage further exploration of possible alternatives.
Metropolitan Tree Growers have started to investigate the potential for clonal selection in Eucalyptus leucoxylon. Several people, notably Dr David Beardsell, have selected desirable forms of this species and these are now available from propagators. In Melbourne Humphris Nursery have been instrumental in this. Dr. Beardsell’s selection “Horsham” is a case in point. In addition to those selections, Metro Trees have made a number of selections from street and reserve plantings where individual trees showing exemplary form or character have been identified. Humphris Nurseries have started grafting these selections. One of these, a form of E. leucoxylon var. leucoxylon has been successfully grafted and nursery experience of it should be gained over the coming summer. A number of other selections have been made this year and these will be grafted over the coming summer. These trees are expected to have more predictable branching patterns and be better suited to urban use than are many current seedling lines, for example the poor form shown in Figure 1. At this point it is not known how many of these selections will find permanent places in urban landscapes as this process is still in its infancy.
Metro Trees have also begun collecting selected forms of Geijera parvifolia and Lophostemon confertus for inclusion in our grafting program. Other commercial lines being grafted in other nurseries at present include selections of various Brachychiton species and hybrids, taller Grevillea selections and arid zone Hakea species.
In the meantime, how should you as arborists approach these “new” plants being put out into the market place? One of the concerns is over the life span of the graft unions of these trees. The evidence available to us suggests that this should not be a major concern to users as grafted native trees of various species have been used in landscapes for several decades now without significant problems. As the range of plants being grafted continues to increase however, this is an aspect of performance that must be continually reviewed. Another issue is going to be that of keeping track of new cultivars as they are released. The number of C. ficifolia cultivars is growing daily it seems and not all of them will find long-term roles. This is almost certainly going to be true of other species as well. What is clear though is that this is an opportunity to improve the quality of some of our most important trees and the real benefits of these improved selections will only become apparent once they are being used and evaluated in the real world of the street and park.