Greater numbers of trees are becoming weedy in the urban environment. Weeds create specific management and budget issues for the urban landscape manager through staff labour, waste removal and, in some cases, genetic problems within remnant vegetation. The Australian Quarantine and Inspection Service have tightened the list of new taxa allowed for import as garden escape weeds become more problematic. Concerned plant growers are closely examining their production lists and are removing specific plants that pose high environmental risks. We believe that careful plant selection will give a wide palate of plants that do not pose the high weed potential of some culprits.
Weed potential is the danger. This potential can be identified as the ability of a plant to thrive in a site, produce viable seed, and have that seed germinate and grow. Many plants from a similar climate to south-eastern Australia will thrive here, and can easily become weedy. Lavandula stoechas the various Hypericum species and especially Echium plantagineum (Patterson’s Curse) show these characteristics clearly. There are also Australian plants, brought in from far-off areas of this continent, that have become weedy in south-eastern Australia ; Melaleuca hypericifolia is a major weed of Victorian coastlines. If we were able to find a sterile form of L. stoechas, then we could use this plant without the worry about its spread.
The Darebin Parklands adjacent to Metropolitan Trees in Alphington has a large population of weedy olives that have seeded from original plantings from the late 1800s. Weediness of olives has been seen extensively through South Australia and Victoria restricting the growth of endemic flora and increasing maintenance costs for this indigenous vegetation. Using low-fruiting and sterile forms of olives in the streetscape or parks can help to eliminate weed risk in native plantings. For many decades ‘Swan Hill’ Olive has been used extensively within the USA as a non-weedy selection due to its pollen-less and fruit-less qualities. A more recent, commercially available, low–fruiting option is the Olea europaea ‘Tolly’s Upright’. This tree comes from Renmark , South Australia and is currently being planted extensively throughout Melbourne . Both of these olives allow the urban manager a highly drought tolerant, smaller growing tree that poses no-to-low risk of weediness.
Melia azedarach (White cedar) is commonly seen in urban plantings and has a naturally occurring distribution from south-east Asia through the Asia pacific and into Australia . Many consider fruit set to be a significant problem, as both a trip hazard and because of its poisionous nature. In Melbourne Melia has weediness potential along our waterways, with seedling germination along the Merri Creek catchment as one example. For many years Adelaide arborists have complained of White Cedar weediness in their city, but plantings have continued because of its drought tolerance, small size and aesthetic value. In 2006 Metropolitan Tree Growers Pty Ltd will have about 1000 sterile White Cedar available. This selection was discovered in a local streetscape. The flower forms to a bud without opening, resulting in no pollen and ultimately no seed. The tree has a similar umbragenous (umbrella-like form) as most Melia azedarach and will develop to a similar eventual height (to about 10m) and spread (to about 8-10m).
Australian vegetation can also be weedy when placed away from its endemic range. For example, Eucalyptus nicholii has been found to be weedy through the Dandenong Ranges . The majority of E. nicholii was used within the Melbourne streetscape as a smaller tree under powerlines, although in many cases its eventual mature height exceeded what was predicted. Suitable replacement trees could include the Austraflora selections of Eucalyptus leucoxylon ‘Eukie Dwarf’ and E. mannifera ‘Little Spotty’ that grow to approximately 7m in height and locally endemic E. radiata growing to 10m.
The commonly planted ash selections (Fraxinus spp.) are well known as weeds. For decades Fraxinus oxycarpa (Desert Ash) have been problematic along waterways, parklands and in some unusual cases the streetscape. Fraxinus ornus (Flowering or Manna Ash) also seeds heavily, creating significant risk as a weed. A very successful non-fruiting selection for Melbourne Streets has been the Claret Ash (Fraxinus ‘Raywood’). Unfortunately they have shown decline over recent years with the extended drought and a proported viroid infestation. The newly imported Fraxinus pennsylvanica ‘Cimmzam’ and ‘Urbdell’ have shown to be either seedless or very low seeding in the USA, proving to be better selections for the streetscape, especially as Fraxinus ‘Raywood’ is no longer being used.
Most sterile forms of trees are more expensive than their reproductive relatives. Many Australian trees, including White Cedar, are seed grown, but the non-fruiting form must be budded. As the propagation costs increase, the overall tree cost also increases. The minimal increase in tree costs is far outweighed by the benefits to the broader landscape.
What Landscape managers can do to reduce tree weed risk?
- Look for low fruiting or sterile selections of those trees you wish to use.
- Selectively place more weed prone specimens away from sensitive areas of remnant vegetation and waterways where tree weed seeds can be more widely distributed.
- Support those Nurseries actively importing low-weed potential alternatives and developing non–weedy substitutes