So where are the weeds of the future going to come from? Many are already here, as garden plants. Perhaps 20,000 species are cultivated as ornamentals or crops. Many are tender and hard to grow, and it's unlikely these will escape. But if the climate changes, some subtropical or dry-climate plants might be able to propagate and spread without human help, becoming new weeds.
|An infestation of Hieracium lepidulum, near Arrowtown, Otago.|
Banksia integrifolia is a weed that seems to be in this early establishment phase. It wasn't included in the Flora of New Zealand volume that deals with naturalised "dicotyledons" (Webb et al. 1988). I remember collecting a wild specimen on Great Barrier Island in 1990. Whether or not that was the first wild record, nowadays this small Australian tree has been picked up in quite a few localities. I collected my second specimen a few years ago growing just outside the predator-proof fence of the Karori Wildlife Sanctuary (now Zealandia). The authoritative plants database of Landcare Research is the best source for plant data because it's collection based (therefore verifiable) and it incorporates the expertise of the professional taxonomists there: they record specimens of Banksia integrifolia from Northland, South Auckland, Taranaki, Manawatu, Wellington, and Nelson.
It's possible Banksia integrifolia is a new woody weed in the making, which will take off shortly and become a problem. However it is a fire-adapted species, needing fire to split the woody seed capsules and release the seeds in large numbers, and being a tree, it should be easy to eradicate before it flowers, unless there are large numbers. This is a plant to be viewed with caution, so a number of Wellington botanists were concerned when Zealandia planted young trees inside the sanctuary because their compact inflorescences could provide out-of-season nectar for birds like tūī, hihi, and bellbirds.
|Banksia integrifolia at Zealandia|
|An inflorescence of Banksia integrifolia|
Leon Perrie commented: "That's amazing that Banksia integrifolia isn't in the 1988 Flora IV, given that adventive populations are now so established and widespread. The New Zealand Virtual Herbarium has over 130 specimens, most of them probably of wild plants. I think this species can definitely be put in the weedy category! I've seen it spreading prolifically in the Northland 'gumlands' (along with Hakea). Also a few spots around Palmerston North."
Mike Bayly from Melbourne tells me the plants there (subsp. integrifolia) probably don't need to have a fire to open their fruits; I guess warm dry weather is enough. He's seen unburned plants with open fruits.
These two comments indicate I underestimated its current establishment and its potential to be an aggressive weed in my post.
Webb, C.J.; Sykes, W.R.; Garnock-Jones, P.J. 1988. Flora of New Zealand, Vol. 4. Botany Division, DSIR, Christchurch.