Wednesday, 20 March 2013

Wednesday wildflower: Banksia integrifolia

Many of our most widespread New Zealand invasive plants came here as weeds, maybe as seed impurities, or in soil or agricultural products.  A majority came from Europe, where the new people came from and where climates and land use practices are similar.  Others were deliberately brought, like gorse and barberry as hedging plants.  But as people started to realise the risks of allowing invasive plants into the country, our controls have tightened, and now it's pretty hard to deliberately or accidentally get something across the border.

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.
Weed naturalisation often takes a while.  There may be a period of decades when botanists find the odd few plants, but it's not certain whether full naturalisation is taking place.  But then, in some, there's a sudden increase and they quickly become common and widespread.  Hieracium spp. did this.  Maybe the slow start is caused by low genetic diversity, lack of pollinators or dispersers, or maybe there's a change in land use (irrigation, fertilisation) that suddenly suits the invader.  In some, it's possible that the establishing population might evolve (allele frequencies change in the establishing population, due to natural selection or perhaps genetic drift).

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
I visited Zealandia today and took these photos.  The sanctuary and the forest are doing well, even in this dry summer, and the bird life was spectacular.  The Banksia have established well, ironically quite close to a display that warns about invasive plants.  They were in flower, which is timely because the only native bird-pollinated flowers I saw were on a single rātā, Metrosideros fulgens, and that seems out of season (normally flowers early Spring).  So, it's good for the nectar-feeding birds, but let's hope the local botanists who advised Zealandia against planting Banksia were wrong about its ability to establish.  Time will tell.
An inflorescence of Banksia integrifolia
Banksia is a member of the Protea family, Proteaceae.  Its flowers are pollinated when birds are attracted to feed on the nectar that flows from small nectaries alongside the base of the ovary.  The stamens are carried on the four petals, and these curl back after the stamens have deposited their pollen on the style.  Later, usually after the pollen has been removed, the stigma expands and becomes receptive, so the style and stigma first present the pollen to visiting birds, then receive pollen in later visits.
Two flowers of Knightia excelsa, rewarewa.  Note the bands of white pollen on the club-shaped tip of the style, and the anthers now tucked away on the coiled petals at the base of the flowers. The stigma will expand at the very tip of the style, later in the life of the flower.
The native tree Knightia excelsa (rewarewa) is also a member of the Proteaceae, and also has large clusters of flowers that function in a similar way, being pollinated by native birds like tūī.


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.

1 comment:

  1. While I understand the undesirable effects of invasive weed species, I think it is possible that people are jumping the gun a bit with Banksia Integrifolia.

    In my observations, these trees are of incredibly high value to our native nectar feeding birds. This is primarily because they offer a high energy nectar source when nothing else is available during the coldest months of the year.
    Sometimes managing a highly modified environment requires a modified approach to conserve species that still have to rely on it. A high energy nectar source like Banksia integrifolia is an absolute Godsend for native birds at a critical stage in their breeding cycle.
    It would be an absolute tragedy if these trees were banned due to irrational fears of what "may" happen when in fact it may only be one extreme end of the country where they could become invasive.

    We live in a highly modified coastal environment that will never see the vast swaths of lowland flax (among other plants) that once existed supporting large numbers of tui/bellbird etc, Banksia integrifolia and its remarkable nectar production could be the very thing to bring these birds back to original numbers.