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Cordyline australis
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This article was published in The Fringe, February 2016

Cabbage Tree - Ti Kouka

The spring flowering of cabbage trees in 2015 turned into a bumper crop with the massed bunches of flower spikes almost obscuring the foliage. Records over many years show such bumper flowering periods happen every 3 - 5 years with particularly large crops every decade or so.

Rather than foretelling the weather pattern ahead, such flowering probably indicates the weather of the summer and autumn just past. It takes an especially sunny, warm and wet period followed by a mild winter without damaging storms or pest explosions for the trees to produce their best displays.

An understanding of the growth structure of ti kouka is required to realise how the flowering occurs. The young cabbage tree has one growing apical tip which remains dominant for 9 or 10 years. After growing to 3-4m with a stem diameter of about 10cm the tree has built sufficient food reserves to allow diversion of some energy into producing flower spikes. The spikes are terminal ones, in that they grow directly out of the growing point and stop future upward growth of the leaf bearing tip. Some months after flowering, new growing tips emerge from below the spike and proceed to grow upwards at a 45 degree angle from the vertical. This establishes the formal and regular pattern that often results in the symmetrical head of branches atop the single trunk characteristic of our iconic cabbage tree. It can be three years before those two new shoots have stored sufficient energy  to flower again, once more producing two or more new shoots per branch resulting in four flowering heads which in turn result in 8 heads then 16, 32 and so on.

Those flowering spikes are complex structures with up to 10,000 flowers per spike, with each flower resulting in one fruit, and each fruit containing up to a dozen seeds. Do the maths and multiply it by the 20 or 30 flower spikes and each tree can be producing a million seeds in a good year. Just imagine the ecosystem support services required to pollinate those flowers and distribute the seeds.

It is a huge functioning system that we are, for the most part, oblivious to, and yet it functions smoothly, year after year; not just for cabbage trees but all plants - everywhere.

Of course, man's activities can prove disruptive to these natural systems and the introduction to New Zealand at the end of last century, probably by accident, of a small delta winged 'passion-vine hopper' was seemingly the cause of 'Cordyline Sudden Decline' disease. An Australian insect, it arrived in New Zealand sometime in the 1920s, but did not impact significantly until its population exploded in the 1970s.  The disease was noted during the 1980s and its spread southwards seemed to follow the dispersal of the passion-vine hopper (Scolypopa australis). The insect population seems to have stabilised in recent years, and the dying cabbage trees can still be seen. But the resurgence of healthy young plants suggests they are more resistant to the disease, and in the future we may again see trees with trunks a metre in diameter with huge branching heads sporting hundreds of flower spikes.

The full story of cabbage trees has been well told by Philip Simpson in his book 'Dancing Leaves' published in 2000. It is an amazing story and gives a much greater appreciation of the strength and values of this amazing tree and its qualities of resilience, its role in a wide range of ecosystems, and its importance to both ancient Maori and today's modern world.


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