The New Zealand Plant Conservation Network

What causes collapse of established flowering Phormium tenax? Not all have collapsed just some.

HI Elaine sounds like FLAX DIEBACK from http://nzpcn.org.nz/flora_details.aspx?ID=2219
'Yellow-leaf' is one of the most serious diseases of harakeke (similar to the 'sudden decline' in cabbage trees). The disease is characterised by abnormal yellowing of the leaves. Scheele (1997) described how "growth of young leaves may be stunted and eventually the whole plant may collapse. Underground, the roots die off, the rhizome tissues collapse and rot spreads towards the crown of the plant".

The cause has been identified as being a phytoplasma (a bacterium), transmitted by the native flax plant hopper. The hopper injects the bacterium into the leaf, while sucking the sap. Yellow-leaf is found in North and South Island, but is more prevalent in North Island (Boyce et al, 1951). For more information read "Integrating New Zealand Flax into Land Management Systems" by Elizabeth McGruddy (2006).

Hi Elaine if your collapse doesn't fit 'Dieback,' P. tenax will go all floppy if it is drier than they are used to and they don't have their roots in a decent water supply. The flower spikes won't stand up again but the fans will mostly recover when it gets wetter. Also, individual fans die after flowering. It's fairly sudden, one good hot day and suddenly they have collapsed outward to half their height or worse.