Wednesday wildflower: lilac oxalis

The war on Oxalis is unwinnable, but that doesn't stop gardeners from trying.  Over several years I think I've eliminated it from a tiny enclosed flower bed beside the front door, but in the rest of the garden it's rampant.

Oxalis incarnata, Kelburn, Wellington

O. incarnata, lilac oxalis, is probably the commonest species of Oxalis around Wellington, though we have a couple of others that will feature in coming months.  Originally from South Africa, it's found throughout New Zealand, but is less common in most of the South Island.  The leaves have three leaflets, and with their notched tips they look like the traditional shamrock.  However the name shamrock is more usually applied to one or other of the many clovers, Trifolium.

Oxalis incarnata leaves, Kelburn

It's a pest because it reproduces asexually, through little fleshy bulbils (B, below) that detach from the roots and stems to establish new plants.  Sexual reproduction in this species is thought not to happen in New Zealand.  The reason takes a bit of explaining.  Oxalis flowers come in three types, called long-style, mid-style, and short-style.  In a long-style flower, the stamens are short and mid length (C below).  In a short-style flower, the stamens are long and mid.  And in a mid-style flower, the stamens are long and short (Darwin 1877).  Pollen from each type of stamen grows best in a style of the same length, and in addition, the plants have a genetic self-recognition system, which means these flowers can't self-fertilise.  And it seems that by chance only one type, the long style form, got introduced to New Zealand.  So although they flower profusely, the seed pods don't form, and the plant must rely on its bulbils to spread (Webb et al. 1988).

Oxalis incarnata.  A, flowers; B, a bulbil; C, section through a flower, enlarged.

While many are nasty weeds, some species of Oxalis are useful.  New Zealanders know the tubers of O. tuberosa as yams, but they're not true yams (yams are Dioscorea, a monocotyledon, and O. tuberosa is more correctly known as oca in its native South America).  Oxalic acid is toxic, or at least irritant, but useful for cleaning up and bleaching mouldy timber.  A few species are ornamentals.

References.

Darwin, C. 1877.  The different forms of flowers on plants of the same species.  Murray, London.

Webb, C.J.; Sykes, W.R.; Garnock-Jones, P.J. 1988.  Flora of New Zealand Vol. 4.  DSIR, Christchurch.