Oxalis--TOW
Mary Sue Ittner (Mon, 10 Nov 2003 14:50:39 PST)

This introduction for the topic of the week from Robin Attrill got filtered
out by the our server and I have no clue why so I am forwarding it on.

Mary Sue
Dear all,

Amongst many gardeners Oxalis tends to be rather stigmatised by the
occurrence of a few weedy taxa amongst the many hundreds of species which
constitute the genus. Fortunately there are also a very large number of
desirable species which merit a place in cultivation. The majority of
these are geophytes, predominately native to Africa and the Americas, and
it is these plants that I will introduce in this weeks TOW.

The most extensive number of species, and diversity of form, occurs in
South Africa, principally in the winter rainfall regions. These plants,
which flower from August through to April in the northern hemisphere, occur
in an extraordinary range of habit, leaf shape, and flower colour, examples
of which are illustrated on the PBS Wiki at
http://pacificbulbsociety.org/pbswiki/index.php/…. We are indebted to the
late Michael Vassar, whose enthusiasm for these plants, and generosity in
distributing them, has done much to popularise oxalis as a whole. A
selection of desirable, and easily grown, species includes O. orbicularis
(early flowering lilac blooms over very attractive foliage with purple mid
ribs and fringes to the leaflets), the O. flava complex (very variable leaf
shape and flower colour, typically yellow but also white and lilac), O.
hirta ( variable flower colour from almost crimson, in the form
'Gothenberg', to almost white), O. versicolor (white flowers with crimson
on the reverse giving a candy stripe effect when in bud), O. massoniana
(orange with a yellow centre), and, towards the end of the season, O.
obtusa which possesses probably the most variable flower colour of all with
the range encompassing pinks, oranges, yellows and white. I find the best
means of cultivation is in pots, kept dry in summer then thoroughly watered
in autumn after which they are watered on a needs basis throughout the
growth period during which they are kept frost free (minimum 5 celsius).
The potting medium should be well drained, and not too rich otherwise
foliage will predominate at the expense of flower. These plants require
excellent light to prevent etiolation. If plants are grown in open ground
those species which produce bulblets at ground level (which disperse
readily) should be sited with caution.

A second group consists of the 'alpines' from southern South America. Three
species are widely available and generally not difficult to grow - O.
adenophylla, O. enneaphylla and O laciniata - all of which are very
worthwhile either in pots or in a rock garden. These come into growth in
late winter and flower in spring and early summer. A considerable number
of species occur further north in the Americas but rather few of them are
in cultivation.

Identification of Oxalis tends to be a rather difficult process due to the
paucity of recent literature. As mentioned by Dave Victor in a posting a
few weeks ago, there is a Checklist, produced by Richard Clifton of the
Geraniaceae group (ISBN: 1-899742-43-3), which assists in sourcing the
literature associated with the named species but it does not contain keys.
The only handbook on the Southern African species, by Salter, is almost
sixty years old - a reprint was produced by Rainbow Gardens Bookshop a few
years ago but I do not believe it is still in print. Copies of the original
are virtually unobtainable as the print run was very small. Despite these
difficulties many of the un-named plants in cultivation do have associated
collection/accession numbers - particularly those originating from the
Vassar collection - which can be used to provide information on their origin.

Sources of Oxalis material are widespread but rather few in number. In the
USA they include Telos Rare Bulbs(http://www.telosrarebulbs.com/index.html)
which offers a good selection, and the PBS exchange lists contain some
excellent plants from time to time. In the UK Monocot Nursery lists a
small but interesting selection. Remarkably -and unfortunately! - the
number of suppliers in South Africa is extemely limited but Cape Seed and
Bulb (http://www.clivia.co.za/) offers a small selection. In contrast to
bulbs, seed is rarely available - a combination of generally limited
viability and exploding seed capsules makes it a rather difficult
proposition - but the Flores and Watson seed lists sometimes contain
interesting South American species.

In summary I believe Oxalis to be a genus with great potential. The plants
require little space and are generally easy to grow. They provide a
beautiful flowers throughout the year in a range of colours that is
difficult to match amongst the monocot geophytes. Why not try them????!

Regards

Robin Attrill