By Miranda Kimberley Thursday, 04 April 2013
Oxalis is a plant that strikes fear into the heart of any gardener. Most of us have suffered from the invasive nature of weedy species in lawns and borders because they can quickly spread via projectile seeds or by the huge number of bulbils that some plants produce.
Fortunately, this is not the whole story. Many species are well-behaved plants that will not spread and are also beautiful ornamentals. It is just a case of knowing which are which. If you are unlucky enough to come a cropper of the main weedy species O. corniculata, O. debilis or O. latifolia, get on top of the problem quickly using chemicals or cultural methods.
There are around 800 species of oxalis and they can be annuals, perennials or shrubs. They often arise from a tuber or bulb, but some have fibrous roots. They are found across the world, but the majority are found in South Africa and South America.
Many oxalis sold as ornamentals are placed in the alpine category. They suit the pot culture of alpine specialists because they generally have a low and spreading habit, with clumps of neat foliage and brightly coloured flowers. The generally funnel-shaped flowers come in shades of pink, red, yellow or white and close up at night or in shade.
Their foliage is usually divided into three leaflets, resembling a shamrock. Some are rounded, others triangular, and they often also fold up at night. There are some nice dark-purple or red-leaved varieties, including O. triangularis, which has purple, triangular leaflets edged with a darker purple-black around the margins and small, pale-pink flowers.
Only a few are reliably frost-hardy, such as O. adenophylla Award of Garden Merit (AGM), O. enneaphylla AGM, O. magellanica, O. depressa and O. acetosella. The woodland sorrels like our native O. acetosella and the American O. oregana will naturalise in woodland or wild gardens and prefer moisture-retentive, humus-rich soil in shade or dappled sunlight.
Smaller decorative species such as O. adenophylla and O. enneaphylla AGM make excellent rock garden plants and even suit a well-drained sunny border. They are also well shown off in raised beds or troughs, as is the dainty O. laciniata, which needs a sheltered position because it is less hardy.
Many of the species from the Cape or South America are either half-hardy or tender so could be tried in a very sheltered spot outside but are best grown in an alpine house. Grow them in pots or wider pans, in a well-drained medium.
The bulbous species should be kept dry in their dormant phase. Remember that those from South Africa go through their dormant phase in our summer. Re-pot them annually as growth resumes — in late winter for spring-blooming bulbous types, in early spring for summer flowerers and in late summer for those that bloom in autumn. Re-pot herbaceous species in spring.
What the specialists say
David Rankin, co-owner, Kevock Garden Plants, Midlothian
"The genus is a mixture of delights, disappointments and terrors. There are a few virulently invasive ones, including our lovely wood sorrel,
O. acetosella, and the yellow O. pes-caprae in Mediterranean climates. There are also many species that are not hardy.
"But some of the bulbous ones originating from the far end of South America are first-rate. They give a lot of colour in early summer and then quietly die down. The emerging foliage is a lovely colour and texture, and the flowers keep popping up. Although they are short-lived, there is a long succession of them.
"O. adenophylla and O. enneaphylla are easy, reliable and give lots of flowers. There are various forms with different colours and sizes. For a trough or small rockery, O. enneaphylla rosea, O. 'Ione Hecker' and O. laciniata are great. These bulk up to create a good mat but are not in any way invasive. They like a well-drained soil, so add plenty of grit to the planting mix, and a sunny position will encourage flowering. There are no significant problems."
Alasdair Sutherland, owner, Ardfearn Nursery, Inverness
"We grow O. enneaphylla, O. adenophylla and O. depressa as well as the choice hybrids 'Ione Hecker' and 'Gwen McBride', which both have pale-pink flowers with darker veining and blue-grey foliage. They are the result of crosses between O. enneaphylla and O. laciniata.
"All of these are either clump forming or moderately spreading and are generally hardy outside as long as they are well drained. They are all suitable for well-drained, moderately fertile soil in part/full sun. Plant them either in the rock garden or a well-drained sunny border.
"O. laciniata is much smaller and slower to grow so we grow it in either a choice spot in the rock garden, raised bed or trough. We focus on growing oxalis species and hybrids that are not invasive and have excellent garden merit, though we do make an exception for O. depressa, which can be slightly invasive if not monitored, because it is worth growing for a late summer/autumn display of flower."
John Winterson, deputy plant buyer, RHS Plant Centres
"We sell a good number of O. adenophylla and O. enneaphylla — in a 7cm pot from our alpine section. They mainly sell in flower during March, April and May, and we have increased the quantities on display.
"But the most popular one from our alpine section is O. lobata (perdicaria), also in a 7cm pot. This one is September and October flowering. It has attractive clover-like spring foliage that dies away and reappears in the autumn with many golden yellow flowers. It will grow to a height of 5-7cm and will thrive in well-drained soil in a sunny position.
"Our customers buy these as an impulse purchase and, because they are in small pots and relatively cheap, they can usually find a place for them in their gardens. They are easy to look after and require no special attention.
"As a houseplant, O. triangularis is fairly popular with its stunning purple foliage. But at present we sell more of the outdoor alpine types."
Species and varieties
O. acetosella is our native wood sorrel. It spreads using runners so is not a plant for the rock garden but for areas in woodlands, wild gardens and among trees. Produces white flowers in early to mid spring. It has a charming pink form, O. acetosella var. rosea. Height: 5cm.
O. adenophylla AGM (H4), commonly known as the silver shamrock, is an Argentinian and Chilean alpine plant. It is a bulbous, slowly spreading type whose grey-green heart-shaped leaves form circular fans of overlapping foliage. Single pink flowers that have deeper mauve-pink tips to the petals are borne in mid to late spring. It is a choice, compact-flowering plant for an alpine sink or raised bed. Height: 10cm. Spread: 50cm.
O. depressa spreads easily through its little bulbils but it is not considered a nuisance species. Fresh green shamrock-like foliage appears in late spring followed by large bright-pink flowers with yellow centres in the late summer/autumn. Looks good in a small trough. Height: 5cm.
O. enneaphylla AGM (H4) is a non-invasive, rhizomatous type that forms a low mat of glaucous leaves, made up of nine leaflets forming tiny ruffs. The funnel-shaped, fragrant, rosy-pink flowers are large in comparison to the foliage. They sit just above the foliage in late spring and early summer. There is a particularly nice form called 'Rosea' with deep rose-pink flowers. Height 10cm. Spread: 20cm.
O. 'Ione Hecker' AGM (H4) is a compact plant with divided, bluey-green foliage that emerges in mid spring in compressed whorls. The trumpet-shaped flowers are borne after the first leaves have opened and are pale-pink with deep-pink veins. Height and spread: 10cm.
O. laciniata is a dainty species that spreads via tiny rhizomes. It has finely divided, grey-green leaves and in summer purple-blue flowers, tightly rolled like umbrellas, open up into wide funnels.
O. magellanica 'Nelson' forms a tight mat of neat, fresh green leaves. In late spring, pure-white double flowers are produced, hiding just under the foliage. Likes an open site in the sun. Height: 3cm.
O. melanosticta produces large soft-yellow flowers from September to December. It has furry, glaucous foliage all growing from a single point. Dormant in summer. Hardy to -10°C. Height: 12cm.
O. oregana is a spreading American woodland species with large soft-pink flowers from April to July. Intolerant of summer dryness but good in shade or partial shade. Height: 15cm.
O. tetraphylla 'Iron Cross' is a spreading herbaceous perennial with four "lucky clover" leaves with a central brown blotch. In spring to summer, it bears pink funnel-shaped flowers. Grown as a houseplant. Height: 15cm.
O. triangularis subsp. papilionaceae 'Atropurpurea' AGM (H1) has very large, striking, purple-red triangular leaves. Pale-pink flowers stand out against the foliage between July and September. Height: 20cm.
Thank you to Floramedia, which supplied the images for this article from its photo library
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