Plants that survive the conditions in our homes (low light, low relative humidity, inconsistent watering) are often quite hardy (or resilient) and can sometimes be very invasive outdoors.
The reckless tenacity of oxalis is worth noting. They are often found in our indoor plants without us even deciding to plant them there. They still manage to survive without any special care and, if not pulled up, reproduce. Generally, people don’t have much good to say about oxalis. Even The Laidback Gardener himself said of them: “This trifoliate plant with yellow flowers (Oxalis corniculata) is strictly a weed: no one would ever think of growing it on purpose.”
That said, if they do appear despite our best efforts, they are easy to keep alive indoors—and some varieties are even particularly attractive…
Origin
Oxalis are found all over the world, except in polar regions, but the plant is believed to originate mainly from South Africa and America (particularly central and southern regions—Brazil has a wide variety of oxalis).
The Oxalis genus has around 550 species and belongs to the Oxalidaceae family. No other Oxalidaceae are grown indoors, but this family includes the pretty star-shaped fruit known as the carambola (Averrhoa carambola).
In some cooler countries, oxalis are annuals whose seeds are scattered to the four winds. Most are small perennials (between 5 and 90 centimeters, depending on the species), except for at least O. gigantea, a bush composed of woody stems that is an exception in this genus of herbaceous (i.e., barkless) plants.
Oxalis are not only invasive in our plant pots; some are very poorly behaved and cause significant problems in environments where they proliferate freely. It is obviously the presence of all these reproductive organs (tubers, rhizomes, bulbs, and even bulbils in the case of O. naequalis) that make oxalis so widespread and difficult to eradicate. The most problematic species are O. pes-caprea, O. corniculata, and O. stricta.
Oxalis sp. has various names: wood sorrel, shamrock, apartment clover, and false shamrock (a more accurate name, since it is not a clover, although Oxalis sp. certainly resembles Trifolium sp.).
Description
Most oxalis are small herbaceous plants with leaves at the end of rather long petioles. Leaves are usually divided into three obcordiform leaflets, i.e. heart-shaped with the tip attached to the petiole. The trifoliate shape of oxalis is reminiscent of clover. Oxalis leaves can also be composed of four to ten leaflets. In some quadrifoliate forms, the leaves are triangular, as in many Oxalis grown indoors.
Growth habit is relatively variable. The most invasive Oxalis grow at the end of small, spindly stems, while others have a drooping habit with much denser foliage. Finally, some do without stems and are composed of long petioles that emerge directly from the ground.
Under the ground, there is also a variety of activity depending on the species (with 550 species, there is bound to be quite a bit of variability!). Those without stems grow from various underground organs, mainly rhizomes or tubers, but also bulbs and corms. [To understand the difference between a rhizome, a tuber, and even a bulb, you can read this article.] These underground organs take various forms, both in terms of their nature (rhizomes, tubers, bulbs) and their size and color. Rhizomes can be covered with scales that give them a distinctive appearance. In cultivated species, there is also variability in the color of tubers (traditionally red, but also yellow, orange, pink, peach, or apricot).
Finally, the plants readily produce five-petaled flowers, which grow at the end of long flower stalks and number between five and twelve. They are most often yellow, but some species are covered with pink, white, or lilac flowers. In some cultivars, they can be double or bicolored. The fruits of oxalids, if they produce any, take the form of capsules that eventually expel the adventurous seeds to the four winds.
Des feuilles qui paraissent vivantes
Almost all oxalis close their leaves when night falls, so it’s normal to see the plant much more curled up in the evening. Some plants also close their flowers under the scorching midday sun, earning them the nickname “Sleeping Beauty”. Finally, some oxalis take this movement a step further and seem to constantly readjust during the day to capture as much light as possible. This is particularly true of O. hedysaroides, which even moves slightly to the touch.
Common Varieties
Although it’s probably pretty easy to grow a wide variety of oxalis in the home, these are mainly three varieties that can be found fairly easily on the market.
O. triangularis
O. triangularis (formerly O. regnellii) is a trifoliate variety whose leaflets, as the name suggests, are triangular. The petioles emerge from a scaly rhizome. When well cared for, it is covered with white flowers all year round.
The basic variety, with its beautiful dark green leaves, is grown less often, in favor of cultivars with bright mauve leaves. Some of these cultivars also have pink or silver markings. These include the common cultivar ‘Charmed Wine’, with each purple leaf set with lighter mauve stripes, and the rarer cultivar ‘Black Beauty’, with almost black leaves. The flowers of purple cultivars tend to be more pink than white.
O. tetraphylla
O. tetraphylla (formerly O. deppei) is a variety with a slightly less dense habit than O. triangularis, whose quadrifoliate leaves are found at the end of slightly longer petioles, which grow directly from the bulbs. With its long, disordered limbs, the plant looks slightly chaotic, which adds to its charm.
The ‘Iron Cross’ cultivar is especially popular, because of the purple markings on its obcordiform leaves. The bright pink flowers are also particularly attractive.
A Brief History of Crosses
It’s amazing how far the research for these articles takes me. When I thought I’d write an article on Oxalis sp. this morning, I didn’t think I’d end up on a page describing the various forms a cross can take. Iron cross doesn’t refer to any iron cross, but to the symbol of the Prussian army, derived from that of the Teutonic Knights. It’s more a question of a “cross pattee”, the shape of which can be recognized in O. tetraphylla (although personally, I think the plant looks more like a Maltese cross).
O. vulcanicola (sometimes seen as O. spiralis) is often seen in flower box arrangements as a drooping “filler” plant. It is particularly easy to grow and its dense habit makes it a good bushy plant. Its flowers are yellow and its trifoliate foliage is often colorful.
We therefore find various cultivars with foliage ranging from neon (‘Copper Glow’) to orange (‘Aurea’, ‘Molten Lava’) and even purple, almost black (‘Zinfandel’) with pink variegation (‘Plum Crazy’). Depending on light and season, the foliage can also change color (e.g. ‘Velvet Sunset’, which goes from neon to red at the end of the season).
Rarer Varieties
There are many other species of Oxalis that could be grown indoors, but as the plant is not particularly popular, testimonials are found only on these:
O. hedysaroides
O. hedysaroides (mentioned above) is best known for its cultivar, ‘Rubra’, whose round, trifoliate leaves are a deep purple, decorated with a slightly silvery vein. The contrast with the yellow flowers only adds to the appeal of this bright cultivar. The plant produces no underground organs. It’s also known as “Firefern oxalis” or hedizarum oxalis (another kind of plant).
O. adenophylla
O. adenophylla‘s obcordiform leaves are much more divided, like small roses. The flowers are much larger and particularly attractive, ranging in hue from white to soft pink, sometimes with a contrasting eye. This oxalis grows from a tuber, and the flower stalks can sometimes take on a reddish hue. This plant tolerates medium lighting without any problems.
O. debilis
O. debilis is an oxalis whose main characteristic is that it is affected by begomovirus, a disease that leaves its leaves crinkled, but pleasantly marked with yellow veins. It’s known as the cultivar ‘aureoreticulata’, but begomovirus can affect the leaves of other plants, so caution is advised. Its flowers are pinkish-purple.
O. herrerae
O. herrerae is a semi-succulent plant with particularly fleshy stems that appear disproportionate to the round, trifoliate leaves. The yellow bloom is characteristic of this variety.
O. acetosella
O. acetosella is sometimes given as a St. Patrick’s Day gift. This white-flowered trifoliate version is reminiscent, of course, of the lucky clover.
O. ortgiesii
O. ortgiesii is a trifoliate variety whose triangular leaves take on purple or burgundy hues, but to a much lesser extent than the cultivars mentioned above. Stems eventually become woody and flowers are yellow.
Variety and Toxicity
Finally, let’s name the variety that is grown specifically for its tubers in the Andes and New Zealand, O. tuberosa. It is used as a starch like other root vegetables, and its taste is apparently very similar to that of potatoes or carrots, with a hint of lemon and sour cream. If you travel to South America and see “ocas” for sale, these are indeed oxalis tubers.
Everything in oxalis is edible, from the tubers to the flowers. The stems are sometimes eaten, which are reminiscent of rhubarb in their sour taste. As for the leaves, I would describe them as sour and astringent—in fact, as my sense of duty knows no bounds, I tasted the leaf of my Oxalis vulcanicola ‘Zinfandel’ myself and can confirm that it is actually quite good. They are used in some countries to quench thirst, and sailors in the past used them to slow the progression of scurvy. Finally, the flowers are described as rather tasteless (I haven’t been able to taste them, as my oxalis plants are not currently in bloom).
However, caution is advised: in large quantities, the oxalic acid contained in the plant can cause problems (there is as much in spinach, so the dangers are probably exaggerated). It is nevertheless recommended that pregnant people avoid eating them.
Plants produced for the summer market or as houseplants are not grown for consumption, and there is no guarantee that the soil or fertilizer used does not contain toxic elements: it is prudent to only eat plants whose growing conditions have been carefully monitored.
Fun fact: the Algonquins used oxalis for their aphrodisiac properties.
Growing Tips
Light
Oxalis are rather flexible when it comes to light. Long-stalked plants (O. tetraphylla, O. triangularis) can tolerate low light levels, but eventually wither away. They grow much better in bright light. With a sufficient supply of water, they can also tolerate full sun. It’s easy to get them used to full sun outdoors. O. vulcanicola grows very well in modest lighting.
Watering
It’s best to water oxalis when the soil begins to dry out slightly. Plants with underground organs are more at risk from over-watering, as their rhizomes, tubers or bulbs may rot. O. hedysaroides is less tolerant of variations in watering. Obviously, the more light the plant gets, the more it needs to be watered!
Atmospheric Humidity
Oxalis benefit from increased humidity, but are content with the relatively low atmospheric humidity found in our homes.
Potting
No special soil requirements. Oxalis tolerate cramped conditions without any problems. With their rather fine root systems, it’s best not to touch the roots when repotting. After repotting, it’s normal for the plant to “sulk” and close its leaves, to show its anger at having been disturbed. Just give it time to get used to its new pot.
Fertilizer
Although monthly fertilization at the recommended dose can help oxalis flower, they’ll grow just fine even without ever knowing the meaning of the word “fertilizer”. They’re weeds for a reason!
Temperature
Temperatures below 10°C (50°F) or above 32°C (90°F) can stimulate a period of dormancy in bulbous (and other) plants. Subterranean organs have varying degrees of cold tolerance, with O. adenophylla, for example, being hardy up to zone 4, and O. triangularis tolerating very brief periods of frost. In general, it’s best not to take chances and to keep them above 10°C (50°F).
Maintenance
This is a very easy-to-maintain plant, requiring very little care. As the leaves follow one another quite quickly, a little regular cleaning is all it takes.
If you need to make drastic changes (for example, to move the plant outdoors for the summer), you can cut it back a few centimetres from the ground: the new leaves will adapt to the surrounding conditions. The same applies to a plant that has just finished wintering and has become etiolated from lack of light.
Several species of Oxalis (O. vulcanicola, O. hedysaroides) do not go through a resting period. For O. triangularis, dormancy is optional, while it is very pronounced in O. tetraphylla. It’s therefore normal for these oxalids to lose all their leaves. You can then place them in a cool location (around 8°C / 46°F) and keep the potting mix barely moist (a quick monthly watering is quite sufficient) for a few weeks to several months before returning them to room temperature. When new shoots begin to appear, you can gradually resume more generous watering.
Propagation
Oxalis multiply in various ways, depending on the type. Species that produce a stem can be cut, quite easily, by taking stem cuttings in moist potting soil directly.
Oxalis that form underground organs multiply by dividing bulbs, rhizomes or tubers.
Problems
- Oxalis encounter few problems. In the event of a major problem, bulbous plants can be cut back to ground level, provided that the problem is not rot.
- Occasional infestations of scale insects, spider mites, thrips, aphids, and whiteflies may occur, but these pests probably prefer other plants to oxalis.
- A specific type of rust can attack the foliage. This is caused by the fungus Puccinia oxalidis, which appears as yellow spots on the upper surface of the leaves. It is quite rare indoors.
- Long, drooping petioles can be a sign of shock (transplanting, acclimatization, watering irregularities), rhizome rot, or lack of light. However, it is also normal for oxalis (especially O. tetraphylla) to have stems that are not completely straight and for the plant to have a chaotic appearance.
- In general, the plant’s leaves close a few hours after shocks (temperature changes, repotting, etc.). This can be impressive, but they should return to their usual position quickly enough. In temperatures that are too low or too high, the plant may go dormant, but it will wake up fairly quickly when temperatures return to normal.
Presentation Tips
I love oxalis and think they’re great as a houseplant in their own right. That said, for the skeptics who don’t find them all that aesthetically pleasing, drooping oxalis make excellent ground covers that can be grown at the foot of taller plants, such as certain indoor trees. Why not take advantage of their natural tendency to invade pots? They’re so flexible when it comes to light and watering, all you have to do is find the right color combination!
Conclusion
I think we can learn from this article that the line between “weed” and “houseplant” is perhaps even thinner than that between weed and outdoor plant. I think that the most colorful oxalis can very well be found on our windowsills and shelves, along with pothos (Epipremnum sp.), spiderwort (Tradescantia sp.) and prickly pear (Opuntia sp.), which, let’s not forget, are also considered weeds in their country of origin!
Here in Central Florida we have Oxalis that grow in our outdoor potted plants and flowerbeds that are considered by many to be weeds but I have always found them to be quite pretty. They can be a bit pushy but that’s part of their charm I think. Now I’m on the hunt for some of these beauties you’ve shown! Thank you!