Oxalis Triangularis vs Regnellii: Why One Goes Dormant and What to Do About It
The purple shamrock (Oxalis triangularis) and the green shamrock (sold as Oxalis regnellii) sit next to each other in every garden center—different pots, different price tags, different labels. Most buyers assume they are comparing two separate species. They are not.
Both plants are Oxalis triangularis. The green form was described twenty years later under the name O. regnellii, but Kew’s Plants of the World Online now treats that name as a synonym. The only meaningful difference is pigment: the purple form produces roughly 15 times more anthocyanin than the green form, according to a 2022 study in Metabolites. Everything else—dormancy, toxicity, care routine—is nearly identical.
This guide breaks down what actually differs, what stays the same, and which form suits your home better.
Quick Comparison Table
| Feature | Purple Shamrock | Green Shamrock |
|---|---|---|
| Botanical name | Oxalis triangularis | Oxalis triangularis (sold as O. regnellii) |
| Leaf color | Deep burgundy-purple | Bright green |
| Flower color | White to pale pink/lavender | White |
| Mature size | 6–18 in tall, 12–24 in wide | 6–18 in tall, 12–24 in wide |
| Light needs | Bright indirect to full sun | Bright indirect; partial shade preferred |
| Watering | When top inch of soil is dry | When top inch of soil is dry |
| Dormancy | Yes—2–4 weeks, triggered by heat or season | Same |
| Difficulty | Easy | Easy |
| USDA zones | 8a–11b (houseplant elsewhere) | 8a–11b (houseplant elsewhere) |
| Toxic to pets | Yes—soluble calcium oxalates | Yes—soluble calcium oxalates |
| Cost (4-in pot) | $8–$15 | $6–$12 |
| RHS Award of Garden Merit | Yes | No |
The Taxonomy Surprise: One Species, Two Labels
Auguste de Saint-Hilaire first described Oxalis triangularis in 1825 from specimens collected in Brazil. Twenty years later, Friedrich Miquel published a separate description under the name Oxalis regnellii. For most of the 20th century, garden references treated them as distinct species.
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Modern taxonomy has collapsed the distinction. Kew’s Plants of the World Online lists O. regnellii among 22 heterotypic synonyms of O. triangularis. Both forms belong to the family Oxalidaceae, both are tuberous geophytes native to Brazil, Argentina, Bolivia, Paraguay, and Peru. The purple form was sometimes separated as subspecies papilionacea, but current Kew classification folds that back into the main species as well.
For growers, this means care instructions that apply to one apply equally to the other. Any difference you notice comes down to pigment, not genetics at the species level.
Why One Is Purple and One Is Green
The purple color comes from anthocyanin pigments—specifically malvidin and delphinidin derivatives concentrated in the leaf tissue. A 2022 study published in Metabolites (Luo et al.) found that the genes CHS and CHI, which drive anthocyanin production, are upregulated at least 700-fold in purple leaves compared to green leaves of the same species.
This is not just cosmetic. Anthocyanins act as a built-in sunscreen, absorbing excess UV radiation and protecting the photosynthetic machinery underneath. The practical result: the purple form handles direct sunlight better than the green form, which lacks this photoprotective layer and can scorch in intense afternoon sun.
Both light and cold temperatures boost anthocyanin production in the purple form. A purple shamrock grown in low light will fade toward green-brown. Move it to a brighter spot and the color deepens within days.
Light Needs: The One Real Care Difference
This is where the pigment difference creates a genuine care gap.
The purple shamrock thrives in bright indirect light and tolerates several hours of direct morning sun without stress. NC State Extension lists its range as full sun to partial shade. Its anthocyanin layer absorbs the extra photons that would otherwise overwhelm the chloroplasts.
The green shamrock prefers the same bright indirect conditions but is more vulnerable to leaf scorch under direct afternoon sun. If your only available spot gets intense western exposure, the purple form is the safer pick. If you have a compact flowering houseplant collection in an east-facing window, either form will be happy there.
Both forms tolerate lower light for short periods but will grow leggy and sparse. If light is a constant challenge in your home, neither shamrock is the best choice—consider a calathea instead.
Every shamrock goes dormant. This catches new owners off guard—the leaves yellow, collapse, and die back completely. It looks fatal. It is not.
Dormancy is triggered by temperatures above 81°F (27°C), prolonged drought, or the natural end of a growing season. The underground bulbs (technically rhizomatous tubers about 2 inches long) store enough energy to sit quietly for 2 to 4 weeks before pushing fresh growth. This cycle is similar to what you see with holiday cacti, though the mechanism differs—oxalis stores energy in tubers rather than succulent stems.
How to manage dormancy:
- Stop watering when the foliage wilts and yellows.
- Move the pot to a cool spot (50–60°F / 10–15°C).
- Give it one light watering per month to prevent the tubers from drying out completely.
- When you see new shoots emerging, resume normal watering and return the pot to its bright spot.
Both forms follow the same dormancy pattern. The purple form’s new growth emerges with a reddish-purple flush; the green form comes back bright green.
Nyctinasty: Why the Leaves Fold at Night
Both forms close their leaves every evening and reopen them each morning. This movement, called nyctinasty, is one of the most striking features of any houseplant.
The mechanism works through the pulvinus—a swollen joint at the base of each leaflet filled with parenchyma cells. During the day, extensor cells on the upper side accumulate potassium ions, draw in water by osmosis, and swell; the leaf opens flat. At night, the process reverses: flexor cells on the lower side fill while extensor cells shrink, folding the leaf downward along its central vein.
This is not growth-based. It is fully reversible, driven by an internal circadian clock, and repeats daily for the life of the leaf. Touch or sudden shade can also trigger temporary folding (thigmonasty). The movement looks the same in both purple and green forms.
Toxicity: Both Are Unsafe for Pets
The ASPCA lists all Oxalis species as toxic to dogs, cats, and horses. The toxic compounds are soluble calcium oxalates, present in every part of both plants. Symptoms include drooling, tremors, and gastrointestinal distress. Kidney failure is possible but rare in small companion animals.
NC State Extension classifies the toxicity as low severity. In practice, the sour taste (oxalic acid is what makes sorrel tangy) deters most animals from eating enough to cause serious harm. Still, keep both forms out of reach of curious pets and young children.
Size, Growth Habit, and Propagation
Both forms are compact mounding plants reaching 6 to 18 inches tall with a spread of 12 to 24 inches. Petioles (leaf stalks) run 6 to 10 inches long, each topped with three triangular leaflets. The overall silhouette is a low dome of clover-like foliage—tidy enough for a desk, windowsill, or shelf.
Growth rate is moderate during active periods. Each tuber produces several petioles, and a healthy clump fills a 6-inch pot within a season. Propagation is simple: divide the tubers when you repot. Each section with at least one growing point will establish as a new plant. You can also separate individual tubers from a purchased bag of dormant bulbs and plant them 1 to 2 inches deep in any well-drained potting mix.
Both forms flower freely. The purple shamrock produces white to pale pink funnel-shaped blooms; the green shamrock produces pure white flowers. Blooms are small (under 1 inch) but appear in clusters above the foliage and last for weeks.
Which One Should You Pick?
Choose the purple shamrock if you:
- Want a statement plant with deep burgundy foliage that stands out on any shelf
- Have a bright spot with some direct morning sun
- Want the RHS Award of Garden Merit cultivar
- Like the dramatic look of purple leaves folding at sunset
Choose the green shamrock if you:
- Prefer a classic, fresh green look that blends with other foliage plants
- Have a spot with bright indirect light but no direct sun exposure
- Want a slightly lower price point
- Are building a green-on-green plant shelf where purple would clash
Get both if: you want to show visitors that these two “different species” are actually the same plant in different color modes. A purple and green pair in matching pots makes a striking conversation piece—and they share identical care, so doubling up adds zero extra work.
Frequently Asked Questions
Are Oxalis triangularis and Oxalis regnellii the same plant?
Yes. Oxalis regnellii is a synonym of Oxalis triangularis according to Kew’s Plants of the World Online. Garden centers use different names for the green and purple forms, but they belong to the same species.
Is the purple shamrock harder to care for than the green?
No. Both have identical water, soil, and dormancy requirements. The only practical difference is that the purple form tolerates more direct sunlight thanks to its anthocyanin pigments.
Why did my shamrock suddenly die back?
It entered dormancy. Stop watering, store in a cool spot, and wait 2 to 4 weeks. Fresh growth will emerge on its own. This is normal and happens to both forms.
Can I grow shamrocks outdoors year-round?
In USDA zones 8a through 11b, yes. Below zone 8, grow them as houseplants or lift the tubers before the first frost and store them in a paper bag in a cool, dry spot until spring.
Are shamrock plants safe around cats and dogs?
No. The ASPCA lists Oxalis as toxic to cats, dogs, and horses due to soluble calcium oxalates. Symptoms are usually mild (drooling, GI upset) but keep plants out of reach.
Sources
- NC State Extension. Oxalis triangularis. plants.ces.ncsu.edu
- Royal Botanic Gardens, Kew. Plants of the World Online: Oxalis triangularis. powo.science.kew.org
- Luo, H. et al. (2022). Transcription and Metabolism Pathways of Anthocyanin in Purple Shamrock. Metabolites, 12(12): 1290. PMC9784199
- ASPCA Animal Poison Control. Toxic and Non-Toxic Plants: Shamrock Plant. aspca.org
- Xu, P. et al. (2024). Mechanism of the Pulvinus-Driven Leaf Movement: An Overview. Frontiers in Plant Science. PMC11083266
