Gadenin日本語Get the appThe most serious hazard is a species in the wrong place
Before any pest or disease, the risk that costs gardeners the most with oxalis is putting a running species into a border. Several members of the genus are among the worst perennial weeds in cultivated ground anywhere with a mild climate, and the reason is the way they multiply. Around the base of each mother bulb they build a dense cluster of bulbils, some no bigger than a grain of couscous. Those bulbils detach at the slightest disturbance. A single pass with a fork or a hoe scatters dozens of them through the soil, each one grows into a plant, and each of those builds its own cluster. Weeding a colony physically increases it.
Several also fire seed. The ripe capsules of some species split explosively and throw seed several feet, which is how a plant that started in one pot turns up in the paving cracks, in the gravel, and in every neighboring container on the bench. Add a rooting creeping stem, in the species that have one, and you get something that cannot realistically be dug out of an established border.
The ornamental kinds grown for purple or shamrock foliage are considerably less aggressive than the notorious weeds, but they use the same machinery and in a favorable spot they do drift. The practical rule is simple and it costs you nothing: grow oxalis in containers unless you know exactly which species you have and you are content for it to spread. Do not accept a plant passed over a fence in a yoghurt pot with no name on it and put it straight into a border. Check any bought plant for stray bulbils sitting loose on the surface of the compost before it goes near open ground, and never tip spent oxalis compost onto a garden bed or an open potting mix heap.
If a running oxalis is already in a bed, do not rotavate or fork it, since both actions multiply it. Suppress the top growth persistently instead, so no leaf ever gets big enough to feed the bulbs, and expect the process to take more than one season.
Spider mites are the pest that actually costs plants
Indoors, and in a hot dry greenhouse, red spider mite is the pest that does real damage to oxalis. The first sign is a fine pale stippling across the upper surface of the leaves, as though someone has gone over them with a very light spray of dust. Turn a leaf over and you find the mites, barely visible specks, along the veins, with fine webbing in the leaf axils once the infestation is established. Warm dry air is the condition they thrive in, so a plant on a windowsill above a radiator is the classic casualty.
Act on the first stippling. Mites reproduce fast enough that a fortnight of inattention turns a manageable problem into a plant that has to be cut to the ground. Because oxalis regrows from bulbs, cutting it right back and resting it is a genuinely effective last resort, and one not available with most houseplants. Clear the foliage completely, dispose of it away from other plants, and restart the pot after a few weeks dry, by which point the mites have nothing to live on.
Aphids turn up on flower stalks and on soft new growth, especially on plants moved outdoors in spring. They are easy to see and easy to deal with, but they distort buds and can cause bud drop, so do not leave them. Fungus gnats appear when the compost is being kept too wet, which on this plant is worth reading as a warning about the watering rather than as a pest problem in itself.
| Problem | What you see | Response |
|---|---|---|
| Red spider mite | Pale stippling on top of leaves, webbing beneath | Treat at first sign, or cut back to the bulbs and rest the pot |
| Aphids | Clusters on flower stalks and new growth, sticky residue | Wash off with water, repeat every few days until clear |
| Fungus gnats | Small flies running over permanently damp compost | Let the top inch (2.5 cm) dry properly between waterings |
| Rust | Orange or brown pustules on the leaf undersides | Remove affected leaves, improve airflow, avoid wetting foliage |
| Powdery mildew | White dusty film on leaf surfaces in still, humid air | Thin the foliage, increase ventilation, water at the base |
| Bulb rot | Sudden gray limp collapse, soft dark stem bases | Tip out, discard soft bulbs, repot the firm ones in gritty mix |
| Vine weevil | Plant loose in the pot, notched leaf edges, grubs at the roots | Tip out, remove every grub found, repot in fresh compost |
Rust and mildew: two leaf diseases with much the same answer
Oxalis is susceptible to a rust that shows as small orange or reddish-brown pustules on the undersides of the leaves, with corresponding pale spots visible on the upper surface. It is unsightly rather than fatal, but it spreads readily by spores through a dense mound of overlapping leaves, and a badly affected plant loses foliage early and goes into dormancy underfed. Powdery mildew arrives in much the same conditions, showing as a white dusty coating on the leaf surfaces of a crowded pot in still, humid air.
The answer to both is airflow and dry foliage. Do not mist the plant. Water at the compost surface rather than over the leaves. Give a crowded pot some space rather than sitting it in a huddle of other plants. Pull out the affected leaves as you find them, at the base, and dispose of them rather than dropping them onto the compost surface where the spores stay in play. If a plant is heavily affected, the bulb habit gives you the same clean escape as with mites: clear the foliage entirely, rest the pot dry, and grow a fresh set of leaves that has no infection carried over.
Rot, which is nearly always about water at the wrong moment
The commonest way an oxalis actually dies is rot at the bulbs, and it happens almost exclusively at two points in the cycle: between planting and the first shoots, and during dormancy. In both cases the pot contains bulbs with no leaves, which means it has no way of using the water that is in it. Compost that stays wet around a leafless bulb for weeks at 60 to 70F (15 to 21C) is close to the ideal condition for the fungi that cause it.
The preventions are all structural rather than a matter of vigilance. Use a compost cut by a third with grit or perlite so it drains within seconds. Use a pot with open drainage holes and never leave it standing in a full saucer or inside a sealed decorative cover. Do not plant more deeply than an inch (2.5 cm). And follow the rule that a leafless pot gets one watering at restart and then nothing at all until shoots appear.
If you catch it, tip the pot out immediately. Rot travels through a bulb cluster, so the earlier the firm bulbs are separated from the soft ones, the more you save. Discard everything soft, hollow or dark, let the survivors surface-dry on paper for a day, and replant them into fresh gritty potting mix. Do not reuse the old compost.
Oxalic acid, pets, and sensible handling
Oxalis takes its name from the oxalic acid in its tissues, which is what gives the leaves their sharp sour taste. In the quantities a person might nibble it is not dangerous, but it is not a plant to eat as a salad leaf, and large quantities are genuinely harmful. The practical concern is animals: a cat, a dog or a rabbit that chews through a substantial amount of foliage can be made ill, with drooling, vomiting and, at high intakes, effects on kidney function. Grazing livestock have been poisoned by weedy species eaten in bulk.
None of that makes it an unreasonable plant to keep. It means putting the pot where a determined chewer cannot reach it, and treating a pet that has clearly eaten a quantity as a matter for a vet rather than for wait and see. Wash your hands after handling it, as with any ornamental, and keep it off kitchen worktops where the leaves might be mistaken for a culinary sorrel or a clover.
Keeping notes that make problems predictable
Most oxalis troubles are recurrences. The pot that gets spider mite this winter is the one that got it last winter, because it is the pot on the shelf above the radiator, and the pot that rots is the one in the cachepot with no drainage. Recording where and when turns that into something you can pre-empt.
Note the date you first saw a problem, which pot and which position it was in, and what you did. Over two or three cycles the pattern surfaces on its own: mites in the dry weeks of winter heating, aphids within a fortnight of moving pots outdoors, rust in the crowded corner of the bench. Then you can act before the symptoms rather than after them, by moving that particular pot in advance or by giving it more space.
If you grow any oxalis in or near open ground, record the exact outline of the planted area and check the edge once a cycle for leaves outside it. Spread of a few inches a year is invisible season to season and unmistakable against a written boundary, and finding it in year two is a different problem from finding it in year five.