Oldenlandia corymbosa aka Flattop Mille Graines

Oldenlandia corymbosa is extremely easy to grow — in fact it is far better known as a weed of lawns, paddies and pavements than as a cultivated ornamental. It germinates readily from self-shed seed, thrives with nothing more than warm weather and occasional moisture, and completes its whole life cycle in two to three months. The main challenge is controlling its spread rather than keeping it alive.

Water moderately so the soil stays evenly moist but never waterlogged. In its native monsoonal climate the plant thrives on regular rainfall, so during hot dry weather a deep soak once or twice a week is usually enough. Established plants tolerate short dry spells but flower best with consistent moisture.

It grows in almost any reasonably drained soil, from sandy loam to clayey sand, and is commonly found in disturbed ground, rice paddies and compacted gravel. A moisture-retentive loam enriched with a little organic matter produces the most vigorous plants. Good drainage matters more than fertility.

The plant performs best in full sun to light partial shade. In open grasslands and lawns it thrives in direct sun all day, while in hotter climates a few hours of afternoon shade is tolerated. Heavy shade produces leggy, poorly flowering plants.

It is a tropical/subtropical species that flourishes in humid conditions, particularly during the monsoon season. Outdoors in warm climates ambient humidity is more than enough and no special misting is required. It still grows acceptably in drier air as long as the soil is kept moist.

Not really — it is essentially a self-sustaining weed and flourishes without feeding, even on poor soils. If you want to push growth in a garden setting, a balanced 10-10-10 fertilizer applied every four to six weeks during the growing season is more than enough. Over-feeding simply encourages floppy growth.

It is a warm-climate species that grows actively at roughly 20–30 °C (68–86 °F) and is frost-tender, dying back completely at the first cold snap. It tolerates the heat and humidity of the tropics very well. In temperate zones it is grown only as a summer annual.

Propagation is almost always from seed: mature plants shed large quantities of tiny seeds that germinate readily once humidity rises, usually within a few weeks of sowing. Stem cuttings pressed into moist soil will also root at the nodes, which is how the plant often spreads naturally. Seed dormancy is broken by humidity degrading the seed coat, so moist warmth is the main requirement for germination.

It is a small, low-growing herb reaching about 10–40 cm tall, with slender stems that often sprawl and root at the nodes to form loose mats around 30 cm or more across. Growth is rapid — the plant flowers 2–3 weeks after germination and completes its whole life cycle in 2–3 months.

The flowers are tiny, star-shaped, four-lobed and pure white, each only a few millimetres across. They are carried on thread-like pedicels in flat-topped clusters (corymbs) from the leaf axils, usually two to five at a time, which is how the species earned its epithet "corymbosa." Flowering typically runs from July to October in temperate regions and throughout the monsoon season in the tropics.

The plant is predominantly self-fertile, which is one of the reasons it establishes so quickly in disturbed ground. Its small white flowers are also visited by small insects, particularly tiny bees and flies, which provide some cross-pollination. This mix of selfing and insect pollination helps explain its weedy success worldwide.

Yes — Kew's Plants of the World Online recognises seven accepted infraspecific varieties: var. corymbosa (the autonym), var. caespitosa, var. erecta, var. linearis, var. microcarpa, var. nana and var. tereticaulis. They differ mainly in habit, leaf shape and indumentum, and most are restricted to particular regions of Africa or Asia.

It is native to the tropical and subtropical Old World, with a range spanning Africa (including Madagascar), the Indian subcontinent and South-East Asia. From this native range it has naturalised widely in the American tropics, the Caribbean, Pacific islands, northern Australia and parts of the southern United States. It is therefore considered a pantropical weed rather than a purely regional species.

Yes, and outdoors is the only practical setting for it — this is a weedy field herb, not a houseplant. In USDA zones 9–11 it behaves as a reliable self-seeding summer annual, and in cooler climates it can be sown in late spring once the soil warms. Be aware that it can easily escape into lawns and beds, so it is best confined to a controlled area.

In its monsoon-driven native climate the plant germinates at the start of the wet season, flowers within a few weeks and sets seed before the dry season returns. In temperate gardens sow or allow self-seeding in late spring, water through summer, and expect the plant to die after flowering in autumn — it is a true annual. Collect a little seed if you want a reliable population next year.

Routine pruning is not required: as a short-lived annual it simply flowers, sets seed and dies within a few months. Dead-heading spent stems before seed ripens can help limit unwanted self-seeding in a lawn or bed. Cutting back during the growing season is rarely necessary or useful.

The species is notably hardy and has few serious pests or diseases — part of the reason it spreads so successfully as a weed. It is itself more often the target of herbicides such as pendimethalin, metribuzin and imazethapyr in rice and plantation crops than a victim of insects or pathogens. Seedlings can occasionally suffer damping-off in very wet, poorly drained soils.

Yes — the tender aerial parts are eaten as a cooked leaf vegetable in parts of Africa and Asia, often mixed with other greens such as amaranth or squash, where the plant is said to soften the texture of the dish. The leaves are a reasonable source of vitamin C, and an ash prepared from burnt stems has been used as a natural vegetable tenderiser. It is a useful wild green rather than a staple crop.

It is an important herb in both Ayurveda and Traditional Chinese Medicine, used as a whole-plant remedy for fevers, jaundice, liver complaints, gastric problems, snakebite and various skin conditions. Modern pharmacological studies have confirmed hepatoprotective, antimalarial, antioxidant, oxytocic and cytotoxic activity in its extracts. Because of the oxytocic (uterine-stimulating) effect, it is generally avoided during pregnancy.

Beyond food and medicine, the roots yield a green dye known as gerancine after mordant treatment. The plant is also grazed by livestock as fodder in parts of Nepal, and its allelopathic compounds hedyotiscone A and B are being studied as natural herbicides for weed control. It has no significant ornamental use.