Bidens pilosa aka Spanish Needles

Bidens pilosa typically reaches 30-60 cm in height under ordinary conditions, but can grow up to 1.8 m in ideal circumstances. It is an exceptionally fast grower, capable of completing its full reproductive cycle in just 57-70 days and producing 3,000-6,000 seeds per year. It grows three times faster than many similar species.

Bidens pilosa thrives at temperatures between 15-45°C and has some frost tolerance down to -15°C. It can grow across a wide rainfall range of 500-3,500 mm annually. In temperate climates, it is treated as a warm-season annual.

Bidens pilosa produces small, composite flower heads typical of the daisy family (Asteraceae). Each head features approximately 4-5 white ray florets surrounding a cluster of yellow disc florets. After pollination, the plant produces black, tetragonal achenes about 1 cm long, each tipped with 2-3 heavily barbed awns that form spherical burrs 1-2 cm in diameter. In temperate regions, flowering peaks in summer and autumn; in tropical climates, it can flower year-round.

No fetched source provides information on named ornamental cultivars of Bidens pilosa. Botanical varieties recognized in the literature include var. minor and var. bimucronata, which are taxonomic variants rather than horticultural selections.

Bidens pilosa is primarily an outdoor plant and is not suited to indoor cultivation. It is native to tropical and Central America and has naturalized across more than 70 countries in tropical, subtropical, and warm temperate regions. It thrives in disturbed habitats including roadsides, field margins, garden beds, and wetland margins. It is considered a major weed and invasive species in many countries. It grows best in full sun, tolerates a wide range of soils (pH 4-9), and can persist in both moist and relatively dry conditions.

Pruning is not a typical horticultural concern for Bidens pilosa, which is an annual species. As a fast-growing weed, it is most often managed by hand-weeding before seed set, or by applying thick mulch to prevent germination. If grown intentionally, removing flower heads before they ripen into burrs helps prevent unwanted spread.

Bidens pilosa does not require leaf-cleaning as a houseplant might. However, the barbed achenes (seeds) cling readily to clothing, fur, and feathers and should be removed promptly from gardeners and pets to prevent further spread of this invasive species.

Bidens pilosa propagates exclusively by seed. Seeds are dispersed passively via barbed awns that cling to animals, clothing, and feathers. For deliberate cultivation, direct sowing in spring is recommended. The plant self-seeds prolifically, producing 3,000-6,000 seeds per plant per year, and germinates readily in warm, moist conditions.

Bidens pilosa is notably fast-growing rather than slow-growing. It can complete its full lifecycle in 57-70 days and grows approximately three times faster than similar competing species. If your plant is growing slowly, the most likely causes are insufficient sunlight, excessively cold temperatures (below the plant's preferred 15-45°C range), or extremely poor soil — though the species tolerates a wide pH range (4-9) and both fertile and infertile soils.

Bidens pilosa is itself considered a major agricultural weed worldwide, affecting at least 30 crops in 40+ countries. It serves as a host and vector for root knot nematodes and Tomato spotted wilt virus, making it a reservoir for crop diseases. As a weed, it is managed through hand-weeding, mulching, and herbicides. When grown intentionally, no specific pest or disease problems for the plant itself are well-documented in available sources.

Bidens pilosa is an important nectar source for bees and other pollinators. The composite flower heads, with their white ray florets and yellow disc florets, attract a variety of insect pollinators. After pollination, seeds are dispersed mechanically via the burr-like, barbed awns that cling to animals and clothing — not by wind.

Bidens pilosa is edible, with a PFAF edibility rating of 4 out of 5. Young leaves and shoot tips can be eaten raw or cooked — they have a resinous flavor and are a notable source of iodine. They are commonly added to salads, soups, and stews, or dried for storage; young shoot tips are also brewed as tea. However, raw consumption carries some risk due to the plant's phototoxic compounds, and cooking is generally safer. Long-term raw consumption has been associated with oesophageal cancer in some regions. Seeds should not be eaten because of the choking hazard posed by their barbed awns.

Bidens pilosa has a long history of medicinal use across Africa, Asia, and Latin America, though PFAF gives it a medicinal rating of 2 out of 5. Traditional applications include wound healing, anti-inflammatory treatment, fever reduction, malaria treatment, digestive complaints, coughs, diabetes, and muscular pain. Leaf juice is applied to wounds and ulcers; crushed flower-heads are used to draw pus from boils. Scientific research has confirmed antimalarial activity (up to 90% inhibition of Plasmodium falciparum), as well as antibacterial, antifungal, and anti-inflammatory properties attributed to polyacetylenes, flavonoids, and ethyl caffeate. The plant contains approximately 200 known compounds.

Beyond food and medicine, Bidens pilosa serves several other purposes. It is an important nectar source for bees and provides seeds for small birds. It has been used as a soil stabilizer on disturbed land. A natural dye can be extracted from the plant, and dried roots have been fashioned into painting brushes in some cultures. Its aggressive ground cover can help stabilize bare soil, though this benefit must be weighed against its strong invasive potential.