Nasturtium (literally "nose-twister" or "nose-tweaker"), as a common name, refers to a genus of roughly 80 species of annual and perennial herbaceous flowering plants in the genus Tropaeolum ("Trophy"), one of three genera in the family Tropaeolaceae. It should not be confused with the Watercresses of the genus Nasturtium, of the Mustard family. This genus, native to South and Central America, includes several very popular garden plants, the most commonly grown being T. majus, T. peregrinum and T. speciosum. The hardiest species is T. polyphyllum from Chile, the perennial roots of which can survive underground when air temperatures drop as low as -15°C (5°F).
They have showy, often intensely bright flowers (the intense color can make macrophotography quite difficult), and rounded, peltate (shield-shaped) leaves with the petiole in the center. The flowers have five petals (sometimes more), a three-carpelled ovary, and a funnel-shaped nectar tube in the back.
Tropaeolum species are used as food plants by the larvae of some Lepidoptera species including Dot Moth and Garden Carpet. A very common "pest" found on Nasturtium in particular is the caterpillar of the Large White (Cabbage White) Butterfly.
In cultivation, most varieties of nasturtiums prefer to be grown in direct or indirect sunlight, with a few preferring partial shade.
The most common use of the nasturtium plant in cultivation is as an ornamental flower. It grows easily and prolifically, and is a self-seeding annual.
All parts of the plant are edible. The flower has most often been consumed, making for an especially ornamental salad ingredient; it has a slightly peppery taste reminiscent of watercress, and is also used in stir fry. The unripe seed pods can be harvested and pickled with hot vinegar, to produce a condiment and garnish, sometimes used in place of capers, although the taste is strongly peppery. The mashua (T. tuberosum) produces an edible underground tuber that is a major food source in parts of the Andes.
Nasturtiums are also considered widely useful companion plants. They repel a great many cucurbit pests, like squash bugs, cucumber beetles, and several caterpillars. They had a similar range of benefits for brassica plants, especially broccoli and cauliflower. They also attract black fly aphids, and are sometimes planted in the hope of saving crops susceptible to them (as a trap crop). They may also attract beneficial, predatory insects.