Nasturtium

Identification and origin

Tropaeolum majus belongs to the family Tropaeolaceae. Native to South America, it is regarded as a plant shaped by cultivation, originating in Peru. Spanish conquistadors found it in Peru and Mexico and brought it to Spain in the 16th century. It has since naturalized in many parts of the world and can become invasive in mild climates. Early English herbalists called it “Indian cress,” because its mustard oil resembles that of watercress. The genus name, chosen by Linnaeus, comes from the Greek tropaion and the Latin tropaeum, “trophy”: the plant recalled a classical trophy. Monet lined the path to his house at Giverny with it.

It is a tender perennial grown almost everywhere as an annual. Depending on the cultivar, it forms low mounds about 12 in tall and 2 ft wide, or trailing stems that cover the ground, banks, or low walls and can reach 10 ft when trained. It grows fast.

The long-stalked leaves are round and peltate: the leaf stalk attaches near the center of the blade, like the handle of a parasol. Smooth, hairless, and bright green, they measure 2–6 in across, with pale veins radiating from the center; some cultivars have leaves marbled or variegated with cream.

From late spring until frost, it bears large, funnel-shaped flowers 2–2 in wide, with five petals and a nectar-filled spur at the back. Colors range from yellow through orange to red, with cream, salmon, or mahogany shades in some cultivars; flowers can be single or double, and a few are fragrant. They attract butterflies, bees, and hummingbirds and make good cut flowers. The fruit splits into three sections, each holding one large seed, and the plant self-sows readily.

Common cultivars include ‘Empress of India’, compact, with dark foliage and crimson flowers; the ‘Alaska’ series, with white-marbled leaves; ‘Tom Thumb’, a dwarf 6–9 in tall; ‘Peach Melba’, creamy yellow with orange markings; ‘Jewel of Africa’, a trailing type with variegated leaves that climbs to 8 ft; and ‘Moonlight’, with pale yellow flowers.

Light

Plant it in full sun, where it blooms most. It tolerates part shade but flowers far less there. In very hot summers, light afternoon shade helps it through the heat, which also makes its leaves and flowers noticeably spicier.

Watering

Nasturtium prefers rather dry soil and tolerates drought, but regular watering through the growing season keeps it blooming strongly and mellows the flavor of the leaves. During germination, keep the seedbed moist but not soggy.

After that, check the soil and water once it has dried out, at the base and without wetting the foliage, since leaves that stay wet encourage disease. In window boxes and hanging baskets, the mix dries much faster than garden soil, so check it more often in hot weather.

Potting mix

It grows best in poor to moderately fertile, light, well-drained soil, sandy or loamy, whatever the pH. Do not fertilize unless the soil is extremely poor: rich soil or nitrogen feeding produces lots of leaves at the expense of flowers.

Temperature

Nasturtium is frost tender: the plant dies at the first hard freeze, and in the ground it behaves as a perennial only in USDA zones 9a to 11b. Elsewhere it is sown every year once all danger of frost has passed. Seedlings come up faster in warm soil. In warm regions with mild winters, it can be sown from spring through fall, or even year-round.

Repotting

Nasturtium transplants poorly, so sow it directly where it is to grow, in the ground or in its window box, rather than moving it later. If you start it indoors, use peat or paper pots that go into the soil whole so the roots are not disturbed.

In containers, use a pot with a drainage hole and a light, lightly fertilized mix. Dwarf types suit window boxes and edging; trailing types suit hanging baskets, walls, and trellises. Because the stems have no tendrils, tie them to their support to help them climb.

Pruning

Remove spent flowers regularly, or pick them to eat: this prolongs blooming. If the plant declines or gets leggy in midsummer, cut it back lightly; it regrows and blooms until the end of the season. In mixed containers, trim stems that crowd neighboring plants, and thin the clump so air can circulate.

Propagation

Nasturtium is grown from seed. Soak the seeds for a few hours or overnight to improve germination. Sow them where they are to grow, 0 in deep because they need darkness to germinate, spaced 10–12 in apart, in late spring or early summer once frost has passed. You can also start them indoors in biodegradable pots four to six weeks before your usual last frost date. Seedlings emerge in seven to ten days depending on soil temperature, and growth is rapid once the weather warms. Seed pods form about four months after sowing, and the plant often reseeds itself.

Common problems

• Lots of leaves, few flowers: soil too rich, too much fertilizer, or not enough sun.
• Leggy plant flagging in midsummer: a light cutback revives it.
• Very spicy leaves and flowers: heat or lack of water.
• Clusters of insects under the leaves: aphids; also watch for whiteflies, mealybugs, and spider mites.
• Chewed leaves: imported cabbageworm caterpillars.
• Winding trails inside the leaves: leafminers, which do the plant no real harm.
• Leaf spots, wilting plant: bacterial leaf spot and wilt, or viruses.

The same symptom can have several causes: look at exposure, soil, watering, and the undersides of the leaves together before drawing conclusions. If you plan to eat it, do not use pesticides on or near the plant.

Toxicity

Nasturtium is not toxic to dogs, cats, or horses, and no cases of poisoning from eating it have been recorded. For people, the leaves, flowers, buds, young pods, and seeds are edible; only the roots are not eaten. Add edible flowers to your diet gradually and eat only plants that have not been sprayed. Handling the plant, however, can cause a skin allergy in sensitive people, so wear gloves if you react to it.