Nasturtium

Tropaeolum majus

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At a Glance

Nasturtium is a vigorous annual grown across UK gardens for its long season of vivid, spurred flowers and its rounded, lily-pad-like leaves. Both flowers and foliage are edible, with a peppery, watercress-like flavour, and the plant is widely used in cottage-style borders, hanging baskets, window boxes and as a quick climber over short supports. Easy from seed and unfussy about soil, it is one of the most rewarding half-hardy annuals for British gardeners, flowering from early summer through to the first autumn frosts.

Overview

Tropaeolum majus belongs to the family Tropaeolaceae and is native to the Andes of South America, from where it was introduced to European cultivation in the sixteenth century. In the UK it is grown as a half-hardy annual: sown in spring after the last frost, flowering through summer, and finishing with the first hard frosts of autumn. It is a popular choice for gardeners wanting reliable, low-maintenance colour, and is equally at home in an ornamental border or in a kitchen garden, where its edible flowers and leaves are a genuine secondary harvest.

The plant's common name, "nasturtium", derives from the Latin for "nose-twister", a reference to the peppery bite of the leaves and flowers. It should not be confused with the unrelated watercress genus Nasturtium, whose common name shares the same root but whose botanical classification is quite separate.

Appearance

Nasturtiums have a distinctive and instantly recognisable appearance. The leaves are rounded and peltate — the leaf stalk attaches to the centre of the underside rather than the edge — giving them a shield or lily-pad shape. Mature leaves are typically 3–12 cm across, a bright, slightly bluish green, with radiating veins clearly visible on the upper surface. The texture is fleshy and water-repellent, and the leaf edges are characteristically cupped upwards.

The flowers are funnel-shaped with five overlapping petals and a short spur at the rear of the bloom, where nectar collects. Individual flowers measure 4–6 cm across and appear in shades of orange, yellow, scarlet, deep red and cream, often with darker veining at the throat. Most varieties are single-flowered, though several cultivars produce semi-double or fully double blooms.

Growth habit varies markedly by type. Compact or dwarf varieties form neat mounds 20–30 cm tall and are well suited to the front of a border or to containers. Trailing and climbing types send out long, succulent stems that can reach 1.5–2 m in a single season, scrambling through pea netting, up trellis or spilling over the edge of a raised bed. The fruit is a three-segmented schizocarp, green at first and drying to a pale beige at maturity; each segment carries a single seed.

Growing Conditions

Nasturtiums prefer a position in full sun, where they flower most profusely, though they will tolerate light afternoon shade — useful in warmer parts of southern England where prolonged heat can cause plants to sulk from late July onwards. They grow best in cool to moderate conditions and flower most freely when daytime temperatures stay below about 25°C; in prolonged hot weather above 28°C, plants often produce foliage at the expense of bloom.

Soil is one area where nasturtiums differ from most ornamental annuals: they prefer lean, low-fertility ground. Rich, heavily fed soil encourages lush leaf growth but very few flowers. A well-drained garden soil of average fertility is ideal; sandy and even slightly stony soils suit them well. The pH should be neutral to slightly acidic, in the range pH 6.0–7.5. Waterlogged or compacted soils should be avoided, as the fleshy stems and shallow roots are prone to rot in saturated ground.

In UK climate, nasturtiums are treated as half-hardy annuals. They will not survive frost and should only be planted out once the danger of cold nights has passed. They do, however, tolerate cool spring conditions well and continue to flower into the first light frosts of autumn — a hard frost ends the display, but short-lived cool nights in September usually do not.

Planting and Care

Sowing is the main method of raising new plants. Direct sow outdoors from late April through June, once the soil has warmed and frost risk has passed. Sow two to three seeds per station, 15–20 mm deep, and thin to the strongest seedling. For earlier flowers, start seeds indoors in 3 cm modules in March or early April and plant out after hardening off. Germination usually takes 7–14 days at 15–18°C. Seedlings are large enough to handle once the first true leaf has expanded.

Spacing depends on the type. Compact varieties should be set 20–30 cm apart; trailing and climbing types benefit from 40–45 cm between stations to allow stems room to spread. Climbing varieties will need a support — pea netting, a small obelisk or twiggy pea sticks all work well.

Watering should be moderate. Allow the top few centimetres of soil to dry out between waterings; overwatering, like overfeeding, suppresses flowering and encourages leafy growth. Container-grown plants will need more frequent watering than those in the ground, especially in warm weather, but allow the compost to dry back between waterings.

Feeding is rarely necessary. Most garden soils contain enough nutrition for nasturtiums to perform well. If growth appears weak, a single light feed with a potassium-rich fertiliser (such as a tomato feed at half strength) in midsummer can boost flowering. Avoid high-nitrogen and high-phosphorus feeds — these promote foliage and suppress blooms.

Deadheading is not strictly required, but removing spent flowers does encourage continued blooming in many varieties, particularly the larger-flowered types. Towards the end of the season, leave a few flowers to set seed if you want to collect seed for next year or to allow self-seeding in mild gardens.

Propagation is from seed only. Nasturtiums do not root readily from cuttings, and vegetative propagation is not used commercially or by gardeners. Saved seed remains viable for two to three years if stored cool and dry.

Common Problems

Aphids are the most familiar pest. The peach-potato aphid (Myzus persicae) in particular colonises shoot tips, leaf undersides and flower buds, distorting new growth. Nasturtiums are, however, often grown deliberately as a sacrificial trap crop near brassicas — the idea being that aphids will colonise the nasturtiums first, drawing them away from the vegetables. A light infestation is best tolerated; heavier outbreaks can be washed off with a strong jet of water or treated with insecticidal soap.

Cabbage white caterpillars (Pieris rapae) will eat nasturtium foliage readily, sometimes stripping plants within days. Netting is the most effective protection; hand-picking is practical on smaller plantings. Slugs and snails are a particular problem for young seedlings, which can be grazed at ground level overnight.

Powdery mildew, caused by Erysiphe species, appears as a white powdery coating on leaves in humid, crowded conditions. Improving air circulation by spacing plants well and watering at the base rather than overhead reduces the risk. Leaf spot diseases caused by Alternaria and Cercospora species produce small brown or black spots; affected leaves should be removed and destroyed, and the bed cleared of debris at season end. Verticillium wilt (Verticillium dahliae) is rare in gardens but causes wilting and browning from the base up; affected plants should be removed and not composted.

Flea beetles (Phyllotreta species) make tiny round holes in the leaves. Damage on established plants is mostly cosmetic; on seedlings it can be more serious, and fine mesh netting is the most reliable protection for a few weeks after planting out.

Popular Varieties

Several well-established cultivars are widely available in the UK. 'Empress of India' is a classic compact variety reaching around 30 cm, with distinctive blue-green foliage and deep scarlet flowers — an RHS Garden Merit holder and a long-standing cottage garden favourite. 'Alaska' is grown for its variegated leaves, splashed and marbled with cream, topped with flowers in the usual orange-to-yellow range; it reaches about 25 cm and is a good choice for containers. 'Jewel of Africa' is a climbing or trailing mix reaching 1.8–2 m, with variegated foliage and a full colour range from cream through yellow, orange and deep red, useful for covering an unsightly fence or scrambling up a tripod.

'Whirlybird' is a popular compact type (about 30 cm) with upward-facing, spurless flowers in mixed colours — the lack of spur makes the blooms particularly tidy and showy. 'Peach Melba' is a compact variety (25–30 cm) with creamy-yellow flowers marked with a deep orange-red blotch at the throat, prized for its unusual colouring and edging the front of a border well. 'Black Velvet' is a more recent introduction bred for its deep mahogany-red, almost black flowers set against bright green foliage — striking in a terracotta pot.

Mature size depends on variety and growing conditions: dwarf types generally reach 20–30 cm, while trailing and climbing types can reach 1.5–2 m in a single season with adequate moisture and light.

Edible and Garden Uses

Although outside the six required sections, the dual ornamental-edible role of nasturtium is worth noting in summary. Young leaves add a peppery note to salads and sandwiches; flowers are used as a colourful garnish. Immature seed pods can be pickled in brine to make "poor man's capers", an established traditional use. The plant is also widely grown as a companion — particularly near brassicas — to draw aphids away from crops, and its flowers are attractive to bees and hoverflies throughout the summer.

Pests and Diseases

For step-by-step help, read Controlling Aphids Naturally and Dealing with Slugs and Snails. Or browse the full plant problem solver to diagnose an issue by symptom.