Nasturtium plants include both perennial and annual flowering species in the Tropaeolum genus, which encompasses more than 80 different plants. These herbaceous flowers are native to South and Central America and known for their rich, saturated, jewel-toned colors. Planted in the spring after the threat of frost has passed, they are fast and easy to grow. In fact, they do best with a little neglect. In general, nasturtium flowers tend to the hot end of the color spectrum. The rounded leaves look like miniature lotus leaves.
I. Appearance and Characteristics
Tropaeolum , commonly known as nasturtium (; literally “nose-twister” or “nose-tweaker”), is a genus of roughly 80 species of annual and perennial herbaceous flowering plants. It was named by Carl Linnaeus in his book Species Plantarum, and is the only genus in the family Tropaeolaceae. The nasturtiums received their common name because they produce an oil similar to that of watercress (Nasturtium officinale).
The genus Tropaeolum, native to South and Central America, includes several very popular garden plants, the most common being T. majus, T. peregrinum and T. speciosum. One of the hardiest species is T. polyphyllum from Chile, the perennial roots of which can survive the winter underground at elevations of 3,300 metres (11,000 ft).
Tropaeolum is a genus of dicotyledonous annual or perennial plants, often with somewhat succulent stems and sometimes tuberous roots. The alternate leaves are hairless, peltate, and entirely or palmately lobed. The petioles or leaf stalks are long and, in many species, can twine around other stems to provide support. The flowers are bisexual and showy, set singly on long stalks in the axils of the leaves. They have five sepals, the uppermost of which is elongated into a nectar spur. The five petals are clawed, with the lower three unlike the upper two. The eight stamens are in two whorls of unequal length, and the superior ovary has three segments and three stigmas on a single style. The fruit is naked and nut-like, with three single seed segments.
Species in cultivation
The most common flower in cultivation is a hybrid of T. majus, T. minus, and T. peltophorum. It is commonly known as the nasturtium (and occasionally anglicized as nasturtian). It is mostly grown from seed as a half-hardy annual, and both single and double varieties are available. It comes in various forms and colors, including cream, yellow, orange and red, solid in color or striped and often with a dark blotch at the base of the petals. It is vigorous and easily grown and does well in the sun. It thrives in poor soil and dry conditions, whereas rich soil produces much leafy growth and few flowers. Some varieties adopt a bush form while others scramble over and through other plants and are useful for planting in awkward spots or for covering fences and trellises.
- The blue nasturtium (Tropaeolum azureum) is a tender species from Chile which has violet-blue flowers with white eyes that can be as much as 4 cm (1+1⁄2 in) across.
- Tropaeolum brachyceras has 2.5 cm (1 in) yellow flowers with purplish markings on wiry, climbing stems. It is a half-hardy perennial from Chile and may remain dormant for several years before being sparked into growth by some unknown trigger factor.
- Tropaeolum hookerianum is a tuberous-rooted species from Chile. There are two subspecies, T. h. austropurpureum which has violet-purple flowers and T. h. pilosum with yellow flowers.
- The Canary creeper (Tropaeolum peregrinum) is a trailing and climbing half-hardy annual species with wiry stalks and palmately lobed leaves. The pale yellow, fringed flowers are borne on long stalks. It originated from Peru but may first have been cultivated in the Canary Islands before being introduced into Western Europe.
- Wreath nasturtium (Tropaeolum polyphyllum) is a prostrate plant originating from Argentina and Chile. It has silvery, deeply lobed leaves and a profusion of small, bright yellow flowers on long trailing stalks. After flowering, the plant dies back. It is a perennial with underground rhizomes which send up new shoots at intervals. It will survive for several years in a suitable sunny location with well-drained soil. It is a very hardy species; the tubers can grow at depths of 60 cm (24 in) enabling the plant to survive at altitudes of as much as 3,300 metres (10,000 ft) in the Andes.
- The flame flower (Tropaeolum speciosum) is well adapted to cool, moist climates and famously does well in Scotland. It sends up shoots that thread their way through hedges and shrubs and, when they emerge into the light, bear brilliant red flowers among small, five or six-lobed leaves. It is difficult to establish but is an attractive garden plant when it thrives. This plant has gained the Royal Horticultural Society’s Award of Garden Merit.
- Three-coloured Indian cress (Tropaeolum tricolor) is another tuberous, climbing species grown for its attractive red, purple and yellow tubular flowers. It comes from Chile and Bolivia and is a reliable winter-growing species.
- Mashua (Tropaeolum tuberosum) is a perennial climbing plant from the Andes grown for its tuberous roots. It has been cultivated since ancient times, and depictions of it are found at archaeological sites pre-dating the Incas. It has leaves with five to seven lobes and small, long-spurred, red and yellow flowers. The tubers have an unpleasant smell when raw, which disappears on cooking. It is frost-hardy and produces crops of 30 tonnes per hectare at an elevation of 3,000 meters (9,800 ft) above sea level. The cultivar T. tuberosum lineamaculatum ‘Ken Aslet’ has gained the Royal Horticultural Society’s Award of Garden Merit.
Species originating from the coastal areas and lower foothills make most of their growth in winter, whereas the true alpine species are summer growers. Tuberous Tropaeolum species are well known for occasionally remaining dormant for one or more years. The species with underground rhizomes and tubers can be propagated from these, while other species are best raised from seed. Many growers favour fresh seed, but dried seed is also often successful. Seed from the winter growing species should be sown in the autumn, while the summer growing species are best sown in the spring in well-drained compost and covered with 1 to 2 cm (1⁄2 to 3⁄4 in) of grit or sand. The containers should be kept at below 4 °C (39 °F) until the seedlings appear in about a month, as too high a temperature inhibits germination.
II. How to Grow and Care
Sunlight
Nasturtiums grow and bloom best when planted in an area of full sun that receives six to eight hours of direct sunlight daily. This plant can tolerate some shade (three to six hours of sunlight), but might not bloom as profusely. When planted in a warm climate, some shade is actually preferred, as hot temperatures may end up browning leaves.
Temperature and Humidity
Some varieties of nasturtium are planted as perennials in USDA growing zones 9 through 11. However, in most North American climates, this plant is treated as an annual, completing its life cycle in just one growing season. They prefers daytime temperatures in the 70s F and can survive a light frost, but not a cold freeze. Additionally, this plant prefers average humidity levels (between 30 and 50 percent), yet it’s not overly particular. Nasturtiums can struggle, however, in extremely dry or extremely humid conditions.
Watering
Nasturtiums typically prefer weekly watering, but can be watered more often when planted in a greenhouse or in a sunny vegetable plot. In these conditions, the water demand can be high from surrounding plants and will dry the soil quickly. Nasturtiums will survive moderate droughts, but blooms will likely diminish and the foliage will begin to look spindly.
Soil
Surprisingly, nasturtium flowers do well in relatively infertile soil with good drainage. Soil that’s too rich will result in abundant greenery, but few flowers. This plant grows best in soil with a neutral pH (6 to 8) and can tolerate dry conditions, although a moderate amount of moisture is appreciated.
Fertilizing
Using synthetic fertilizers is usually not recommended, as many gardeners grow them to eat. Organic gardeners rarely need to amend the soil before planting, unless conditions are very poor or the surrounding vegetables need soil impavements. Just remember, robust soil properties can cause nasturtiums to put out more foliage and fewer flowers, so a good balance is important.
Pruning
Trailing nasturtium varieties are prone to legginess and usually need pruning mid-summer, and then again in late summer, to stimulate new growth and blooms. Trim the longest stems back by 6 to 12 inches on trailing varieties and thin browning leaves and flowers by pinching them off at the base. For bushy varieties, trimming stems is not generally needed, but pinching off spent flowers and dried leaves will aid in the plant’s overall aesthetic.
Propagation
After pruning your nasturtium, it is possible, and easy, to propagate the cuttings. You may also want to grow a new plant from a cutting if a large portion breaks off or if you want to replant the same variety in another part of your garden.
Here’s how to propagate nasturtium from cuttings:
- Gather garden shears, a pot with drainage holes, potting soil, and rooting hormone powder (optional).
- Fill the pot with potting soil.
- Select a stem from your mature nasturtium plant and cut off a 4-inch (or longer) piece with at least three leaves intact. Dip the cut end into rooting powder (this is not necessary, but may speed up the process).
- Poke a hole in the center of the soil and insert at least 1 inch of the stem into the hole.
- Backfill the hole gently with your fingers and water the pot carefully in the sink.
- Allow the pot to drain and place it in a sunny window or under grow lights.
- Check for roots in about one week. Once strongly rooted, transplant the stem into your sunny garden and continue to keep the soil moist.
How to Grow From Seed
Most gardeners choose to sow nasturtiums annually from seed. Seeds can be sown directly in the garden once the soil has warmed, or started indoors two to four weeks before the average last spring frost date. For outdoor planting, sow seeds directly into soil that is at least 55 to 65 F, and plant them 1/2 inch deep and 10 to 12 inches apart. Keep the soil continuously moist until seeds sprout, which should take about a week to 10 days.
When planting in pots, sow seeds in peat or paper pots, as nasturtiums can be finicky about transplanting. This will reduce transplant shock.
Potting and Repotting
Sowing seeds directly into pots is the best method for growing nasturtiums in containers. You can also sow seeds in peat starter pots, and then move them into a larger vessel once the seedlings sprout. A good quality potting mix will do, but don’t opt for one with added fertilizer if you want more blooms. Nasturtiums grow best in natural stone or clay containers with ample drainage. You can also add a layer of stones or pebbles to the bottom of the pot, before adding soil, to increase the drainage capacity.
How to Get to Bloom
Nasturtiums begin to bloom when temperatures rise, so make sure your plant is in a sunny, warm location to assure maximum color. If your nasturtium hasn’t started blooming within four to six weeks after sprouting, then you may need to wait for the outdoor temperatures to rise, or for weather conditions to improve.
Plants with abundant foliage but no blooms may be an indicator of rich soil conditions. While it’s usually not recommended to fertilize this plant, under these circumstances, a plant food high in phosphorous may help promote flowers. Opt for natural products, like bone meal or rock phosphate, especially if you plan on eating the flowers. Pruning your plant may also help if you planted a variety that holds flowers under its leaves.
Pests and Diseases
Common Pests and Diseases
Nasturtiums are prone to aphid infestation, consequently making them a good companion to, and deterrent for, vegetables also vulnerable to aphids. If your plant’s foliage looks crinkled, or otherwise unhealthy, aphids might be the problem. Reduce the population with a strong blast of water from your garden hose. You can also opt for a non-toxic spray made at home from diluted vinegar or essential oils like lavender and peppermint. Better yet, plant catnip alongside nasturtiums to deter the pests altogether.
This plant can also contract bacterial leaf spot under improper conditions. Small brown or black spots will show on the leaves if this is the issue. You can minimize or prevent the problem by providing ample airflow between plants and watering with drip irrigation, as opposed to sprinklers, as this condition spreads by splashing water.
Common Problems
Some varieties of nasturtium actually obstruct flower production due to the plant’s physical structure. If you are growing nasturtium at ground level, as opposed to one that trails or vines, choose a variety that holds its flowers above its leaves, allowing the sun to get to the buds.
Root rot can also affect nasturtium if the soil is waterlogged. This plant prefers almost sandy soil, so make sure there is ample drainage in the location you select.
III. Uses and Benefits
- Medicinal uses
T. majus has been used in herbal medicine for respiratory and urinary tract infections.
In Germany, licensed physicians can prescribe the herbal antibiotic Angocin Anti-Infekt N, made from only nasturtium and horseradish root.
- Culinary uses
All parts of Tropaeolum majus 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 flowers contain about 130 mg vitamin C per 100 grams (3+1⁄2 oz), about the same amount as is contained in parsley. Moreover, they contain up to 45 mg of lutein per 100 g, which is the highest amount found in any edible plant. The unripe seed pods can be harvested and dropped into spiced vinegar to produce a condiment and garnish, sometimes used in place of capers.
Mashua (T. tuberosum) produces an edible underground tuber that is a major food source in parts of the Andes.
- Companion planting and biological pest control
Nasturtiums are used as companion plants for biological pest control, repelling some pests, acting as a trap crop for others and attracting predatory insects. While companion planting is a widespread notion and often adopted by home gardeners, there is little but anecdotal evidence to support these claims.