How to grow perennial nasturtiums

What are perennial nasturtiums?

Perennial

Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs.

nasturtiums belong to the genus Tropaeolum and they originate from Central and South America. Their leaves may be rounded and lobed or divided into several leaflets, depending on the species. These are very decorative in their own right, though the scrambling T. polyphyllum produces so many yellow flowers that the pretty blue-green leaves barely show! The flowers have five petals and are usually funnel- or trumpet-shaped, with a long spur extending from the back of the flower. The canary creeper, T. peregrinum, is quite different with two larger, fringed petals held like outstretched wings and the other three much reduced to ribbons – looking indeed like a canary in flight. Many species produce tubers to help them survive in the ground when dormant.

Choosing perennial nasturtiums

Summer-flowering T. speciosum is hardy enough to grow outdoors all year. Some other species, such as T. pentaphyllum and T. ciliatum, may survive in sheltered gardens in warmer areas of the country. T. peregrinum will flower in its first year from seed and is often grown as an annual. The rest are best grown in pots so they can be moved to a protected location for winter. You can also select plants according to their flowering season or your soil conditions.

Choose by planting location

In the ground outside:

T. speciosum (flame flower), with scarlet flowers and pretty divided leaves, is hardy and will weave its way up and over shrubs, reaching 3m tall. 

T. peregrinum, with its bright yellow flowers, is often grown from seed each spring and can reach 4m on an archway, obelisk or pergola.

In pots:

You could choose any of them, but pots are ideal for tender species that can be kept on a sunny patio in summer, then brought into a frost-free greenhouse, garden room or conservatory for the cold season.

Choose by flower season:

For summer: T. speciosum, T. tuberosum, T. polyphyllum and T. peregrinum

For winter and/or spring: T. azureum, T. hookerianum, T. beuthii and T. tricolor

Choose by soil pH

This describes how acid or alkaline soil is, and is expressed as a number between 0 and 14. Soil with a pH of 7 is neutral, an acid soil has a lower value, and an alkaline soil has a higher one. Most plants grow best in soil that’s within a specific pH range, so it’s useful to know your soil’s pH. Testing kits are easy to use and widely available in garden centres and online.

:

Preferring acidic conditions: T. speciosum, T. tuberosum and T. hookerianum

Choose by soil type:

For sandy soil: T. speciosum, T. tuberosum, T. polyphyllum, T. peregrinum and T. hookerianum (which needs especially good drainage)

For clay soil: T. speciosum

Buying perennial nasturtiums

Canary creeper (T. peregrinum) is widely available as packeted seeds, or occasionally in spring as young plants. Flame flower (T. speciosum) plants are widely available to buy mail order. Other species are more likely to be found at specialist nurseries. 

When choosing, look in particular for varieties with the RHS Recommended: Award of Garden Merit. These performed well in trials, so should grow and flower reliably for you. 

To browse photos and descriptions of many species, or to track down specific varieties, go to RHS Find a Plant.

When and how to sow

T. peregrinum can be sown indoors in spring and planted outside after the last frost. Or, because they flower so quickly from seed, they are also successful when sown outdoors directly in the soil in early summer. When sowing directly outside, clear the area of weeds, then rake the surface to a fine, crumbly texture. Sow the seeds about 1cm deep and water well.

How to sow seeds indoors

How to sow seeds outdoors

When and how to plant

The best time to get plants established in the ground or in pots is when your chosen species is just coming out of dormancy

Dormancy refers to a period of inactivity in plants, during which they slow down or stop their growth, conserving energy and resources. This is a natural process that helps them survive harsh conditions, such as winter or drought. Dormancy in seeds is a mechanism that prevents them germinating until environmental conditions are favourable for growth.

and starting to grow. This timing varies according to the species, but plants are also most likely to be available to buy at this stage.

How to plant a climber

How to plant up a container

Where to plant

Nasturtiums need moist but well-drained soil. Their position should provide full sun for their best display, either as ground cover in the case of T. polyphyllum, or trained up a support or over other plants for both T. speciosum and T. peregrinum. T. speciosum will enjoy cool, reliably moist but well-drained soil – tucked beneath a leafy shrub can be ideal.

Watering

Check and water seedlings

A seedling is a young plant grown from seed.

and newly planted out nasturtiums regularly for the first few weeks. After that, plants growing outdoors should only need watering during very dry or hot spells. However, T. peregrinum doesn’t have time to develop an extensive root system, so do keep a close eye on it. 

Plants in containers will need regular watering while the plants are in growth – they can dry out very quickly in warm conditions.

Watering

Container maintenance

Feeding

Plants growing in any reasonable garden soil will not need additional feeding. 

In containers, apply a potassium-rich liquid fertiliser, such as tomato feed

A potassium-rich liquid feed to encourage flowering and fruiting in various plants, not just tomatoes. Take care to follow the instructions on the packet.

, every couple of weeks during the flowering season. You can also add slow-release fertiliser, such as fish, blood and bone, to the compost

Can refer to either home-made garden compost or seed/potting compost: Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

as plants are coming out of dormancy

Dormancy refers to a period of inactivity in plants, during which they slow down or stop their growth, conserving energy and resources. This is a natural process that helps them survive harsh conditions, such as winter or drought. Dormancy in seeds is a mechanism that prevents them germinating until environmental conditions are favourable for growth.

.

Perennial

Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs.

nasturtiums use twining leaf stalks to cling to supports, so they are happy scrambling over shrubs or climbing up fine structures such as wire mesh, metal obelisks or string. On thicker supports, such as a wooden trellis, some tying-in may be required.

They do not need pruning, though it can be worth pinching out the growing tips to promote branching. Allow plants to die down naturally at the end of their season before clearing away the old growth.

Buy fresh seed each year for T. peregrinum. For other species, divide established plants

Established plants have been in their current location for two or three years and so have well-developed root systems able to support strong growth with healthy foliage and flowers.

during their respective dormant seasons. For species that develop tubers, follow the steps below:

• Start the dormant tubers into growth indoors, either in early spring for summer-flowering ones, or in autumn for winter-flowering ones, in a shallow tray of peat-free potting compost
  

  Can refer to either home-made garden compost or seed/potting compost: Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

• When the shoots are 2–3cm tall, lift the clump of tubers and divide it into several pieces, making sure each piece has both roots and shoots
• Pot up each section into a separate container and grow on in a bright, frost-free place 
• For summer-flowering types, gradually acclimatise the plants to outdoor conditions by hardening them off before moving them outside

Under glass, and increasingly outdoors in recent years, they may be susceptible to glasshouse whitefly and glasshouse red spider mite. Outdoors, cabbage whitefly can be a problem.

If you’re a member of the RHS, you can use our online Gardening Advice Service, via MyRHS for all your gardening questions.