At a glance
Thunbergia alata – the black-eyed Susan vine – is one of the most cheerful summer climbers available to UK gardeners. The vivid orange, yellow, cream and white flowers with their distinctive dark chocolate-brown or black centres appear in continuous succession from June right through to the first frosts, covering supports, hanging baskets, obelisks and container trellises with colour throughout the season. The flowers are relatively small compared to other climbers but appear in such numbers that the overall effect on a well-grown plant is striking.
As a tender perennial grown as an annual in the UK, thunbergia needs a warm start indoors to give it enough growing season to reach its potential. Plants sown in February or March will be in full flower by June; those sown in April will catch up quickly but start flowering a few weeks later. This guide covers everything from the sowing conditions that give the best germination through to the feeding, watering and support regime that keeps the plant flowering prolifically across the full British summer.
About thunbergia
Thunbergia alata is native to tropical eastern Africa and is naturally a perennial in its home climate, scrambling through the undergrowth and up into trees and shrubs with twining stems that can reach 2-3 metres in a season. In the UK it is treated as a half-hardy annual, started under glass in late winter and grown on as the season warms. It is not frost-tolerant and will be killed by even light frosts, so timing of outdoor planting after the last frost date for your region is important. The name black-eyed Susan derives from the dark centre of the flower, which provides a strong visual contrast against the warm orange and yellow petals. This is a characteristic shared with the unrelated North American wildflower Rudbeckia hirta, which goes by the same common name, so when searching for seed it is worth using the botanical name to ensure you are buying the climbing species rather than the herbaceous perennial.
Sowing and growing on
Thunbergia seeds germinate best at temperatures of 20-25 degrees Celsius and require this consistent warmth for reliable results in the UK – a heated propagator on a warm windowsill or a greenhouse bench is ideal. The seeds are round, relatively large and easy to handle. Sow individually into 9cm pots or cells at a depth of around 1cm, water in and keep at the required temperature. Germination typically occurs within 7-14 days in good conditions. In lower temperatures germination slows significantly and seedling vigour is reduced, so maintaining warmth is worth the effort.
Once seedlings are established and showing their first true leaves, grow on at 15-18 degrees in good light – a bright windowsill or cold frame that is kept frost-free works well. Pot on into a 1-litre pot when roots start to fill the first container, taking care not to disturb the root ball. Begin hardening off three weeks before your intended planting-out date by giving the plants increasing exposure to outdoor conditions. In most of England this means starting hardening off around late April for a late May planting date. In Scotland and northern England, delay outdoor planting until mid-June to be safe. The hardening-off period is worth doing properly with thunbergia – plants that are moved directly from a warm indoor environment to exposed outdoor conditions in a cool May will sulk for weeks before resuming growth, effectively losing more time than the hardening-off process costs. Gradual exposure over 10-14 days, bringing plants in at night if temperatures are forecast to drop below 10 degrees, gives a much smoother transition and keeps growth momentum going.
A heated propagator repays its cost with thunbergia. The 20-25°C required for reliable germination is difficult to maintain consistently on a windowsill in February and March without supplementary heat. A basic propagator with a thermostat brings germination rates up significantly and reduces the waiting time. If sowing later in April, a warm unheated windowsill usually suffices.
Care, containers and support
Thunbergia is one of the best climbing plants for containers in UK gardens. Its relatively modest vigour compared to large-flowered clematis or climbing roses makes it suitable for large pots and window boxes where a 2-3 metre climber can be managed without the plant overwhelming the space. A 30-40cm pot filled with good quality peat-free compost and positioned in a sheltered south or west-facing spot provides ideal conditions. The deep container allows the root system to develop properly, which is important for flowering performance.
Feeding is important for container-grown thunbergia because the restricted root volume cannot draw on soil nutrients the way a border-grown plant can. A weekly liquid feed with a high-potassium fertiliser from the moment the plant is established outdoors through to September keeps the flowers coming. A tomato feed works well as the formulation supports continuous flowering rather than leafy growth. For plants grown in the ground in a sheltered border, the soil provides a wider nutrient base and feeding can be every two weeks rather than weekly.
Watering container-grown plants consistently is the most important single care task through summer. Thunbergia in a 30cm pot in a warm sunny position can need watering once daily in a heatwave, and will drop buds and flowers noticeably within 48 hours of the compost drying out completely. A moisture-retaining perlite mix in the compost helps, as does grouping containers together with other plants to reduce moisture loss from the pot sides. Deadheading is beneficial but not strictly necessary as spent flowers fall cleanly and the plant generally tidies itself without intervention. If you want to collect seed for the following year, leave a few seed pods to ripen fully on the plant in September before harvesting – the seeds are viable and store well in a dry envelope over winter for the following spring.
Varieties and related species
Most thunbergia sold in the UK is T. alata or T. alata hybrids. The range of flower colours has expanded considerably through breeding and mixed packets now offer a palette from deep orange through buff, yellow, white and even lavender-toned forms – though the classic orange with a black eye remains the most popular and the strongest grower in British conditions.
Problems and troubleshooting
Thunbergia is relatively problem-free in UK gardens once established in a warm sheltered position. The majority of failures relate to cold conditions or insufficient sun rather than pest or disease pressure. Spider mite can be a problem on plants grown in warm dry conditions – particularly on south-facing walls in a hot summer or in glasshouses – and manifests as fine webbing on the undersides of leaves with pale mottling on the upper surfaces. Increasing humidity around the plant and ensuring good ventilation reduces risk significantly. Aphids occasionally colonise new growth but at a lower frequency than on many other summer annuals, possibly because the small leaf size and rapid growth outpaces mild infestations. Slugs and snails can damage young plants immediately after planting out, so protecting the base of the plant for the first two or three weeks is worthwhile if slug pressure is high in your garden.
Thunbergia alata combines well with other warm-colour annuals in border and container plantings. The orange and yellow tones work particularly well alongside marigolds, nasturtiums and the buff and cream shades of ornamental grasses. For a purely climbing display, combining it with ipomoea morning glories on the same support creates an interesting contrast between the vivid orange thunbergia flowers and the blue or purple morning glory trumpets, both appearing through summer and autumn on the same structure.
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