Black-eyed Susan : sowing, planting and care

Description and botany

The Thunbergia, more poetically known as “Black-eyed Susan” or also “Eye of Susan” or “Zanzibar Smile”, is a climbing and voluble plant from the Acanthaceae family, native to the tropical regions of East and Southern Africa, Madagascar, and India. In its natural state, it grows in forests, on rocky walls, winding around trees or bushes.

It has naturalised in many countries including Australia, Haiti, and Florida, where it is considered an invasive perennial.

From its tropical and subtropical origins, it retains a certain sensitivity to cold (hardiness between 10 and 8°C), and is grown as an annual or kept protected from frost in winter in our climates.

The genus Thunbergia includes about 100 annual or perennial species with evergreen foliage, climbing or bushy, including Thunbergia alata with orange to yellow flowers and Thunbergia grandiflora (or Bengal Trumpet) with dark blue flowers, which are the most commonly cultivated. The latter comes in several cultivars, including Thunbergia grandiflora ‘Alba’ with white flowers. Less commonly, Thunbergia erecta can be found, a bushy species with blue-violet flowers and a yellow throat, or T. gibsonii with bright orange flowers.

With its climbing habit, this fast-growing liana can form a beautiful bush up to 2 meters high in all directions in just one summer, sometimes even more. The heights of the plant are very variable. In its natural habitat, Thunbergia can reach up to 12 m high (Thunbergia grandiflora), much less so in our latitudes. The tallest woody specimens can live for several years in a greenhouse during the cold season, where they can become magnificent, reaching several meters for container-grown plants.

Thunbergia alata – botanical illustration

From a woody stump, Thunbergia develops on voluble stems that climb by wrapping around any support using the petioles of the leaves. Velvety, thin, and quadrangular, sometimes purplish, they cling on their own until they intertwine, forming a beautiful mass. They lignify over time.

On these stems, the lush foliage, bright green to dark green, develops. It remains evergreen when the plant is wintered away from frost. It consists of opposite leaves, entire or lobed and serrated, up to 20 cm long and 15 cm wide. Rough and downy, they are heart-shaped with sagittate tips and measure between 2.5 and 20 cm long. In the alata species (winged), they are borne by a slender winged petiole at the base of two small leaves, which gave the plant its specific name.

Particularly graphic, these long leafy stems add a certain lushness to the plant and highlight the exotic colours and voluptuous shapes of the flowers. Thunbergia is appreciated for its original and long flowering that extends from June to the first frosts. The Black-eyed Susan is one of the most spectacular vines for the beautiful season. The bushy vegetation is adorned with a profusion of large trumpets.

The flowers, encased in a velvety calyx and suspended by a short peduncle, bloom all summer long, continuously, either solitary or in small groups at the axil of the leaves, all along the stems. Around a small dark purple eye resembling a pupil at the centre of an iris, which earned it the nickname “Black-eyed Susan”, each petal unfolds in various often vibrant colours.

The tubular flowers, measuring 2 to 8 cm, open into five spreading lobes in shades of golden yellow, yellow-orange, sulphur yellow, orange, brick red, cream yellow, most often, but less commonly white or even lilac blue (Thunbergia grandiflora) or blue-violet (Thunbergia erecta) with a throat marked in brown or yellow. Thunbergia ‘Tangerine Slice’ stands out for its bicolour shade and the unusual pattern of its red-orange flowers striped with mustard yellow. The Thunbergia grandiflora or Thunbergia with large flowers bears the largest flowers (6 to 8 cm in diameter).

Nectariferous, this aubergine purple eye almost black at the centre attracts pollinating insects. After pollination, in warm climates only, the flowers develop into fruits, globular capsules that catapult small round seeds over a distance.

The Black-eyed Susan is usually grown in our country as an annual in open ground or in pots as a houseplant due to its fragility and low hardiness (-8°C) against frost. If you want to keep it: bring it inside!

Thunbergia, several flower colours.

Easy to grow, this tropical perennial needs heat, light, water, and a light, draining, humus-bearing soil to thrive. To bloom well, it will appreciate a sunny or shaded position sheltered from winds and cold drafts.

The Black-eyed Susan grows very quickly and can cover a trellis, a gazebo, a lattice, or a pyramid stake in the middle of a summer bed in just a few weeks. If not trained, it can also form an original ground cover at the foot of shrubs.

Grown in pots, in hanging baskets or window boxes, it flowers terraces and balconies. It can also find a place in the conservatory where it will thrive in warmth.

Main species and varieties

Les plus populaires

Nos préférées

Thunbergia alata Tangerine - Black-eyed Susan

Fast-growing, it will reach 2m in the space of a summer and will continuously flower until September.

• Flowering time July to November
• Height at maturity 2 m

See the product sheet

Thunbergia alata - Black-eyed Susan Vine Superstar Orange Seeds

With its very bright orange flowers, it spontaneously wraps around any type of support: whether artificial (mesh, tipi, trellis, fence) or natural (maize, large grass).

• Flowering time July to October
• Height at maturity 2 m

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Thunbergia alata

An annual climbing plant with cheerful yellow flowers with a black heart. It is suitable for pot cultivation as well as in the ground, in a sunny position.

• Flowering time August to November
• Height at maturity 2 m

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Thunbergia Tangerine Slice - Black-Eyed-Susan

This new American variety stands out for its bicolour shade and unusual flower pattern. It clings to trellises in large pots by itself, or generously cascades from hanging baskets.

• Flowering time July to November
• Height at maturity 2 m

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Thunbergia alata Collection - Black-eyed Susans

An essential mix of yellow and orange Suzanne seeds with black eyes! A voluble but light climbing plant, it pairs well with a perennial climbing plant and can also be used as ground cover.

• Flowering time July to November
• Height at maturity 2 m

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Thunbergia alata - Black-eyed Susan Vine Salmon Shades

These black-eyed Suzanne seeds in different colours: cream, apricot, salmon, orange quickly create a very soft display from June to October.

• Flowering time July to October
• Height at maturity 1,50 m

See the product sheet

Sowing and planting

Where to Plant Black-eyed Susan

The Black-eyed Susan is sensitive to cold and frost is fatal to it. It is a tropical plant suited to warm climates that does not tolerate frost. It cannot withstand temperatures below 8°C, which is why, in our climates, it is usually grown as an annual in the ground or planted in pots to be brought indoors, protected from frost during winter. In October, before the first frosts, move the pots to a bright and warm place (the temperature should be above 10°C to preserve its beautiful foliage). You can take them outside during the beautiful season.

It needs full sun to flower well. In the south of our country, it will fear the scorching sun and will accept partial shade protected from the burning rays of noon.

This beautiful climbing perennial grows quickly and should be planted in light, rich, and well-draining soil. In fertile and cool soil, its growth will be multiplied and its flowering very abundant.

The Black-eyed Susan can reach 2 m in the space of a summer. The height of the plant is very variable: plan for an appropriate support and enough space. It needs to be trained or staked if you are using it as a climber.

This remarkable climber is easy to grow, both in the ground and in pots on the terrace or in a greenhouse.

It can be used in borders or as a standalone, left to itself in a warm spot or trained on a trellis or fence, a tipi, a pergola, a railing, a pyramid stake. It can even weave around bushes or abundantly cover walls.

Its flowering continues from May to the first frosts, making it an excellent choice for quickly flowering large, lush pots or stunning hanging baskets that can be kept on the balcony all summer or in the conservatory where it can thrive in warmth during winter.

When not guided on a support or staked, in the garden, it can also form an elegant bushy and trailing mass over a low wall or a unique ground cover at the foot of shrubs.

When to Plant and Sow Thunbergia or Black-eyed Susan

Plant the plug plants in the ground, only when the risk of frost has passed, during the month of May. Seeds can be sown in a warm place, starting in March, in pots or trays. You can also sow directly in the ground from April to June.

How to Plant and Sow Black-eyed Susan

• Planting in the Ground

Upon receipt, pot up and store our young plants in plug plants of Black-eyed Susan under cover in a warm and bright place (conservatory, greenhouse, cold frame…) at a temperature above 14°C for a few weeks. Once the risk of frost is definitely over, plant them in the ground outdoors.

Install 2 to 3 Black-eyed Susan plants per m², or about every 30 cm.

Provide it with a trellis or a stake at planting around which it can entwine. If the soil in your garden is too poor, enrich it with potting soil and compost.

• Dig a hole three times wider than the root ball
• Make a good gravel bed to ensure perfect drainage
• Place the plug plant in the centre of the hole, with the collar level with the soil
• Bring back the extracted soil mixed with potting soil to encase the roots
• Firm down with your foot
• Water regularly

• Sowing

In Pots:

• Soak the seeds before sowing
•  From late February to March, under cover at a temperature of 18-20°C, sow two to three seeds in 6 cm pots in a light mix of peat and sand in equal parts
• Do not bury the seeds more than 1 cm deep
• Firm down and maintain at 20°C in sunlight until germination (between 14 and 21 days)
• Three to five weeks later, separate the seedlings and pot them into individual 12 cm pots placed under cover
•  In May, when the soil is warmed and the risk of frost has passed, plant the seedlings in the garden or in pots or containers of 20 cm in diameter (more for taller varieties) in soil enriched with compost
• Provide them with a support immediately to wrap around

In the Ground:

You can also sow directly in place, in warmed soil during the months of April to June, at a rate of 2 or 3 Thunbergia seeds per cluster every 30 cm. Stake or let it run on the ground. Feel free to mix different varieties together to create warm, colourful displays.

• Pot Culture

All Black-eyed Susan varieties are suitable for growing in large pots or in hanging baskets (the trick to avoid having to stake the beautiful climber!). They can be taken outside in the beautiful season and stored as soon as the first frosts arrive or kept in a conservatory warm all year round.

• Plant in a rich, well-draining mix consisting of 1/3 potting soil, 1/3 river sand, and 1/3 garden soil
• You can also add a bit of compost
• Mulch generously at the base to retain moisture
• Stake if necessary
• Water generously, keeping the substrate always slightly moist
• Fertilise regularly
• Place in light, without direct sunlight to preserve its colours
• In autumn, bring the pots into the conservatory to protect them from frost

→ Learn more about the cultivation of Black-eyed Susan in pots with advice from Leïla

Black-eyed Susan is well suited for pot cultivation.

Maintenance and care

Little demanding, the Black-eyed Susan really requires little maintenance, fearing only frost. It just needs to be protected from the cold. It proves to be easy to grow after careful planting, provided it does not lack food, water, or sunlight, as this tender plant loves warmth!

Remember to stake your plants as soon as they are planted in the ground or in pots.

In summer, keep the soil cool, especially during prolonged dry spells. It is advisable to mulch generously at the base right after planting: it will tolerate a short drought fairly well, as long as it is watered regularly during the hot summer days.

From November onwards, pull up the Black-eyed Susans that are planted outdoors.

In pots

If necessary, plant a stake in the centre of the pot to facilitate the winding of the lianas.

The Black-eyed Susan is a hungry plant. Throughout the growth period, apply flowering plant fertiliser every 15 days.

Water generously as soon as the root ball dries out, keeping the soil always slightly moist.

In winter, space out the waterings, allowing the soil to dry out between two waterings.

From October onwards, as winter approaches, bring the potted Thunbergia indoors to a very bright place that is not too cold, protected from frost, in a greenhouse or conservatory where the temperature does not drop below 10°C.

Pruning the Black-eyed Susan

Regularly remove faded flowers to encourage the renewal of flowering.

In August, if your Thunbergia loses some vigour, lightly prune the stems, and it will bounce back beautifully.

Pruning is truly only done on plants that have been overwintered or grown indoors, which extends their lifespan. This is more about annual thinning pruning. This allows for balancing or reducing the branches, renewing the stems, and promoting growth and flowering. It can also be useful if your Thunbergia becomes invasive: sometimes it is necessary to contain its growth!

• In September, before bringing them in for warmth or between February and March, before growth resumes, use pruning shears to remove crossed, thin, or dead stems at their base, keeping only the most vigorous ones, and cut back the spent shoots.
• You can also slightly reduce the clump.

Flowers of Thunbergia or “Black-eyed Susan”.

Pests and potential diseases

In open ground, Black-eyed Susan has no enemies; only plants grown in greenhouses may be more susceptible to infestations of mealybugs, whiteflies, or red spider mites.

Mealybugs can be eliminated with a cotton ball soaked in 90°C alcohol, followed by rapeseed oil sprays to be repeated two or three times at 15-day intervals.

To eliminate whiteflies, spray a soapy solution made from black soap mixed with a little vegetable oil.

Against red spider mites, which are common when the plant lacks water, spray a nettle maceration.

Powdery mildew sometimes whitens its foliage in case of excess moisture: spray decoctions of horsetail or nettle manure.

Multiplication

How to propagate Black-eyed Susan cuttings?

This method can be somewhat hit or miss. However, it is worthwhile to take cuttings of Thunbergia as after two years of indoor cultivation, the plant tends to flower less.

• In August-September, take semi-woody cuttings (turning into wood), 10 to 15 cm long just below a node
• Remove the leaves located at the bottom of the stem and take off the flowers
• Dip in plant hormone
• Insert them three-quarters into individual buckets filled with a light (turf and river sand) and well-draining mix, kept moist but not soggy
• Place under a warm frame (18°-25°) during winter, in light but out of direct sunlight under a bell or plastic
• Repot into 10 cm pots
• Transplant them into the ground in spring, in May when they have enough roots and the soil is warm enough

Associate

The Black-eyed Susan is a stunning climbing voluble plant with warm and vibrant colours that adapts to all desires, whether in the ground or in pots. Within a few weeks, it dresses any support provided with a touch of exoticism, adding a whimsical flair to somewhat rigid flower beds.

Its always vibrant hues are essential in summer for all flower beds or containers, harmonising well with plants in warm colours.

Left to its own devices, it will form a lush original carpet at the base of bushes alongside the pretty red, elongated flowers of a Nasturtium.

You can find it in our beds of summer perennial plants alongside the large tomato-red or blazing orange flowers of Echinacea, the large daisy-shaped flowers in a beautiful intense red of Helenium, or even gladioli around which it will twine. Meanwhile, some Grasses like Calamagrostis will temper the display with their golden hues.

An example of a warm-toned association: Thunbergia ‘Tangerine Slice’, Gladiolus ‘Robinetta’, Helenium ‘Ranchera’, Calamagrostis acutiflora ‘Avalanche’, Echinacea ‘Tomato Soup’.

The Thunbergia easily associates in a bushy bed with a variety of flowering shrubs of a mild climate featuring the red blooms of Oleanders, the Dwarf Pomegranate, or the sky-blue of a Cape Fuchsia.

Its original flowering will echo the trumpet-shaped mandarin flowers of a Campsis radicans or Trumpet Vine or another herbaceous climbing plant like a herbaceous clematis.

It will also make a significant impact in a hanging basket or a large pot alongside petunias, ivy geraniums, Lobularia (or Sweet Alyssum), or Mandevilla.

Pair it with tagetes, gauras, or sages that will keep the soil cool at its base.

→ Discover more association ideas with the Black-eyed Susan in our advice sheet!