Black eyed Susan: varieties, hardiness & more

Growing black eyed Susan from seed is easy even for novice gardeners. It is not hardy, but if given a sunny location, will delight you with a large abundance of flowers.

The black eyed Susan vine is a profuse flowering climber and belongs to the genus Thunbergia. This was named after the famous Swedish botanist Thunberg. It is the most commonly cultivated species with the most varieties and is given the botanical name Thunbergia alata. When black-eyed Susans bloom, how best to propagate them, and what other Thunbergia species there are, we explain here.

Black eyed Susan: flowering time and properties

The genus Thunbergia, also called skyflower, belongs to the acanthus family (Acanthaceae) and is native to Southeast Africa. The most popular species in our gardens is the black eyed Susan vine, which is colloquially known as black eyed Susan. It owes its name to its flower structure. The funnel-shaped base is darkly coloured inside, making the flat corolla appear with a dark “eye”. The wild type of black-eyed Susan forms orange-yellow flowers, although cultivated forms may bear other flower colours. The leaves are heart-shaped and toothed on the edge.

The black eyed Susan is popular with hobby gardeners mainly due to its rapid and lush growth, which allows it to climb as much as two to a maximum of three metres. This climbing plant can thus be left to grow up the walls of houses with trellises or sticks or used in hanging baskets and window boxes as a magnificent hanging plant. The flowering period of the black-eyed Susan is between June and September.

Is black eyed Susan perennial?

Yes, black eyed Susan is actually a perennial. However, in our part of the world it is often cultivated as an annual, as it loses its vigour from the second year. Due to its declining vitality, it is recommended to sow new seeds every year.

Is black eyed Susan bee-friendly?

Bees are rarely observed on black eyed Susan. Nevertheless, it is an important food source for insects such as butterflies and bumblebees, whose longer proboscises allow them to reach the nectar at the base of the flower. Therefore, it is not without reason that it is one of our most popular insect-friendly climbing plants for the garden.

The most beautiful species and varieties

The wild variety of black eyed Susan has orange-yellow flowers, although it is worth getting acquainted with other varieties with different colours of flowers:

• Thunbergia alata ‘African Sunset’ is distinguished by its multicoloured shaded flowers. With this variety, you can look forward to yellow, orange and dark red hues in one flower.

• Thunbergia alata ‘Arizona Rose Sensation’ is low-growing and will enchant you with its bright pink blooms.

• Thunbergia alata ‘Alba’ beautifies the garden with brilliant white flowers and belongs to the low-growing varieties.

In addition to Thunbergia alata, there are other Thunbergia species that are popular in gardens:

• Thunbergia gregorii is a striking variety with its bright orange flowers without a black eye in the centre. In the wild, it reaches a height of 3 m, cultivated forms can grow up to 5 m. Like its relative Thunbergia alata, if it is planted in a sunny spot, it will thank you with an abundance of flowers.

• Thunbergia grandiflora is also called blue skyflower and owes its name to its large blue flowers. It likes a sunny to semi-shady place and grows to a height of 5 m.

Is black eyed Susan hardy?

No, black eyed Susan is not hardy and is very sensitive to frost. Due to its original African origin, it is accustomed to warm temperatures. At below 8 °C its shoots die. If you want to overwinter the black-eyed Susan, there are a few things to consider. It is assumed that the plant has been cultivated in a tub or pot in order to be able to propagate the black eyed Susan in a suitable place.

Before that, cut off all the shoots 5 cm above the ground. Black eyed Susan does not like it too warm in winter, so it is necessary to ensure a temperature of 10-15 °C in the winter quarters. Also, make sure the location is bright and avoid waterlogging. It does not need to be fertilised over the winter months.

In the spring, after the last frost, the climbing plant can be brought back outside. If these conditions can not be met, there is no need to worry, because black-eyed Susan is easy to propagate and sow.

Propagation

There are two ways to propagate black eyed Susan – either by cuttings or by generative propagation, i.e. seeds.

Propagation via cuttings

• Select young, green shoots
• Take the cuttings between late summer and autumn
• Leave at least 3 upper pairs of leaves on the shoots, remove the remaining leaves
• Put cuttings individually in small pots with growing medium
• Choose a bright and comparatively warm location between 15 and 20 °C
• Cover cuttings with a film to prevent heavy transpiration
• The substrate must be kept moist but never wet

As the growing roots of the cutting do not need a lot of nutrients, they can be cultivated in low-nutrient soil. Our Plantura Organic Herb & Seedling Compost provides cuttings with nothing more and nothing less than the nutrients they need and can also be used for seeding young plants. In addition, it is peat-free and consists of 100% natural raw materials, which is why CO₂ is saved during production. If the cutting forms new leaves, this is a sign of successful rooting. Now you can get started pricking out the black eyed Susan and plant three to five plants together, depending on the size of the pot.

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Propagation from seed

If generative propagation is chosen, black eyed Susan seeds can be obtained from the developing pods after flowering in late summer. If the dark-coloured seed pods pop open when pressed, it can be assumed that the seeds are ripe. Seed harvesting is worth it, because, fortunately, the seeds of the beautiful climbing plant are very germinative.

If you do not plant the seeds directly you should dry them in a shady place. When storing, ensure a cool temperature of about 10 °C and low humidity.

Note: Thunbergia alata is cross-pollinated by insects, the newly obtained seed may not be pure in variety.

Is black eyed Susan poisonous?

The black eyed Susan is not poisonous. It is even edible, so it is great for locations that are accessible to children. The flowers look pretty on salads or in drinks, for example. Likewise, the climbing plant is non-toxic to dogs and cats and can be planted without hesitation.

Black eyed Susan, along with several other plants, makes an excellent flowering screen along fences or house walls. You can read about the best climbing plants for fences & walls in our dedicated article.