Soil: sand to clay

Light: Full sun to part sun

Bloom colour: Yellow

Bloom period: August

Height: 7 feet

Moisture: Medium

Attracts: Sweat bees and bumblebees, butterflies, and birds.

Notes: Wild senna is a fantastic perennial that will reach 7 feet high when grown in part-sun well drained soil with regular moisture. It is versatile enough to thrive in a range of garden conditions, but it may not be as tall; therefore, this is an easy plant to grow. It remains quite upright and does not need too much support. In theory, you could make a simple hedge out of these plants because the foliage is quite dense, but it also works well in small clumps as shown. In this case, it has been surrounded with geraniums, but it would do equally well with great blue lobelia. It does have the potential to spread through rhizomes, so be prepared to cut back any new growth in places where it is not wanted. The plant is native the Eastern half of North America including Ontario and the areas around the great lakes.

Closely related to Acacia plants, wild senna shares a number of interesting ecological and aesthetic properties. The distinct composite foliage, which is typical of the plants in this family, remains in good condition right up to the end of summer. It provides interest for the whole season and the seedheads remain attached throughout the winter.

In July, extrafloral nectaries appear on the plant in advance the flowerheads. This is an unusual feature, especially for a temperate plant. Most plants do not invest considerable resources in protecting their flowers when they produce so many of them. These nectaries are visited mostly by ants who may chew them down a bit to obtain the nectar. It is thought that the ants may play a role in protecting the flowerheads directly from herbivorous invertebrates or the extrafloral nectar could be providing an alternative nectar source to discourage ants from farming aphids. The flowers are not covered with ants as this might be a hindrance to pollinators and the crucial service that they provide. Therefore, a possibility exists that wild senna has some chemical means to discourage the ants from walking onto the inflorescences.

Foot long racemes of flowers crown the plant in August and the individual flowers require sonification by bumblebees to release the pollen. The three anthers next to the white hairy stigma are functional. Their larger size enables bees to grip the anther properly while carrying out buzz pollination. While most anthers open along the lateral slits, the anthers in wild senna are poricidal. They open only at the tips and relatively few bees can get access to the pollen by sonification. The bees have to vibrate the anther at the right frequency during which copious amounts of pollen fly out. There is a set of anthers at the top that are definitely not producing pollen. The redundance of the anthers might be because the flowers evolved a more efficient means to attach pollen to the right pollinator.

The plant itself is a host for two sulphur butterflies in Ontario and its seedheads, which remain on the plants until the following spring, feed several birds.

Wild senna is a handsome plant with plenty of interesting features as well as high ecological function. Perhaps more people should consider this species as an option for their garden.