Today, I’m going to talk about what is a pretty nice looking shrub to small tree, but is a weed here in Sydney. It is a native of South America, specifically Brazil and Paraguay.

Plant description

The leaves are composed of 3-6 pairs of oblong leaflets, which are up to 5 cm long with rounded tips and yellowish margins , there is also a gland between the lowest leaflet pair. The leaves are dark green in colour and are arranged in an alternate arrangement on the branches.

The flowers are bright, yellow and fairly unmissable in the landscape. These flowers have two or three long stamens and have five petals. They appear in clusters at the tips of the branches. They are usually present from summer to autumn, but can be present all year.

The fruit are followed by pods that are green when young and pale brown to straw-coloured when mature. They are usually 10-20 cm long and contain up to 40 seeds. These seeds are brown, round and flattened and 6-6 mm in diameter.

Usually, senna is multi-branched. The branches are green and sparsely hairy when young, but woody and brown when mature.

Preferred growing conditions

Senna prefers tropical, sub-tropical and warm temperate climates, being happy to grow in full sun and shade. Senna is happy to grow in many soil types, including sandy and salty conditions. Frost is tolerated and the base of the plant can re-shoot if the upper parts are damaged by frost (which it can also do after fire).

In NSW, this weed can be found on disturbed ground like roadsides, in woodland and forest-especially on its edges, hind dunes along the coast and in gardens.

Why is this plant a weed in New South Wales, Australia?

This is classed as a weed for a few reasons, as follows:

• It spreads into native bushland areas, especially those along the coast
• It grows fast enough to out-compete native plants
• This in turn reduces the available habitat and food for native animals

Sometimes I will hear arguments that we should just leave plants like senna, and others that do more damage, to do as they naturally do. As for as I’m concerned, they should be removed as much as possible. I say this because the wildlife here is unique and all wildlife should have the chance to thrive-just because many people are too lazy, unwilling to either give time or finances to help sort out the problem or not helping to educate people on how to prevent other plants from becoming such a problem in the future doesn’t mean that native species should suffer for it! This inertia just makes things worse in my view.

Senna, as with many weedy plants, will spread quickly and easily, which is a weed prerequisite (and why they are so successful at what they do). With senna, seeds are produced once the plant reaches the age of 2-3 years old. Each tree has the potential to produce 1000 or more seeds a year, all of which can be viable within the soil for five years. Seeds are easily spread by water, contaminated soil and by the dumping of garden waste (rather than sensible disposal).

Control

With small plants, they can be pulled out by hand easily. If possible, remove the roots, to prevent regrowth. Again, if possible, with larger plants, ensure the whole crown is removed, with seed pods (if present) put into bin bags to prevent them staying behind!

Admittedly, weed removal is an utterly thankless task-I’ve been there and done it many times. The main thing is to be as diligent as possible in removing as much as possible, every time you attempt removal, as it does get rid of problem plants over the long term. Taking pride in doing a good job, is the way forward-a quick, half-hearted attempt is no attempt at all!

References

https://www.business.qld.gov.au/industries/farms-fishing-forestry/agriculture/biosecurity/plants/invasive/other/easter-cassia. Accessed 28/02/24

NSW Department of Primary Industries, NSW WeedWise, 2021, written by Wendy Gibney, edited by Brigitte Verbeek, found at:

https://weeds.dpi.nsw.gov.au/weeds/cassia. Accessed 28/02/24

All photos are my own