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Jacarandas then and now

- Jennifer Fitchett

Monitoring the timing of recurring biological events is key to understanding the effects of climate change.

Jacaranda trees were introduced to South Africa from Brazil in the early 1800s, with the first trees planted in Pretoria. Since then, many Jacaranda trees were planted in Johannesburg and fewer across the rest of the country. Some of those Johannesburg Jacarandas are on the Wits campus, the most notable framing the Great Hall.

The Great Hall at Wits University © Shivan Parusnath |

Urban legend on campus is that if the Jacarandas start blooming and you’ve not yet begun studying for year-end exams, it’s too late to pass! Conversely, if a Jacaranda flower lands on your head, you’ll ace your exam.

Indeed, the purple haze across Gauteng in early summer has been well timed with the November examinations, and provides a beautiful display for those staring out of the window while trying to cram in a year’s worth of work. 

Today’s students might find this urban legend puzzling – in 2019, Jacarandas started flowering in mid-September, a full month and a half ahead of the start of the Wits exams, at the beginning of the fourth teaching block. While some students should probably start studying that far in advance, many probably would not be doomed to fail if their notes weren’t in order at this point. Does this mean that previous generations of students needed more time to study? No, it means the Jacarandas are blooming earlier than they used to.

Phenology refers to the timing of annually recurring biological events. For plants, these include the timing of flowering in spring, fruit development through summer, and leaf colouration and fall in autumn. 

In animals, phenology is more diverse and includes the timing of migration, hibernation, egg-laying and hatching. Seasonal changes in the environment trigger these phenological events, which most often relate to the temperature change in the shift from winter to spring.

As the world’s climate warms, temperatures previously experienced during the spring months of September and October are now occurring frequently in the late winter months of July and August. This means that phenological events in plants and animals who are triggered by spring events are now occurring in winter. Similarly, early summer is now being experienced in mid-spring.

These advances in the timing of phenological events have been observed across a range of flowering plants around the world, including our Wits Jacarandas.  

We tracked the timing of Jacaranda flowering in Gauteng over the past century using a collection of articles from The Star, Rand Daily Mail, Beeld, and Wits’ own Vuvuzela. Our study confirms that the phenological advances seen globally are occurring for our Jacarandas. In the past decade, there has been a 2.6 day per decade advance in flowering from 1919-2019, from an average peak flowering date in mid-November in the 1920s to late-September. 

These shifts demonstrate the plants’ adaptation to the warming climate. However, this advance in the timing of flowering cannot occur indefinitely, because it progressively affects the trees’ capacity to take up water, cycle nutrients, and withstand stressors.

As Jacarandas are invasive trees, for which replanting is prohibited, the long-term effects on tree health threaten our purple city in early summers.