"The owner of the farm shouted at me sometimes and said if I didn't get all the work done I'd get sacked". Sawuma left school at 15. She had to, as there was no-one to pay the fees - both her parents died and she had 4 brothers and sisters to look after.
"I got some work on a rice farm", she says. "I was given a plot and told to plant the padi. I was paid around Tsh 200 (10p) for half a day's work, but the pay varied according to how much work I did.
"It was very hard being out in the padi field, with the sun. I had to stand in the water with no shoes. Sometimes I got fungus between my toes, so I had to stop work. Once I couldn't work for two weeks. I wasn't paid for that time.
"The owner of the farm was very strict. He shouted at me sometimes and said if I didn't get all the work done I'd get sacked, so I had to work very quickly."
After two years, Sawuma was lucky enough to be selected for a trade union-funded training scheme for vulnerable young people. She now earns a living tailoring clothes.
Casual workers are acutely vulnerable to exploitation by their employers. Without written contracts and often unable to unionise, the fear of being told not to come back the next day often prevents them from complaining about their conditions.
It's common practice in many countries, including the UK, for workers to be kept on rolling temporary contracts, often working for the same employer for years at a time. This enables employers to evade their responsibilities to provide statutory benefits and entitlements, as well as equitable wages.
Retailers have a responsibility to ensure that their buying practices allow their suppliers to predict the volume and timing of their orders where possible, so that they can increase the numbers of workers on permanent contracts.
They must also ensure that suppliers are not abdicating their responsibilities as employers by keeping workers on rolling temporary contracts.