Bitter fruits: On the anniversary of the abolition of the slave trade, Emma Townshend considers how this dark chapter of our history influenced the world's plant life

When you go into the garden, you expect to get away from it all - not ponder shadowy historical matters. But actually the gentlest corners of our gardens are full of hidden history, some of it troubling. This week we commemorate the end of our slave trade; 200 years ago, on 25 March 1807, Parliament passed the Abolition of the Slave Trade Act. Even within our own tranquil English gardens there are reminders of that history, which are worth contemplation.

Slaves were overwhelmingly taken to be used as growers. They were transported in their millions to work on plantation crops such as tobacco and sugar. English entrepreneurs used them as labour to compete in burgeoning commercial markets, but they were always paranoid that other nations such as France were sneaking ahead by growing more profitable crops. The French and English both had their eye on the spices that had made so much money for medieval merchants, and many planned to get rich by growing pepper or cinnamon in the Caribbean, with slave labour. 'I hope I am not asking too much,' wrote one Jamaican hopeful in 1805, 'when I request from Kew Gardens a plant or two of the nutmeg, providing they can be spared; they would be a valuable acquisition to this island.'

I went to look at the handwritten volumes recording which plants came in and out of the gardens, which reveal the strenuous efforts made by the British to outplay the French. Lists of plants sent in the 1790s to Jamaica and Barbados display the hope that those islands would soon be planted with coffee and cinnamon, and - more optimistically - peaches, nectarines and gooseberries.

But the competition was only just hotting up. 'We have reason to believe,' wrote Johann Reinhold Forster in 1772, 'the French to be in a fair way of getting the spices in their plantations, as Mr de Poivre has actually planted at St Barths some hundreds of cloves and nutmeg trees.'

Slave owners began to see the logic of involving their slaves in deciding what to grow - the French were even rumoured to be finding economically useful plants by, 'being instructed by the negroes that arrive from the different parts of the African continent', as one letter-writer in the 1780s put it. The planters of Jamaica, concerned that the French were getting ahead, voted in 1775 to create two botanic gardens to investigate all such possibilities.

There wasn't just the question of what crops the slaves should grow. Feeding slaves as cheaply as possible was a great preoccupation of the time. In the Caribbean, planters wanted to use their best land for sugar, which meant that vegetables which could crop on poor soils were in high demand.

Sir Joseph Banks, who was then President of the Royal Society, and head of Kew Gardens, spent much of his life thinking about such matters. We often celebrate Banks's energy and vision, and the many introductions that we owe to his collectors - for example Strelitzia, the bird-of-paradise plant, Kniphofia, the red hot poker, and Banksia, first collected by Banks himself in Botany Bay. Much more ambiguous were his efforts to find a way of feeding the slaves. Banks had a memory of eating breadfruit - a starchy plant full of calories - while travelling with Captain Cook in Tahiti, and dreamed up a scheme to bring it to the Caribbean.

He secured a ship and had the decks reorganised to hold the many trees he wanted. The Bounty set off loaded with small plants, heading for the botanic gardens at St Vincent and Jamaica, and the famous mutiny was the result. In the end, slaves hated the taste, yields were low, and the foodstuff was slow to catch on. But you can still see breadfruit at Kew, where they are grown in the Palm House. 'In this building,' says historian Lucille Brockway, 'we we feel the extent of the British maritime and colonial penetration of the entire world.'

The New Cocoyam, Xanthosoma violaceum, was another valuable plant for the slave-owner. Introduced to West Africa from Central America, its starchy tubers were cultivated along that coast to stock ships preparing to make the long voyage to the Caribbean. This was the cheapest, simplest way of feeding the hundreds of people squashed into each boatload. And it doesn't even need much sunshine. At Kew, the Cocoyam is planted down a dark, descending staircase in the Princess of Wales Conservatory. I stood underneath its heavy, purplish leaves and felt the nausea of a sea voyage in chains, and with death occurring all around.

Elsewhere in the Princess of Wales Conservatory you will also find the dumb cane, Dieffenbachia picta. This is a member of the Aroid family - and has those wonderful leaves so typical of this group. Despite its splendid appearance, it too has a sad story, according to Aroid expert Deni Bown, author of the most important book on the subject. If the poisonous leaves come into contact with the mouth, there is immediate heavy swelling - hence the name dumb cane - the plant actually silences you. Every year there are still people admitted to casualty who have been burned by their houseplant - and the damage to the larynx and oesophagus can be permanent. Ostensibly, the plant was grown for use in the process of granulating sugar, but it was also used as a punishment for slaves, and even once to silence a witness in a court case. Most poignant of all, says Bown, 'Among black Americans in the slave era it was a chosen method of suicide'.

It's important to remember our history, certainly, but we can still find solace in a garden. According to James Walvin, Professor of History at the University of York, that's exactly what slaves did, carving out the smallest personal space for themselves by tending to their gardens and vegetable plots after their long working hours ended. And we could always choose to commemorate this historic anniversary by planting Canna indica, for me always a reminder of the Caribbean's other story, its beauty and its peace .

'Aroids, Plants of the Arum Family', by Deni Bown, is published by Timber Press, priced £25

If you do one thing... start the sunflowers

If you have any sun in your garden at all, think about sunflowers. They are so absolutely spectacular, and also attracts insects and birds in numbers. Start them now in pots, keeping a close eye on slug and snail predators. Once they get bigger, you can plant them out, but the molluscs will be on the case, so at least grit around the planting area. Your reward won't come till later in the summer, when their big smiley faces will greet you, for weeks on end.