'There are scary moments' Knapp says nonchalantly in her lilting Californian voice. 'Like when you see a poisonous snake. You think: is it going to bite? One time I jumped out of a tree and almost ended up on top of one. I also hate travelling in small aircraft. I've had to land on a grass strip with ocean at one end and jungle at the other. If the pilot had mistimed it . . . that would have been it.'
For Knapp, finding a new plant may mean hours of painstaking sifting through lovingly pressed collections in the calm of the Natural History Museum Herbarium. Or it may mean trekking over mountains with only her collecting gear (a plant press and clippers), a sack of rice, and the determination to do whatever it takes to get her plant.
'If you want to be a tropical botanist, you have to be prepared to go into the field,' Knapp says matter-of-factly. 'There is nothing to beat the thrill of finding something new and seeing it alive.'
One of her most exciting discoveries was made in Panama. A companion pointed out a small flower washed up at the river's edge. 'I took one look at this passion flower and thought 'That's it'. '
A less formidable woman might have been satisfied with one bloom, but the tall and wiry Knapp plunged into the chest -high water and fought her way upstream until she saw the whole plant overhanging the river. She tried to climb the tree but kept slipping. Her companion finally reached it down. 'He was really cross because by this time it was night, and we had to cross the river in the dark.' She pauses. 'You have to seize the moment.'
Knapp's passion for her subject stems from her childhood. Her father is a nuclear physicist and she grew up in Los Alamos, New Mexico, site of the first atomic bomb tests.
'That really influenced me. I grew up surrounded by scientists. My parents have always been supportive of my work. They've even visited me in the field.' In fact, all three of Knapp's younger siblings have gone on to become scientists - a physicist, a meteorologist and an applied mathematician.
It was always assumed she would go on to university, but to study mediaeval French literature, not botany. A class attended out of curiosity changed all that. 'I was mesmerised by looking at things in their natural habitat.'
She decided to make her career in systematics - the classification and naming of plants - taking a doctorate at Cornell University on, of all things, a type of rain-forest tree related to the potato. This was followed by time spent collecting for the Missouri Botanical Garden, often going to areas like the La Serrania de Maje in Peru, where few botanists had been before. The work requires stamina. 'You need to be able to put up with a lot, not just physical hardship, but mental torture too.
'When we were in Peru, Mrs Thatcher was at the height of her powers,' she explains. 'Every peasant farmer we met, even in the back of beyond, wanted to talk about her. It was just awful. It took a great deal of endurance to have the same conversation about the Iron Lady four thousand times.'
But if Knapp had a bellyfull of Thatcher in Latin America, she also found romance. She met her future husband, Jim Mallet, a butterfly expert, in 1980 while they were both working in Costa Rica.
The marriage did have a detrimental effect on her fieldwork. Two months pregnant, Knapp began to bleed during a mountain expedition. Only the herbs of a local healer helped her reach medical aid.
'I had been rather silly,' she says. 'The gynaecologist said I should stop working in the field for a while. I had to agree.'
The couple returned to England. Fortunately, Sandy soon saw a job advert for a tropical botanist at the Natural History Museum and applied. The job, although not desk-bound, (she still undertakes five trips a year) is somewhat less strenuous. Even so, she admits to missing her three young children when working abroad.
However, she seems to have found the ideal solution. When she takes off to Ecuador on her next mission, her children will go along.
'They are going to be my assistants,' she explains. 'They do a lot of collecting for us at home. Though, having been exposed to so much biology as children, I expect when they grow up they'll want to be accountants.'