It is a stiff challenge for Alexis Datta and her team of seven gardeners to keep the celebrated 3ha pre-war garden in tip-top condition.
"We spend a lot of time on the lawns, as they set off the rest of the garden," says Datta. "The greatest wear and tear is on the grass, so we keep that well-watered." Regular sprinkling, oversowing with sports turf seed and a high mowing height have so far succeeded in keeping grass looking lush, even during the South East's driest summer in years.
High maintenance at the Kent garden is combined with minimal chemical use. "I am keen to be as environmentally friendly as possible," she says. "We use a few chemicals, but only fungicides - we have used only organic insecticides for three years. Plant invigorators are helping to keep the plants healthy."
In line with the trust's policy nationally, the garden has been peat-free since 2000. "People say it's difficult to grow without peat. I think it's just different, you have to get used to it," says Datta.
Growing media is a particular issue at Sissinghurst because the garden is largely self-sufficient in plants, thanks to an extensive nursery and glasshouses, which generates a surplus to sell at the estate shop.
Only roses are routinely bought in. They come from David Austin Roses, which grows in peat-free in order to meet the trust's policy. Roses are among the garden's more demanding plants, with the bower in the "small and intimate" White Garden, for example, having to be re-trained each season.
"We have quite a lot of volunteers - 15 regular ones and a team who come in only to deadhead the roses, which is really useful," says Datta. "There's no way we could do it all ourselves."
The garden also provides a recurring three-year training programme for an apprentice under the National Trust's careership scheme. The current incumbent, a computing graduate, is at the end of year two. "It's a well-rounded programme in floral gardening, though it doesn't take in things like using tractors," Datta explains, adding that places are much sought after, with the three-yearly vacancies attracting 50 to 60 applicants.
People skills are as much part of the job as plantsmanship, she adds. "A lot of gardeners don't like making a noise when the public's in, but they might find it interesting to see how we maintain the place - otherwise we would never get it all done.
"The garden doesn't open until 11, so we have some respite until then. Also, a lot of the garden is quite small and narrow, so we can't work there when it's crowded. When the public are in, we get questions all the time. They also send in questions by post and email. You have to be polite."
This partly explains the appeal of the garden to Datta, who celebrates 20 years of service next year. "People relate to it better," she says. "They think: 'I could do that at home.' That's what's lovely about Sissinghurst."
Little of Sir Harold Nicolson and Vita Sackville-West's original 1930s plans for the garden was ever formally written down, but this has not been a problem for successive staff because it has been under continuous maintenance.
In addition, the couple's two head gardeners, Pamela Schwerdt and Sibylle Kreutzberger, changed the garden a lot. "It's as much their garden as Vita's," says Datta. The two were kept on when the trust acquired the property in 1967, with Kreutzberger retiring only in 1991 - the same year as Datta started at Sissinghurst, originally under then head gardener Sarah Cook. "She knew a lot about Vita from Pam and Sybille, so really there's a continuity right through," adds Datta.
Being a 20th century garden means "a different set of rules" to other heritage gardens, she says. "People say: 'Don't you find it restrictive?' But I have free rein to put in new planting - although it has to look as though it belongs here. Vita was all for the plants of her day, but you can't put in anything too modern-looking. I try not to pick up on the fashions in horticulture. And the colour schemes are a given - I can't make the White Garden pink."
The property's unusual status as a "Special Trust in Credit" helps sustain it financially, she adds. "The money made here, stays here. We make tens of thousands of pounds on plant sales - that helps. While some other gardens struggle, we are in credit."
Nicolson and Sackville-West's grandson, the writer Adam Nicolson, lives on-site with his wife, the broadcaster and garden writer Sarah Raven. A BBC4 television series last year, which featured Datta, documented the couple's efforts to bring the wider estate into production.
Datta also lives on-site - "not everyone's cup of tea" - and says of her chosen lifestyle: "You don't get very well paid, but you have an enjoyable job in a lovely place."
1970s: Gardens apprentice, Greater London Council
1983-91: Gardener, Cliveden, Buckinghamshire
1991-2005: Assistant head gardener, Sissinghurst
2005 to date: Head gardener, Sissinghurst