"I never intended to be here so long - but I still enjoy it," he says.
He is now seeing his fifth US ambassador at the US Ambassador's residence, Winfield House. "Some take an interest in the grounds, while others appreciate it but are happy just to let me get on with it," he explains.
The 5ha garden, nestling within Regent's Park, is a remnant of John Nash's ambitious Regency design. The house itself, although 20th century, adopted period style and is replete with antique furniture - and a host of plants.
He describes his role as "the custodian, running it like a National Trust property" - albeit one criss-crossed with infrared security beams.
Having a staff of only two, Crisp tailors garden maintenance accordingly. In the formal gardens near the house, consisting of a series of "rooms", he avoids staking plants - "no time", and aims for planting schemes that "lush up" enough in season to prevent weeds getting a foothold.
He replaced a rose garden, designed in 1981 by former Landscape Institute president Sir Peter Shepherd, with a geometric planting design inspired by the stained-glass windows of US architect Franklin Lloyd Wright.
"With a bit of care you can create interest for 10 months of the year," he says. "And it's so much less work than rose beds, which I loathe and despise - you spray them, prune them, deadhead them, and they still look rubbish."
Within the wider "Reptonian" landscape, rolling lawn gives way to woodland underplanted with shade-tolerant shrubs and perennials, where Crisp has laid meandering paths.
He is also responsible for the house's interior horticulture. "Generally I like the country-house style," he says. "But if there's something special happening in the house, the flowers and plants can contribute to the theme of the event."
A series of nursery glasshouses and all-weather tennis courts add to his responsibilities.
"It's quite full-on here, and I don't get out that much," he says. "But I like to keep an eye on trends from shows, books, magazines, even seeing what's in season at Habitat. It's important to be aware of what's going on rather than just regurgitating the same old stuff."
Contact with other gardeners is also important he says - he aims to visit at least one other garden a month. He is a member of both the Professional Gardeners' Guild and a fellow of the Worshipful Company of Gardeners.
Networking is an overlooked asset for gardeners, he says. "It's not something they teach in colleges, but pivotal things have happened in my career through being in contact with people. I got the job here because the former Homes & Gardens editor Peter Coates recommended me to the then ambassador and his wife. I've been lucky - but sometimes you have to make your own luck."
Contacts in the wider world also allow him to put something back. "I wrote to Westminster Council, initially tongue-in-cheek, about a stretch of canal that had become a no-go area," he says. "It ended up with £300,000 being invested to improve it. Constructive suggestions from an informed outsider can influence people like councillors."
Complementing this is his role as a Britain in Bloom judge. "It gives councils an incentive to invest in their public spaces, which not all of them do," he says.
He believes his role exemplifies much of what professional gardening should be like, but currently is not.
"I'm treated as a professional, paid well and given a degree of liberty to just get on with it," he says. "Too often gardeners are treated as a piece of equipment rather than someone with passion and experience to contribute.
"Most people are not in it for the money, so you have to value them in other ways. My staff are trained and motivated - and they wouldn't get that in the public sector.
"They tell me at the beginning of the day, 'This is what I am going to do', rather than looking to me to tell them. You give guys respect so they'll still get on with the job when you're not around."
Gardeners' pay does not generally reflect their skills, he says. "You need to have so many skills now, compared to 50 years ago - IT, marketing, PR, events management ... A pittance and a tied cottage is not enough, as a lot of people are finding."
He compares the situation with that of chefs. "Once you get a name for yourself, employers will seek you out and offer you a good package.
"But pay in gardening is low because there's effectively a cartel and no one will step out of the ring."
This is leading to a shortage of craftsman gardeners coming through, he explains. "Garden designers realise that without a good gardener, their new designs won't survive and thrive - they need someone who understands their aspiration."
1977-79: Studied for an RHS Certificate, Wisley
1980-81: Attended the International Gardener Training Program, Longwood Gardens, Pennsylvania, US
1981-82: Obtained a work/study scholarship at Tresco Abbey Gardens, Isles of Scilly
1982-87: Gardener, Leeds Castle Foundation, Kent
1987-present: Head gardener, Winfield House, London.