I had come for a speech by a friend, Senator Tim Wirth - chairman of the first Clinton-Gore campaign and now the rather grandly entitled Under Secretary of State for Global Affairs - on the US stance at negotiations on curbing global warming. (It was pretty depressing, but that's another story).
I arrived just in time to be included in his personal guided tour. As we went round, the director, Professor Sir Ghillean Prance, showed us the stamps of the gardens' twin plagues - Canada geese and badgers. The lawns were littered with the birds' droppings and raked by the beasts' claws. A special goose-proof fence has had to be erected around the magnificent Palm House, and rare plants are being endangered by badgers snuffling in their mulch.
While Brock has had a good press ever since The Wind In The Willows, Canada geese are not easy to love. Thugs with wings, they drive out other birds and even attack children. Combining prodigious appetites and regrettable digestive systems, they each produce a long, slimy, green dropping every three minutes.
Forty years ago there were just 2,000 in Britain: now there are 60,000. Their numbers are doubling every eight years and a government committee has sat for half a decade to discuss what to do.
It's not easy. Wandsworth Council planned some years ago to shoot 200 geese to protect Battersea Park, causing such an outcry from Linda McCartney, Carla Lane and others that Rentokil - presumably not the most squeamish of firms - refused to do the job, fearing "irreparable damage" to its reputation.
In 1993 government marksmen secretly shot 100 geese at dawn in St James's Park - the very place where Charles II first released them. Milton Keynes has clandestinely removed eggs from nests, hard boiled them in a giant tea urn and returned them, hoping the parents wouldn't notice. Kew says it destroys eggs by pricking them or dipping them in paraffin. Has it killed any birds? "We will not divulge what we do to keep the numbers down."
Badgers are also booming - their numbers have almost doubled, to more than 440,000, in just nine years. There are now nearly twice as many badgers as foxes - but they enjoy as much if - not more - protection as Britain's rarest wildlife.
They are the only species to be safeguarded by a special Act of Parliament all of their own. You can't even use machinery within 20 metres of a set, or clear scrub within 10 (except with a special licence), on pain of six months in jail. In July this prohibition against disturbing badgers halted house building in the grounds of a listed building in Dorset after orthodox representations had failed. And when John Gummer as Environment Secretary torpedoed the nuclear industry by refusing to allow initial work on a national waste dump site, he cited inter alia damage to the habitat of a local badger clan.
None of this impresses farmers who believe badgers give their cattle TB. And they're not cheering at Kew, badger-free till eight years ago, which now harbours about 50.
Keith Collins, a reader, of Whitton, Middlesex, writes to ask if he can shoot magpies - also rapidly increasing - which he hates for killing songbirds. The Royal Society for the Protection of Birds says that by law he can, providing he does it in his own garden and can show that they have preyed on other wildlife.
But it pleads for understanding. "If you're a small bird a magpie can seem a pretty nasty piece of work," it says. "But to us, it's just trying to make its way in a countryside denuded of food."
Mr Collins is unmoved, saying he'll have to learn to shoot. At present he keeps three tennis balls to throw at the birds but, he says, they usually don't take the hint.
the final word must go to a company called Acheta, which has sent me details of "pest control" courses "tailored to the requirements of individual customers". The pests it lists include: "urban birds", "rats and mice", "insects and mites" and - I kid you not - "receptionists".