Not Kew, although it is a giant conservatory, the second largest in London. Look closely and the concrete perhaps gives it away. It's at the Barbican, of course, at the centre of a maze of passages above the main arts centre. I think I've only ever stumbled on the front door twice, even though the location looks blindingly obvious when you get there, and only once on a day when those doors were actually open. Sundays and bank holidays only, eleven until five (which are slightly longer hours than it says on the notice outside). For the rest of the week the Barbican Conservatory falls silent apart from the occasional special event, so do bear it in mind if you have a corporate soirée to organise.
The entrance is outdoors round the back of the theatre, although you'll probably find it most easily by following signs to the 3rd floor from inside the Barbican Centre. Negotiate the swing doors and you're in, to meet the gaze of a welcoming volunteer. It's their job to give advice as necessary, to click the numbers up one for every visitor, and to try to stay awake throughout their shift. But you'll be looking elsewhere by this point, at a high pyramidal atrium filled with plants, topped by glass and lined by concrete.
This is the more geometrically laid out side of the conservatory. Tropical palms and ferns rise from rectangular raised beds, each made from reassuringly 1960s-style brown bricks. Many plants have been selected for their loftiness, because there's a lot of space to fill, and also to part-cover the far grey wall. This shields the theatre's fly tower - the void above the stage where scenery is suspended - and the conservatory was indeed built as a deliberate act of camouflage. Across to the left is a small cage filled with birds, mostly finches but also doves, sparrows and quail. They don't have far to fly, but they do have a surprisingly varied environment in which to live out their lives.
To reach the other side of the conservatory requires wallking along a rather narrower path, a little more like a back passage from a sports centre, but still green. Expect to see leftover evidence that gardeners work here (and I'd guess quite a few gardeners given the luxurious loveliness of the place). Quickly you enter the less formal more densely-packed half, with a forest of lush leaves and high trunks all over. Hunt around and you'll find a fountain and a small footbridge as part of an internal water feature. You might even be lucky enough to find space on a bench to sit down and soak the whole place in, although they're rare, so arrive early.
As befits the very best garden conservatories, several changes of level are possible throughout - a few short flights of steps to raised pathways, and one proper staircase up to an elevated walkway. There's a futuristic Seventies feel to the concrete overbridge, inasmuch as you might expect a character from Logan's Run or Blakes 7 to come striding across at any moment dressed in clingy nylon. A spiral staircase ascends to the very top of the fly tower, strictly staff only, but you're already high enough to enjoy a grand overview of the verdant plantation down below.
There's one last free treat on the top level, which is the arid house. Step inside to enjoy a range of cacti and succulents, again impeccably looked after, and some rather splendid-looking orchids. I was hoping to peer more closely at some of the specimens, but the room had already been half taken-over by a group of girls intent on filming an amateur fashion shoot against this exotic backdrop. They put up with a few early morning intruders, but you could tell they were keen for us to stop gawping and get out of shot.
I hope I've tempted you to take a look inside the Barbican Conservatory. You do very much have to be in the right place at the right time, which is why it's taken me years to go inside, but it was worth the wait. The mix of brutalism and botany is the key attraction, but half the fun is exploring every nook and cranny to discover what further delights (ooh, koi carp) lie beyond. Just don't allrush this Sunday, will you, else the rarefied atmosphere of the Barbican's secret garden might be lost.