Urban Jungle: Hidden treasures in the Barbican maze

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Pat "the palm" gave me the tip about visiting the conservatory at the Barbican centre. Pat's tips, so long as they don't involve horses, are usually worth following, so I phoned the Barbican to set up a visit. I had detailed instructions about where to go and I felt confident, even though the Barbican Centre always gives me the fear. The fear of getting lost. I know it's an old observation about the Barbican but I have managed to get myself lost there so many times in the past that it was inevitable the nightmare would recur. And it did. Luckily, a kind person took pity on me and showed me the way: through the garden room (level 3) to the conservatory (level 3 in the brochure but I'm sure it's really 2.5).

I'm glad I persisted, because Pat's tip was a good one. As you enter the conservatory, you notice the tallest plants first: they are at eye- level before the steps down to the main area. It reminded me of a trip I once made to Belize when I climbed a Mayan ruin to look down on to the jungle's canopy. There is an enormous phoenix dactylifera (date palm) with its classic palm shape and fibrous trunk. This one is so tall that it's touching the glass roof. A three-year plan is under way to move it to a site in the conservatory where it will have some headroom in which to grow. The plan involves digging half way round the palm's roots in the first year, digging out the other half during the following year and then moving the three-ton plant in the third year.

The conservatory also houses a giant howea forsteriana - the kentia palm - which is more usually grown as a houseplant. I had no idea these could get so big; there are several here and the largest has also reached the roof. Kentia palms have elegant foliage, which starts off upright and then droops to near horizontal. Tree ferns (dicksonia antartica), getting on for 16ft tall, add to the jungle atmosphere.

The conservatory was built as an "add on" to the main Barbican building in 1980. It covers 23,000 sq ft and is situated on top of the theatre's fly tower (where sets are stored) and was originally designed to disguise this rather overbearing structure and to improve the view for the local residents. Its design envelops the central tower, cleverly incorporating the towers' balconies to create a "hanging gardens of Babylon" effect.

As you start to explore, the space itself seems to grow. Decisions need to be made. Up the stairs? Or through the balconies? Along a path which leads to a bridge over a fish-filled stream? The feeling of being on a journey keeps each composition fresh. Every planting opportunity has been exploited to include palms, trees, climbers, trailers, clingers, ferns, ground-coverers and even air plants floating about.

Cereus peruvianus, with its silver-blue leathery skin, bears a strong resemblance to euphorbia eritrea. A tall and upright plant straight out of a Western movie, it takes you by surprise as you turn a corner on the way to the arid house - segregated from the rest of the plants as they enjoy much drier conditions (without any water at all between September and March) and wouldn't tolerate the humidity of the main conservatory.

My favourite discovery was a plant that looks like a succulent but is actually an epiphyte. Epiphyllum cranatum is a real dangler. It would normally live in the humid rainforest by rooting itself in pockets of leaf mould found in the hollow of a tree. Here it is grown in a hanging basket where its flattened and deeply notched leaves trail down at least six feet to brush your head as you walk past. Some of my other favourites here include three different figs: the Bengal fig (ficus bengalensis), the Fiddler fig (ficus lyrata) and the biggest weeping fig I've ever seen (ficus benjamina). Though they are all from the same genus, these plants, in terms of bark, leaf shape and size, couldn't be more diverse.

There are around 2,000 different plant species here, making it almost impossible to create the optimum environment for all of them. Most of the plants look pretty healthy and have adapted well to their artificial environment, but low light levels due to the shade of the tower mean that some of the plants are getting leggy as they search for light. I actually like the feel this gives to the conservatory, because this is exactly how the plants would react in their natural habitat. By contrast, fluctuations in temperature in the conservatory are far more extreme than most of the plants would naturally be used to - summer temperatures at night in the Barbican can drop to 11C, and shoot up to 38C during the daytime.

The watering system is crucial to the health of the plants as it helps keep up the humidity. It incorporates a nitric acid dosing system, which reduces the pH, keeps the soil acidic (which these plants like) and stops the calcium deposits, found in hard London water, from blocking up the irrigation nozzles.

Thanks Pat - it was well worth the trip and I would thoroughly recommend it. One thing to remember, though, is that if you're going to lose yourself in the Barbican try to make sure it's in the conservatory rather than somewhere between the car park and the main reception.

The Barbican Conservatory, Silk Street, London EC2Y 8DS is open on most Sundays and Bank Holidays from 10am-5.30pm. Call 0171-638 4141 for details

Joe Swift owns The Plant Room, 47 Barnsbury Street, London N1 1TP

Duff Hart-Davis is away