ART / An unnatural device: Tom Lubbock on Ha-ha, a display by 14 artists in the grounds of Killerton Park, an 18th-century National Trust garden

THERE'S A joke in here somewhere. A ha-ha, in the normal way, is a rural illusion, an invisible trench that separates parkland from grazing fields. The beasts can't get across, but the trench is sunk in such a way that from the park side it looks as if park and fields are continuous, and as if the animals keep their distance from a sense of natural tact. It's also, at the moment, the collective title for a display of work by 14 contemporary artists, set around the grounds of Killerton Park, an 18th-century garden just outside Exeter, now the property of the National Trust. Contemporary art in this case means things like non-traditional sculpture, installation work, conceptual larks: not, then, the kind of stuff you'd normally associate with the National Trust. So - ha ha?

If the laugh was meant to be on the National Trust, one could have a little sympathy with that. (Perhaps some heritage silliness is creeping in here - the staff in the tea-room wear milkmaid outfits.) But actually, there's a more serious attitude problem. It's the idea of a garden that Ha-ha isn't happy with. It finds it suspicious. For as the Ha-ha brochure informs you - you need to have one, to locate all the items - a garden like Killerton's is not a product of unaided nature. No, it is the result of human artifice. It's a surprising observation, because who could conceivably have thought otherwise? Putting it like that makes a garden sound like a fraud. But then, contemporary art is in general very hot on detecting fictions, and seeing through them. A vague climate of knowingness and suspicion constitutes its essential world-view. And, to its eye, a garden is an especially sneaky thing because it does look a bit natural (trees and that), while being, of course, completely made up.

But a garden can't possibly be understood like this. It can't be understood (and couldn't have been made in the first place) without some idea of cultivation, of human design and natural growth working together. Without that idea, it does indeed become a strange and alien environment: not a place to which one could add, but a only site in which one can intervene.

Admittedly, there are very few sites where the contemporary artist does feel at home. Everywhere - even the studied neutrality of the art gallery - has become suspicious. Whatever the place, intervention is the way in; just as, whatever the subject, posing doubts or raising issues is the approach. It's no surprise to find that when the Ha-ha artists have 'responded to the site' - and most have in one way or another - they have responded with another question. It's just the way it always goes. But still, these things can be done well or badly.

The title piece, Cornelia Parker's Ha-ha, isn't a bad case in point. Killerton does have its own ha-ha, and it's an effective one. You need to get right up to it before you realise that the mild dip in the ground running along the bottom edge of the garden is a deep and uncrossable ditch. And at one point on the further side of the ditch Parker has planted some weeds - plants traditionally excluded from gardens, right? - in the shape of a pair of scissors: cut along the dotted line, so to speak. The point is gracefully made, and while it queries it, this item - uniquely - probably could become a permanent addition to the garden. On the other hand, there are Sarah Statton's Viral Buds, globular, sci-fi growths made of coloured translucent resin, plonked subversively in the middle of a real flower-bed, and heavy with various cliches from the contemporary copy-book: disease, mutation, techno-paranoia.

Interventions can at least have the virtue of surprise, and Peter Appleton's group of four elongated bird-boxes set on poles is quite a nice one. There are speakers inside them: sensitive (somehow or other) to the proximity of people, they emit recordings of birdsong, wind, words and musical sounds when approached. Anthony Gormley's Post is another. A tree has been pollarded right down to its naked trunk and becomes a 30ft-high pedestal for - if you look up - one of his life-size iron men. It's an arboreal Nelson's Column. It doesn't have enough ground round it to make this point very strongly, but presumably there wasn't much choice in available trees. This is always a problem with interventions: you can have a good idea, but the environment is not at your disposal, and things look makeshift. (Then again, Peter Randall-Page's organic stone carvings, fine in themselves, not made for this site, and perfectly portable, just seem to have been put in the wrong places.)

The other stuff is, in the main, not sufficiently enticing to draw the viewer into any reflection whatever - almost too dull to describe really, if only because description is likely to make it sound more interesting than it is. Georgina Starr's concept, for instance, sounds like it might be neat. She's intervened in the Acoustiguides. You're given a set at the entrance, if you ask, and through the headphones you hear voices tell of a mysterious incident in the locality. It is so boring, and so incompetently done, that five minutes of this tape is the maximum bearable. Or there's Bill Furlong's Time Garden. When you read the brochure, it seems one to look forward to: 'grasses from 12 time zones. Arranged in trays to create a 'time- table', this work is a botanical, geological and chronological atlas.' When you track it down, there are 12 pieces of turf, about half of them dead, with a kitchen clock beside each one, telling the time in the zone of origin: it's an idea that would be more successful if not realised.

One might also mention Anya Gallacio, one of two artists who aren't in the garden but in the Spacex Gallery in Exeter. Like other people, Gallacio has cottoned on to the fact that if you cover a floor with a large amount of the same kind of stuff, never mind what, it usually looks quite good. This time, since we're out responding to the countryside, it's raw wool, smelling strongly of sheep. A very townie idea. For the rest, whether the artists have shown themselves elsewhere as talented or talentless (both sorts are represented), their work on this site doesn't find a good reason to be there.

In general, I feel that the game should now be up altogether with 'responding to sites', because it's extremely rare for artists to have any vital response to the sites alloted to them. (They suddenly discover an interest in gardens, or naval hospitals, or the carpet industry.) Something can usually be cooked up, and it looks good on the cv. But it simply becomes a way of unnecessarily generating art - with some added tincture of worldly relevance - like a school project, or an answer to the old question 'What shall I draw, Mum?' It gives the not very gifted something to fall back on (a bad plan). And it's no big improvement on the old outdoor-sculpture practice of dotting already existing statues around a landscape. No doubt, if there was an artist with a real interest in (say) Killerton Park, something worthwhile might come out of it. Meanwhile it's a very fine garden, if you're round that way.

Ha-ha at Killerton Park, Broadclyst, Exeter, to 31 Oct. Open 11am-6pm (0392 881345)

(Photograph omitted)