Not every laburnum walk need have a frame. A double row of free-standing laburnums is a fine thing and, if it leads somewhere interesting, so much the better. But one has to think of the size these trees will grow to if left to their own devices.
The next step up in formality is the laburnum walk as devised by Sir Geoffrey Jellicoe for Horsted Place in Sussex, which appears from photographs (the garden has apparently been changed) to have included a curved path along which individual arches were set at intervals, just like rose arches. This kind of walk would suit many a town garden, but it would not have that tunnel effect which makes the formal laburnum walk so staggering.
The late Nancy Lancaster, who in old age had her butler go out every morning to record the dawn chorus, had a laburnum walk at Haseley Court, Oxfordshire, whose photograph may be seen on the front of Stephen Lacey's The Startling Jungle. It is, or was, underplanted with Siberian wallflowers and Welsh poppies.
Rosemary Verey famously underplanted hers at Barnsley House in Gloucestershire, with purple alliums. One other laburnum tunnel I would mention as a precedent is at the old royal palace at Kew. This is the most formally controlled of the ones I have seen, and also the most convenient to study if you were about to take the laburnum walk plunge.
The thing that might put anyone off creating such a garden feature is the expense of the metal structure. Agriframes make a version, but it is not cheap and I do not entirely like its look. For a metal tunnel of some size, it may work out cheaper (as it did for me) to go to a small metal workshop, describe the thing you want and see if you can afford it. The kind of workshop I mean is one that specialises in agricultural metalwork (the modern version of a blacksmith) and which does not charge fancy prices. All you want, after all, is a plain, sturdy frame.
You might, of course, use wood for the uprights, but the hoops which form the roof of the tunnel would still need to be metal of some sort if the structure was of any great size. A wooden frame which was designed to rot away in time would have to last for about 10 years, and any such frame has to be well cemented into the ground.
I take it that 10 years is the minimum to establish a rigid structure of live laburnum. But that is not the amount of time it takes for the tunnel to give pleasure. In year one, admittedly, you might be impatient, but from year two onwards there is plenty of flower and growth to enjoy, provided you do not do what I did, which was to buy the plants here and there, as the opportunity arose, in garden centres and nurseries, until I had the number I needed.
Do not do this. It is a waste of money and time, and you'd be astonished at the varied quality of the plants. The last thing you want is to be throwing out no-hopers in the second or third season.
The laburnum you want is L. x watereri 'Vossii', a hybrid that produces long racemes of flowers and does little seeding, which makes it less of a danger to children. Laburnum seeds are poisonous enough to kill a child. Their resemblance to peas in a pod might make them attractive as playthings. So any child should be taught to identify and not eat them.
L. x watereri 'Vossii' is common in garden centres, but it is absolutely essential to get reliable, uniform plants. Rather than waste money and time, order them from a reliable nursery such as Hilliers or Scotts of Merriott. For a two- metre bare-rooted whip, Hilliers charges pounds 19.99.
If the plants are good, they do not take long to take off. And when they take off, they can shoot up to the top of the tunnel in a single season. This is not one of those projects which requires immense patience over the years. Once a tree is established it will send forth great long whippy shoots, like a willow, in a matter of weeks. These have to be tied into place.
The aim is not to have a lot of lateral nor to clothe the sides of the tunnel, but to create a curved roof. Supposing the trees are planted 6ft or 7ft apart, all you need for the roof is four or five well-spaced branches fanning out over the tunnel, depending on how thick a shade you want. The rest of the growth is pruned to two or three buds in February, and the tree flowers from its myriad armpits.
The neater your pruning regime, the more striking the winter shape of your tunnel. If you do not like seeing nature so trained, then relax the regime. But you do not want to exclude too much sunlight: the underplanting is going to be a great part of the pleasure of this exercise.
Ideally, the width of the tunnel is determined as follows. You want a path down the centre, which is supposed to allow two people to walk side by side. This is not a tunnel for a solipsist, although it will become a fine place to retire to when seething or for smoking out of sight.
On either side of the path there should be space for flowers. So a total width of 12ft and a height at the centre of 10ft (one does not want those racemes down the back of the neck) seems a fair calculation. The ideal alignment of the tunnel is east-west, so that when you send out the butler to record the dawn chorus he can enjoy the rising sun as seen down the tunnel's length.
The underplanting is a great source of pleasure. Mr Lacey, in the book mentioned above, says he would underplant with hot colours. I jump the other way and go for pale blues and creams. Wallflowers feature in everybody's plantings because they are scented and coincide so well with the season. I go for ivory white wallflowers and forget-me-nots.
I read somewhere of an Irish laburnum walk featuring camassias, which are tall, pale-blue bulbous flowers, and I thought that perhaps my visitors would be less likely to have seen that combination than Rosemary Verey's alliums. So I nicked the idea and it works very well. No one has found me out yet.
Wallflowers also suggest tulips. Last year we added the peony- flowered 'Mount Tacoma' to the mix, but a couple of passing muntjak made a meal of them.
These elements of underplanting are only a part of the story. The object you are creating is a shady garden. Mine runs between a tall wall and a yew hedge. Clearly it has its weeks of glory in May when the laburnum flowers, and the bedding plants are geared to that season. The glory of the laburnum tunnel comes into its own the weeks before the roses come into their own. But before then it has the usual plants associated with a winter garden: snowdrops, hellebores, pulmonarias and suchlike.
After the laburnum is over, the wallflowers and forget-me-nots are removed (the camassias being left to die back at their own pace) and we put tobacco plants in the gaps - true tobacco, Nicotiana tabacum and N. sylvestris. Both reach 5ft or 6ft and are still flowering now.
There is never a moment when the beds are stripped bare. The bedding plants form a repeated design in between the perennials and small shrubs.
In the space between the tunnel and the wall, there are things which may be viewed from afar through the structure of the tunnel, without creating a desire to get up close; simple things like sweet rocket, honesty and foxgloves. And the wall itself supports nothing more elaborate than honeysuckle and some ivies.
The tunnel has just completed its fourth season and the canopy is almost there. It looks as if it will close up next year. After 10 years, the yew hedge will screen the whole thing off from the rest of the garden. You won't know it's there until you turn the corner and, voila] - a gallery of dripping yellow: dazzling, fragrant and cool.
Hilliers Nurseries Ltd, Ampfield House, Ampfield, Romsey, Hampshire SO51 9PA (0794 368733). Scotts Nurseries (Merriott) Ltd, Merriott, Somerset TA16 5PL (0460 72306).