"Two solutions will work. Metposts are iron ferrules, used for fence posts and available at the bigger DIY sheds and nastier garden centres. You bang them into the ground and drop the post into the square slot. (Yes, I know iron is also impervious, but these have drainage holes.) The only difficult bits are (a) getting them to go in truly vertical and (b) getting someone younger and beefier to do the banging. There's less root disturbance, too.
"If you don't want to go underground, you could keep your posts from leaning over in graceful parallelograms by cross-bracing them. (I must admit I have never done this with a pergola, but it works brilliantly with shelving.) If they are firmly fixed to the top horizontal, one crossbrace should hold a row of posts square.The carpentry is quite simple and the effect is faintly Roman. I imagine that, on a similar principle, making tripods instead of posts would also be pretty stable."
Alan Crocker of Sandwich, Kent, is also a Metpost man and made two important points about them. The post should be slightly smaller than the timber, so that any water running down the timber falls outside the steel tube rather than in. And the Metpost should stick out at least 50-75mm above the ground so that the timber itself is never in contact with the earth.
Metposts featured again in Brian Allt's recommendations, contained in a densely written, fully illustrated broadsheet, not so much a letter as a complete DIY manual. If concrete must be used, he said, and his stern tone suggested that I was extremely misguided ever to take this course, then "four very stout nails, sticking out about 3/4", should be hammered into the bottom of the post. Further, fast-rotting "pegs" should be inserted at the base and sides (pushed into the earth) so that water can run away. Rolled-up, rubber-banded corrugated cardboard would do."
Graeme Allen of Eaton Bishop, Herefordshire, thought my elm branches and larch poles "quite unsuitable. Use rough-sawn timber, 3" x 2" or 4" x 2", pressure treated against rot. Connections should be made with bolts and nuts (galvanised), preferably with toothed washers in between. No nails."
John Tordoff of London E8 has an entirely different solution to the problem. "Working on the premise that before long the structure would be so smothered in foliage that its appearance was immaterial, I used steel reinforcing mesh. This is used in the preparation of concrete floors and can be obtained in sheets of 8ft x 12ft from any builder's merchant. From the same source come the uprights - 3/4" steel poles in 20ft lengths. Cut them to the required height and allow an extra 2ft for banging in with a sledgehammer.
"For an arch or pergola, three pieces of mesh are needed: two side panels and another for the roof. The mesh is easily cut to size with a hacksaw. A curve for the roof is achieved by laying the piece flat on the ground, standing on it and gently pulling up the edges into the desired shape.
"After positioning the uprights, side panels and roof piece are attached to them by a binding of green, plastic-covered wire. The finished structure has proved enormously durable, supporting, without any sign of stress, the ramblers 'Brenda Colvin' and 'Paul's Himalayan Musk'. It is an extremely cheap solution.
"There are two other pluses. First, compared with the ready-made pergola or arch, reinforcing mesh is extremely cheap; the other is that what little of it is visible acquires a natural patina of rust. The steel is thick enough not to corrode and therefore it never needs painting."
Thank you to everyone who wrote in on this subject. Perhaps we have invented a whole new genre. After the disaster movie, the disaster gardening column, with readers providing solutions to the gardening correspondent's problems.
Just watch this space...