Garden design: It takes two

I often wish I had a pet garden design expert living in my shed. I could then turn to them at any hour of the day and ask their advice about the various problem areas in my plot. Why doesn't this corner work? What do I need to give that border a bit of wow factor? How can I make the shapes of the plants contrast with each other, instead of having one huge blob? I think every gardener, however experienced they are, stumbles over these dilemmas at some point.

I do have the next best thing to an expert in my shed, however. I've known garden designer Pamela Johnson since I first joined the National Gardens Scheme in 2007. Pamela has opened her own garden in Tooting, south London, just down the road from me, for around 20 years and she has been a generous source of advice.

Pamela is the sort of person who can explain things in a very clear, logical way, without seeming patronising or dictatorial. Her passion for design is matched by her love of plants, so I was intrigued when she told me she was publishing a book, with fellow designer Jill Anderson, that was intended to demystify planting design.

Jill and Pamela met as members of the Society of Garden Designers. Pamela has an art background while Jill trained in amenity horticulture before gravitating towards garden design. They found they shared the same philosophy, spoke the same language. These days, they even finish each other's sentences.

They both believe very firmly that the two aspects of design and horticulture are equally important when it comes to creating a wonderful garden. This may sound obvious, but quite a lot of designers (and the garden owners commissioning them) get caught up in the landscaping, and the plants seem to be an afterthought.

Pamela and Jill's book, Planting Design Essentials, is aimed mainly at experienced amateur gardeners, but it's also written with garden designers in mind. Pamela explains: "It's quite common for people to choose garden design as a second career. Most do a one-year diploma, and there is such a lot to learn, it's impossible to do everything in depth."

The result is a guide that takes you from the planning (and thinking) stages, through practicalities such as aspect (is your garden sunny or shady?), soil type (acid or alkali, clay or sandy?) and local weather conditions, to working with colour and selecting a plant list.

Sprinkled in among the sections are lists of planting suggestions, practical tips – such as using a trolley at the garden centre to try out different combinations of plants – and lots and lots of colour photographs.

Why all the mystique, you might ask? Surely you just buy the plants you like, plonk them in and hope for the best. Well, up to a point. Buying plants that don't suit your soil is a complete waste of money. You might as well shove a £10 note in the hole.

Plants that are going to outgrow their space very quickly are never going to be satisfactory, because they're either going to be elbowing something else out of the way, or pruned to within an inch of their lives.

Personally, the thought of drawing a scale plan of my garden makes me want to go and lie down, but as Pamela points out: "If you don't know how big your garden is, or how big a border is, how do you know how many plants you need?"

Good point. Perhaps that's why I always come back from a shopping spree at a nursery with either too many plants or too few.

Jill feels that if you regard plants as merely ornamental, you are missing a dimension. You need the structural stalwarts to show off the more decorative planting, in the way that you need to have walls in order to show off paintings.

"Many people are seduced by colour and flowers," she says. "You often hear people say, 'Oh, my garden only looks good in spring'. Well, that's because they only go to the garden centre in spring. They never go to the garden centre in autumn or winter, so they miss out on what will be looking its best in those seasons."

This doesn't mean that everything in the garden has to perform all year round. Jill says: "Many people think that if a rose isn't repeat-flowering, it isn't worth having. I think the fact that something is only there for a short time can be rather nice."

All gardeners are susceptible to the impulse buy, but Jill and Pamela make a strong case for trying to avoid this classic pitfall of the plantaholic. You have to stop looking at your plants as a collection of individuals, they advise, and see them as components of a whole scheme.

If you haven't thought through where you're going to put a plant, they argue, it usually ends up being planted where there is a gap, rather than in the best possible situation for its needs.

Another common mistake, they say, is not leaving enough space for planting. If you want a classic summer border, burgeoning with shrub roses and herbaceous perennials, a meagre foot-wide strip around the lawn is never going to suffice. "People realise that plants grow vertically," says Pamela, "but they don't seem to remember that they will also get bigger in width."

With years of experience as garden designers between them, Jill and Pamela are realistic about the challenges posed by urban gardens in particular, where very few gardeners start off with a blank canvas. Deciding whether to keep existing plants, they say, can be as difficult as deciding on which new plants to bring in. You should ask yourself three key questions: is it healthy? Do you like it? Will it fit into your new scheme?

The book lays great emphasis on sustainability. Each chapter stresses the need to know your local environment; to garden with nature, rather than against it; how to combat the vagaries of a changing climate.

At a time when gardeners are told one minute that the climate is getting hotter, only to find that the next minute they're facing one of the coldest winters on record, what's the solution?

Use tough, adaptable plants to form the structure of your garden, say Jill and Pamela, and think about using natives, or plants which have been bred from native species. When choosing plants from another continent, select those which have adapted to a similar geology and climate.

'Planting Design Essentials', by Jill Anderson (andersonlandscapedesign.co.uk) and Pamela Johnson (pamelajohnson.co.uk) is published by Crowood Press, £14.99.