Gardening can be such a pleasure but choosing the best plants for your garden can be a trial. I don't know about you but if I'm at the garden centre, I often choose a plant because I like the look of it, without really thinking about where I can put it in my garden. I then discover I don't really have anywhere suitable so I go out and buy yet another pot for the terrace to accommodate said plant.

The moral of this story is don't buy on impulse. If you know you're heading for the garden centre, take a look at your garden first. I know that sounds crazy, but you'll be surprised when you discover that there are no empty sunny spots left, or shady spots or places where a shrub can spread. Don't forget those dry or boggy patches or the place that the wind sweeps across.

Apart from the aspect and the dryness or otherwise of the soil, don't forget the pH level, which is a measure of whether the soil is acid or alkaline. To determine the pH level, you can get a testing kit.

Most plants survive best in soil with a fairly neutral pH of say 6.0 - 7.0 (1.0 being the most acid and 14.0 the most alkaline). Others, however, such as azaleas, rhododendrons and others of the same family prefer an acid soil with a pH of less than 6.0, in which iron is more readily available.

You can neutralize alkaline soil by adding peat moss or aluminium sulphate (very carefully) and you can neutralize acidic soil by adding lime. However, you may be better off just choosing the best plants for the soil that you already have.

Okay, now you know what type of soil you have and where you have spaces in your garden so you have all the information you need to go and buy the most suitable plants. However, before you fill your trolley at the garden centre, have you thought about quantities? If you're planting shrubs, they will bush out and fill quite a large space but if you're planting ground cover or annuals, you won't want just one here and one there; you'll probably want groups of four or five.

One more thing before you make your final choice - what about colour? Do you just want a jumble of mixed colours or would you prefer a theme such as blue and white or red and orange. Blue and white give a cool relaxed effect but red and orange are fiery and not so restful. Think about what you use that part of the garden for and choose your colour scheme accordingly. Don't forget to use grasses and foliage in for structure and a change from bright flowers or choose some shrubs with variegated leaves, for contrast.

Before you actually dig the holes for your newly acquired plants, you should place all your purchases where you think you want them to go, still in their pots. Move them around and group them in different ways until you're happy with the result. Only then start digging.