The first thing to remember is that a garden center is not a supermarket. Well it may be, but some of the worst mistakes are made, the sort that can really cause a great deal of damage to the garden in the future, when people walk around behind a trolley, as though they were in one.

While you may do your weekly, family stock-up armed with a shopping list, there's always room for some impulse buying. But when in a garden center, never buy a plant on impulse. The plant should only be purchased if it has a part to play in the planting scheme and the generel garden design. It is on a shopping list that has been carefully considered and researched. Of course nobody is suggesting that the planting plan is unalterable, holy writ. There's nothing wrong with some flexibility. However if you suddenly see a plant and say to yourself," that looks so nice. I must put that into my flower bed," do not be tempted into purchasing it – on principle. Instead, make a note of the name of the plant, botanical names are better than common names, because of their universality, and do some homework. This is what you should be looking for:

* Is the particular species likely to enhance the garden composition, or could it detract from it? If judged by the six principles of design, how does it stand up? Is it in scale with the rest of the garden, does the plant maintain or disturb balance, and if it contrasts with the rest of the plants, does it have at least something in common with them? What in short is its compositional role? Emphasis, contrast, or support?

* Does it have undesirable characteristics such as aggressive, invasive roots, that could do damage to the house?

* Is the plant liable to become a noxious weed? Could it spread uncontrollably throughout the garden.

* Is it suitable in terms of its cultural requirements, such as full sun or shade, acidic or alkaline or neutral soil, and hardiness to frost or heat?

* Is the plant notoriously vulnerable to pests and disease? The most effective way of controlling pests and disease, is to avoid plants that are susceptible in the first place.

The right time to consider personal taste is when these questions have been answered positively. It's all somewhat counter intuitive, because the natural starting point is to say, "That's nice," or "I like that". Yet if you can resist the temptation and instead, go through the thinking process described here, you stand a better chance of avoiding the serious errors that many people make, and sometimes heavily regret, when it comes to including a plant specimen in their garden.