This article is aimed at novice gardeners, sometimes described as those with brown thumbs, but it is worth reading to remind ourselves of certain basic principles which we ignore at our peril. Of course we learn by making mistakes even deliberate ones when we do something we know to be wrong like placing a plant in an unsuitable location and hoping it will grow despite the odds. The common mistakes described below come from an article by Pat Ferguson which I found on The Master Gardeners website.
Ah, the cheerful, pleasant vision of planting a beautiful garden. Hold that thought! If you are fairly new to gardening, you will need to know a few things that will help keep you out of trouble. Even armed with gardening know-how and gardening advice, being a gardener means, among other things, making mistakes.
First of all, before placing any flower, vegetable, vine, tree, or shrub into the ground, you must prepare the soil. You can purchase the highest grade plant material available; but if your soil is not properly prepared, your garden may be doomed to failure. Know your soil. You can start with top soil if you wish; but because by itself it is too heavy and dense and is limited due to porosity, water retention and lack of nutrients, you must add lighter material to it.
You may add any of the following materials to top soil to make it “good” gardening soil: sphagnum peat moss (aerates the soil to promote strong root systems, helps retain moisture and nutrients); perlite (white, lightweight little “pebbles” that improve aeration and drainage and help promote root development); vermiculite (similar to perlite); humus (rich, dark soil); composted cow manure (contains a wide range of minerals and nutrients, adds to the composition of the soil and holds moisture). Do not use fresh manure as it can burn young plants; composted manure has no offensive odor. A generally acceptable mixture is two-thirds soil and one third amendments, well mixed.
Know your conditions. For example, do not plant impatiens where they will receive 6 to 8 hours of sun daily because they will not survive. A better choice of annuals for that space would be petunias, verbena, geraniums, daisies, vinca, or cosmos. Plant your impatiens in a place that receives either a little morning sun with shade the rest of the day or where they receive dappled shade all day.
See more at The Master Gardeners