Season of bloom: A big mistake that many gardeners make is to buy only what they see blooming in the garden center in the spring. Their gardens end up looking lovely in spring and early summer, but lack color during the rest of the summer and fall. A balanced flower garden has about one-third of its plants in bloom at any given time. Divide your flowering season into thirds (or fourths, if you live in a long-growth season climate) and choose plants that bloom in each part of the season. Visit garden centers in mid to late summer to fine attractive plants that bloom in those seasons

Flower color and form: Gardeners usually make flower color their top priority when deciding which plants to purchase. Popular garden themes that revolve around color include single-color plantings, such as white gardens, soft pastels, bright crayon-box colors, or motifs to match the color of your house. Although you really can't go wrong in mixing flower colors, some hues naturally go well together.

Color wheels, which you can find at your local art supply store, show the rainbow as a circle of colored slices. Color wheel opposites, such as red and green, orange and blue, purple and yellow complement each other. Colors that form triangles on the color wheel, such as blue, green-yellow and red-purple, also make good combinations. A single hue (such as red) has many lighter and darker colors (such as pink and scarlet) within its family and combining these make single-color theme gardens more interesting.

Red, yellow and orange - called hot colors - jump out in the landscape and can appear closer than they are. Blue, green and purple - called cool colors - blend into the garden and look farther away. Use these colors to achieve certain effects. Cool colors in a small garden can make it appear larger, for example, while hot colors draw more attention to street-side plantings. White also stands out in the landscape, especially in dim light and is useful for planting with more colorful flowers to brighten or moderate the mix.

Flower size and shape contribute to the plant's overall appearance, too. Add variety and interest to your garden by blending plants that produce masses of small flowers with those that bear larger or single blooms.

Plant height and spread: Most gardens have a front, back and middle. To arrange plants by height, put the shortest ones in the front and tallest in the back, just like the lineup for a family photograph. Pay attention to the mature width of your plants and give them the space they need. But watch out for aggressive plants that travel unbidden throughout your garden. These usually creep rapidly above- or underground or spread by numerous seeds.