The first thing you need to consider when planning your garden is what growing zone you live in. This is based on both the temperature range of your climate and the amount of precipitation. Take a close look at the area in which you are going to plant your garden. If the ground tends to be very moist, choose plants that can tolerate constantly wet soil, and even standing water. If you live in an area that suffers from frequent droughts, however, select plants that can tolerate going long periods without water, especially in light of the frequent watering restrictions imposed on such areas. If you are lucky enough to live in an area that has a balanced climate, you have a wider range of choices for your plants.
Low Water Requirement Plants
Plants that require low levels of water are often called drought tolerant. Drought-tolerant plants can thrive in hot, dry conditions with very little water. They include both perennials and annuals. Most drought-tolerant plants only have to be hand-watered when they are planted and while they are establishing themselves. After that, they can be left to the natural cycle of the elements. Popular drought tolerant trees include the red cedar. live oak, crape myrtle, and the windmill and saw palmetto palm trees. All citrus trees are also drought tolerant. Many homeowners in areas prone to drought, such as parts of the southern United States, use shrubs and ground covering vines as part of their landscaping. These include Texas sage, orange jasmine and Chinese fountain grass. There are not many perennial drought-tolerant plants, but amaryllis is one that is very popular, along with the African iris. Popular drought-olerant annuals include marigold, cosmos and the Dahlberg daisy.
Mid-Level Water Requirement Plants
Most plants land in this range when it comes to water requirements. These plants do not need to be watered every day, but they need to be watered when the soil has been dry for over a week or two. Sometimes these plants are classified as plants lying in the "occasional water zone". These include popular plants such as geraniums, most roses, wisteria, clematis and other vine plants, sunflowers, spring flowering bulbs, and most flowering perennial shrubs. Note that flowering annuals planted in containers will need watering at least once or twice a week, while annuals planted in the ground will need watering less often.
High Water Requirement Plants
Some plants require large amounts of water. These plants typically grow in marshy areas or bogs, or along the banks of rivers, streams and lakes. The soil for these plants should always be kept moist. Standing water is not a concern for these plants, so you don't have to worry about root rot. Perennials are especially good for wet areas because they don't have to be replanted year after year, which can be difficult in marshy areas. Popular perennials for wet soil include iris plants, cannas, bee balms, ferns, and bog salvia. Aquatic mint is a pleasant ground cover that likes wet soil. The red osier dogwood does very well in wet conditions. Most annual flowering plants also do well in constantly moist soil.