Deciding what plants to use in a perennial garden and where to plant them can be a daunting task. Many new gardeners tend to overplant their perennial gardens with a wide variety of colors, shapes and sizes, which ends up looking disorganized and uncoordinated. Others fail to consider the soil and sun conditions and end up planting the wrong perennials for their location. By breaking down the planning and planting process into smaller steps, you can easily consider each element and compose a garden that is well balanced and coordinated and supports the healthy growth of your plants.
Consider the conditions in your garden. While you can amend the soil pH or add fertilizers to supplement the nutrients, it's very difficult to change the soil composition or sun conditions of your existing garden. Instead of trying to change the location, take an inventory of what portions of your garden are full sun, partly shaded or completely shaded, and notice what type of soil you have (light and loamy, sandy, clay-based, etc.).
Make a scaled drawing of your garden (using graph paper will make it easier to stay in scale), noting which areas get continuous sunlight. Take note of how much area you have with full or partial sun; this calculation will be important when you choose your plants.
Consider what type of perennials you'd like to plant. Consider the size of your garden, and stick to four or five varieties of plants. Choose some flowering perennials, like iris or peonies, and some green "filler" perennials, like hostas. Choose plants that grow to varying heights and consider how the colors of each flower will look next to one another.
Make a list of the perennials you want in your garden, and check the soil, water and sunlight requirements for each. This information is readily available at any garden store or from an organization like the National Gardening Association (see the resources section for their website, which details sun, soil and water requirements for most plants). Make a note of each plant's requirements. Eliminate any plants that will not grow in your garden's environment.
Fill in your diagram with the plants you have chosen. Place shorter plants near the front of the garden, so they are not hidden by larger plants, and disperse flowering perennials among green filler plants. Group plants according to their water needs, so plants that need thorough watering are not next to plants that thrive on occasional droughts.
Plant your perennials according to your diagram in the early spring or late fall. Some perennials, like peonies, should be planted in the fall, while others, like Indian Summer, do best with spring planting. Consult the National Gardening Association website for planting directions, or ask an employee at your local gardening store. Follow the spacing directions, usually printed on an information card or sticker and attached to perennials before they are sold, to know how far apart to space your plants. Do not overcrowd perennials, as they will usually grow and spread out each year.