The first thing to consider before investing in perennial plants is winter temperatures. Some perennials can withstand bitter cold winters that will kill others. To tell which perennials will overwinter where you garden, determine your USDA Hardiness Zone. In New York, these zones range from frigid Zone 3 in parts of the Adirondacks (where temperatures can drop to -40 degrees F), to the relatively mild Zone 7 on parts of Long Island (where the temperature seldom drops below 0 degrees F).
Look for hardiness information on plant labels or catalog listings. If a plant will survive winters in your zone, it is said to be hardy to that zone. Tender perennials are those that are not hardy in your zone, but can sometimes be overwintered by digging up their roots and storing them in a cool place or moving the plants into a greenhouse.
Match your plants to the amount of sunlight your site receives. Many perennials need full sun -- six or more hours of direct sun a day. Some, such as hostas and coral bells, do well in the shade. Also consider the balance between good air circulation (which helps prevent disease) and protection from strong winds (which can dry out soil and plants).
Because your perennials will grow in the same place for at least several years, it is particularly important to do a good job of preparing the soil. Most perennials require well-drained soil. Avoid putting beds in low-lying areas where water pools after heavy rains, or be sure to choose perennials that prefer boggy areas. Similarly, there are perennials that will tolerate other less-than-ideal sites, including those with dry soil, low fertility, and other conditions.
If your site has not been planted before, start improving the soil in the fall before spring planting, or in early summer before fall planting. Kill the sod with organic or plastic mulch, herbicide, or by turning it over with a shovel. Hoe out any weeds or grass that survive. The site should be level or only gently sloped to keep soil from eroding.
Work in three to six inches of organic matter (such as well-rotted manure) to improve the soil. This is particularly important to improve drainage in heavy clay soils or improve water-holding capacity in sandy soils. Apply about 2 pounds per 100 square feet of low-nitrogen fertilizer such as 5-10-5 or 5-10-10 or an equivalent organic source and work it into the top 4 to 6 inches of soil.
Contact your local Extension office for information about how to test your soil to find out your pH and nutrient levels. They may suggest a more complete soil test from the Cornell Nutrient Analysis Lab. Follow the directions on your soil test report about adding lime to increase pH or adding fertilizer to correct nutrient deficiencies.
If deer are a problem at your site, choose plants that deer tend to avoid. Other alternatives include regularly applying deer-feeding deterrents to plants or installing 6- to 8-foot-tall deer fencing or other barriers.