What is a Perennial?
Perennials are plants that grow for 3 seasons or more. Some are short-lived plants, such as columbine (Aquilegia), which last only a few years, but happily seed around. Others, such as peony (Paeonia), may last 60 years. Perennials are herbaceous (non-woody) plants. Some are evergreen; most die back in the fall and reappear in the spring. While annuals tend to have non-stop flower power throughout the season, perennials, on average, have shorter bloom times. They often make up for this with wonderful foliage and a certain elegance that is lacking in most annuals.
Winter Hardiness Zones
The U.S. Department of Agriculture Plant Hardiness Zone map (see link in right column of this page) divides the United States into 11 zones defined by the lowest temperature reached during the winter. Zone 1 is the coldest, with temperatures down to -50°F, while Zone 11 is the warmest, with winter temperatures no lower than 40°F. New York City is in Zone 6 to 7. This means that plants listed as Zone 6 or lower will usually survive the winters in this area, while Zone 7 plants might need some protection. Plants listed as Zones 8 to 11 should be taken indoors in the winter or treated as annuals. Most reference books and nursery catalogs list hardiness zones for all plants. But remember, except at the extreme ends of the scale, hardiness zones are guidelines, not absolutes.
What Do We Look for in Perennials?
There are many factors to consider when choosing a plant. The "right plant in the right place" always holds true; you can only alter your growing conditions so much. If you have sandy soil, try plants that not only tolerate but thrive in sandy soil. If you have partial shade, don't plant perennials that require 8 hours of sun a day; you'll be disappointed with the results. Don't forget design considerations; if you have a bed of hostas, add ferns, bleeding hearts or other shade perennials for contrasting color, texture and height.
- Amend your soil with organic matter such as compost, composted manure or peat moss. Dig 4 to 8 inches deep (usually the depth of your border fork or spade).
- Water your perennials in their container before planting. Make sure that the ground you are planting them in isn't too dry or too wet.
- Tease the roots of the perennial if the plant is pot-bound.
- Plant at the level of the crown (growing point at the base of the plant; the stems grow up from the crown and the roots grow down). If you plant too deep, the crown will rot.
- Gently firm around the base of the plant once it has been planted; good contact with the soil is important, but you don't want to compact the soil.
- Water perennials immediately after planting. Keep them well watered for the first month (at least once a week) until the root system gets established. Thereafter, water infrequently and deeply rather than frequently and superficially. You will get stronger plants with better root systems. If plants look like they are wilting, water immediately.
- Mulch around your perennials to prevent weeds and to keep their roots cool. Don't pile mulch around the crown of the plant; you will encourage rot and disease.
- Granular fertilizers or slow-release fertilizers work best for perennials. Try a low concentration such as 10-10-10 or 5-10-5. You can also add super-phosphate to encourage root growth. Scratch fertilizer in the surface of the soil around the plant. Follow the directions on the label and do not over-fertilize. In established perennial gardens, wait until the plants are 2 to 4 inches tall before you fertilize them.
- When dividing perennials in the spring, make sure that they have at least one month to get established before the intense summer heat. Divide fall-blooming perennials in the spring and early spring bloomers after they flower.
- Stake plants early before they get too tall.
- Deadhead plants throughout the season to prolong bloom and to tidy the plant. Leave ornamental seed heads when warranted, on plants such as coneflower (Echinacea).
- Once large or floppy perennials such as false indigo (Baptisia), catmint (Nepeta) and blue star (Amsonia) have bloomed, groom them by shaping them with shears or pruners.
- For fall-flowering asters and chrysanthemums, cut back by ½ in early June when the plant is 1 foot tall to prevent it from getting too floppy. Otherwise, start pinching it back at the end of May and then every 2 to 3 weeks until July 4th. Sedum, Montauk daisies, Russian sage, Joe Pye weed and hardy hibiscus will all become bushier, more stable plants with a significant cut back early in the season.
- Make sure you properly space your perennials. Small perennials should be planted 8 to 12 inches apart. Larger perennials should be spaced 12 to 24 inches apart; otherwise they will become overcrowded. For the first growing season, you can plant annuals to fill in bare patches.
- Leave space in your perennial beds for maintenance access, bulbs for early-season interest and annuals for full-season color.
- For plants that suffer from powdery mildew, such as border phlox (Phlox paniculata) and bee balm (Monarda), make sure there is good air circulation, avoid overhead watering and don't over-water your plants.
- When cutting back perennials in the fall, leave a few inches on each plant so you know where it is and don't step on the crown or plant bulbs on top of it.