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Growing Perennials

Perennial border

Perennial plants are the backbone of nearly every flower garden. Unlike annual plants, which must be replanted each spring, herbaceous perennials die to the ground at the end of the season, and then regrow from the same roots the following year. People grow perennial flowers because they are such easy-care, dependable performers, and because they offer an enormous variety of color, texture and form. Here are the basics of garden design, plant selection and care.

The lifespan, bloom time, culture and form of perennial plants varies greatly. Some species, such as lupines and delphinium, are so called "short-lived" perennials, with a lifespan of just three or four years. Others may live as long as fifteen years, or even, in the case of peonies, a lifetime. Bloom time may last for only two weeks each year, or may extend over two or three months.

Some perennials, such as primroses, require deep humusy soil and plenty of shade, while others such as threadleaf coreopsis and cushion spurge wither away unless they grow in well-drained soil and full sun. Some perennials contain themselves in a nice, neat mound, while others, such as gooseneck loosestrife, will take over your entire garden. Some species should be cut back in midsummer, while others, such as hybrid lilies, may die if you remove their foliage.

There are so many different species and cultivars of perennial flowers to choose from that few people ever become completely familiar with all the options. For the perennial gardener, books are an invaluable resource. They provide photographs for identification (and inspiration!), cultural information, a description of growth habits, bloom time, color and characteristics of special cultivars. Invest in a good how-to book that has cultural information, and a color encyclopedia to help you identify plants and plan your selections.

What's in a Name?

It may be hard to believe, but scientific plant names are used to avoid confusion, not create it. They are developed by taxonomists to ensure that the same plant is called the same name throughout the world, regardless of language. Scientific plant names are usually a combination of Latin and Greek.

Common names, such as "bleeding heart," are often used to refer to all the plants in a genus and are useful unless you want to ensure you are purchasing a 24-inch high, spring-blooming bleeding heart (Dicentra spectabilis) rather than the ever-blooming species known as the fringed bleeding heart, which is only 12 inches high (Dicentra eximia). To learn more about botanical names, look for a copy of Gardener's Latin by Bill Neal (Algonquin Books, 1992).

Dicentra spectabilis 'Alba': (old-fashioned white bleeding heart)
Dicentra: The first name is the genus. It is always capitalized.
spectabilis: The second name is the species. It is not capitalized.
'Alba': The third name, which appears in single quotes, is the cultivar (cultivated variety).

Fellow gardeners are another great source of information about perennials. They can give you firsthand details about bloom time, height, hardiness and cultural requirements, and, if you visit their gardens, you can also see for yourself what the plants really look like up close. Nothing beats seeing a plant in a garden setting, where you can observe how it is being used. You may even go home with some pass-along plants for your own garden.

There's just no way to know how a plant will do for you unless you give it a try. If it turns out to be too tall, the color is wrong, or the plant doesn't thrive, you can always move it and try something different.

Perennial Planting Styles Few if any "perennial gardens" contain only herbaceous perennials. Woody plants, such as shrubs, roses, and trees, are often incorporated to provide a backdrop for the perennial plants, or are used to fill in and give mass to the bed or border. Many gardeners include annuals or biennials in their perennial gardens to provide splashes of dependable color throughout the season. Bulbs are added for early spring color and ornamental grasses for their interesting textures and late-season beauty.

Traditionally, perennial gardens have been laid out in one of two ways: a border or an island bed. A border is typically a long, rectangular flower bed that is about two to four feet deep. The classic English perennial border, which was so popular in the first half of the 20th century, was often as much as eight feet deep and 200-feet long. But for most home gardeners, a better size is about three feet deep and about 12 to 15 feet long.

Borders are usually viewed from only one side, and are located in front of a backdrop. This backdrop may be created with shrubs, a hedge, a fence or a stone wall. A well-defined front edge is important. You may design a solo border, or a matched pair. When selecting plants, keep in mind that borders usually look best when there is a repeating theme of plants and colors.

An island bed is a garden that floats in a "sea" of lawn. The shape is irregular, with gentle curves and no sharp corners. It is usually designed to be viewed from all sides, with the tallest plants positioned along the center line of the bed, and the shortest plants around the edges. Island beds look best when they are generous in size. A good size for an island bed is 8-by-15 feet, with the tallest plants reaching a height of about five feet.

Of course perennial flower gardens sometimes look nothing like a traditional border or island bed. Rock gardens break all the rules, for the objective is usually to create an irregular, natural-looking rock outcropping where tiny alpine plants can be featured.

Shade gardens are often irregularly-shaped, because they follow the natural shade patterns of the trees above. Another emerging style for perennial gardens is the large, free-form garden. In this case, the garden is defined by a series of meandering paths that lead the viewer right into and then through the plantings. Perennial flowers can also be mixed in among shrubs, planted around your mailbox, used in woodland or streamside plantings, or even planted in containers.

Arranging Your Plants

The appearance of a perennial garden depends as much upon the shapes of your plants and how they are arranged, as upon their colors.

Height: You'll want to place the tallest plants in the back of the border, or in the center of an island bed, then work down in height, ending with the shortest plants around the edges of an island bed or the front of a border. Books and labels usually list the average mature height for a plant in bloom. Remember that many plants hold their flowers well above the foliage. This means that when the plant is out of bloom, it may be much shorter than the specified height.

Heights are also an average. When grown in poor, dry soil, a plant may be only half as tall as the same plant grown in rich, moist soil. Be prepared to move your plants around once you see how tall (or short) they really grow. Even the most experienced gardeners rearrange their plants (usually more than once!).

Width: A plant's width, or spread, is just as important as its height. Width figures given in books or on labels are also an average. The actual width of a plant will vary depending on soils, geographical location and the age of the plant. Be careful about locating slow-growers very close to rapid spreaders. The former may all but disappear by the end of the first growing season.

Spacing: Patience is a virtue, but when most people plant a perennial garden, their goal is to create a full effect as soon as possible. The challenge is to plant thickly, but not break the bank, or create a crowded, unhealthy situation two or three years down the line. When planting a grouping or "drift" of the same kind of plants, you can put them closer together to create a massed look more quickly.

Another trick is to place short-lived plants between slower-growing, long-lived plants. Most peonies, for example, have an ultimate spread of three feet, but it may take seven years for them to reach this size. While you're waiting, you could interplant with Shasta daisies, a fast-growing, short-lived plant that will provide a full look and plenty of flowers while the peonies get themselves established.

Drifts versus specimens: A garden planted with groupings of five or more plants of the same variety will display drifts of repeating colors and textures. In this type of garden, plants are used primarily as design elements that add up to a pleasing and integrated visual effect.

At the opposite end of the spectrum is the collector's garden, filled with onezies and twozies of all different kinds of plants. These are the gardens of people who simply love plants and want to have one of everything. The look of this type of garden may be a jumble of colors and textures, and maintenance is usually more challenging, but these gardens are about plants first, and design second.

How to Select Plants

When it comes to deciding which perennials to plant, most of us are not very deliberate about our choices. We succumb to a luscious photo in a catalog, stumble upon an irresistible beauty at the nursery, or a neighbor sends us home with a bag full of cast-offs. If you ever do set out to make an informed and deliberate choice, here are some of the things that you should think about.

Your site: Perennials, like all plants, will live longer and be healthier and more floriferous if they are planted in a location that suits them. Does your garden have sandy soil or is it heavy clay? Is it in the sun or shade? Is the soil moist or droughty? Is the pH high, low, or neutral? Is the site flat, gently sloped, or steep? A good reference book can help you figure out which plants will probably be happy in the growing conditions that you can provide.

Hardiness: If a plant is not hardy in your growing zone, it will not survive the winter. If you don't know which zone you live in, check a USDA Hardiness Zone Map. Though knowing your zone is very important, altitude, wind exposure, soils and snow cover can have a dramatic impact on plant hardiness, effectively shifting the hardiness rating for your garden by as much as a full zone.

For best results, choose plants that are well within your zone. You will probably be tempted by those that are at or even just beyond your growing zone. If you can afford to take the gamble (financially and emotionally), it can be very rewarding to discover that you can grow a couple of Zone 5 plants in your Zone 4 garden. Where snow cover is not dependable, a winter mulch of leaves or straw can help marginally hardy plants survive a cold winter. Well-drained soil is also a benefit. Heavy, wet soils will often heave and damage plant roots.

Northern gardeners concern themselves with the minimum temperatures that a plant will tolerate, but Southern gardeners must also pay attention to zone ratings. Many popular perennials, including lupines, peonies, and garden phlox, must be exposed to a period of subfreezing temperatures to produce a good display of flowers. Other perennials will simply not tolerate long periods of heat and humidity.

Color: In working with color, aim for a balance of integration and contrast. Too much of the same color can be monotonous, yet a cacophony of different colors can be jarring rather than pleasing to the eye. You may want to organize your garden around one color; or choose a theme such as pastels, cool colors, or hot colors. You can also experiment with different color themes in different parts of your garden—hot colors by the front door and cool colors in a quieter part of the yard.

Remember that few perennials are in bloom for more than a couple of weeks each year. Most of the time, plants are green, and it is their leaf form and foliage texture that are the "color" in your garden.

Bloom time: A perennial may be in bloom for two weeks a year or for as long as three months. If your objective is all-season color, choose several plants from each bloom season. When selecting plants for a spring garden, concentrate on those that bloom during April and May. After that peak, the garden may lack color for the rest of the season, but you will have achieved a spectacular spring display. For best effect, group at least two or three different varieties of plants together that will bloom at the same time.

Remember that specified bloom time is only an average. In California, April may be the peak bloom time for bearded iris, yet in Vermont, the same plant will not bloom until early June. Recording the bloom times of various perennials in your garden will become an invaluable reference. No book, no matter how good, will be as accurate as your own observations about when plants bloom and how they perform in your own garden.

Seedling, potted or field-grown: When purchasing perennials, try to get the largest, most mature plant that you can afford. The bigger the plant, the more quickly it will fill out and the sooner it will begin blooming. Typically plants are available in pot sizes ranging from 3-inch diameter to 12-inch diameter. Pot-grown perennials can be planted from spring through fall, and will suffer minimal transplant shock.

Some mail-order companies ship their plants bareroot (without soil). Bareroot perennials are usually available only in early spring when the plants are still dormant. The roots must be kept moist, and the plant should be put into the garden as soon as possible (within a couple of days). Once the plant is in the ground and has emerged from its dormant state, it will take hold relatively fast.

A few local nurseries still offer field-grown perennials. These plants are dug up when you come for them and they need to be transplanted immediately (within a few hours) to minimize transplant shock. Field-grown perennials are usually the largest and most mature plants around, but today most nurseries only offer container-grown perennials.

Vigor: Vigor can be good, but it can also create problems. Plants that are too vigorous can invade neighboring plants and gradually take over your entire garden. Determining a plant's propensity for invasiveness can be difficult, because poor growing conditions can render a normally invasive plant relatively tame, whereas in fertile soil, a normally restrained plant may exhibit invasive tendencies.

Look closely at plant descriptions and be wary of those described as "vigorous." This may be a euphemism for an invasive plant that you'll wish you never set eyes on. Perennials with a reputation for invasiveness include: bamboo, Macleaya cordata (plume poppy), Physostegia virginiana (obedient plant), Monarda (bee balm), Artemisia ludoviciana (Silver King artemisia), Lysimachia clethroides (gooseneck loosestrife), Tanacetum vulgare (tansy), Aegopodium (goutweed), and Boltonia asteroides.

Maintaining a Perennial Garden

Though most flowering perennials are dependable, easy-care performers, all perennial gardens require some maintenance. Here are the eight most important steps to ensure a healthy and floriferous garden:

1. Fertilizing
Most perennials are not heavy feeders and they will be happy with one spring application of a low-nitrogen, high-phosphorus fertilizer (5-10-5). For established plantings, scratch in a good handful of fertilizer around each plant. Annual or biennial applications of aged manure or finished compost will restore trace elements and improve soil texture and water retention.

2. Watering
A perennial garden does not require as much water as a vegetable garden. Depending on where you live, if you select plants suited to your site, and mulch them well, you may not need to water at all. If you live where summers are very dry and you do need to water, try to water deeply and avoid getting water on the foliage (soaker hoses and drip irrigation systems are great for perennial gardens).

3. Mulching
By early summer, a densely planted perennial garden will shade out most weeds. But a new garden, a spring garden or a garden that is more sparsely planted, will benefit from some kind of mulch. The mulch will keep weeds to a minimum and help retain moisture in the soil.

The aesthetics of the mulch are as important as the function. Your garden will look best with a finely textured material such as shredded leaves, dry grass clippings, peanut shells, cocoa hulls or shredded bark. Big chunks of bark, newspaper or straw will overpower your plants.

4. Neat Edges
A neat, cleanly defined edge between your lawn and flower bed will give your garden a professional look. You can achieve this in one of two ways: get a nice sharp edging tool and recut the edge several times during the growing season; or install some permanent edging. A defined edge will also help keep grass and weeds from growing into the bed.

5. Pinching
Some kinds of perennials, including asters, chrysanthemums, phlox and salvias, benefit from being pinched back. Pinching creates a bushier plant that produces more blooms and is less likely to flop over. Pinch back the growing tips--using thumb and forefinger--once or twice during late spring. Not all kinds of perennials should be pinched. If in doubt, pinch a little here and there, and see what happens.

6. Deadheading
Some plants drop their spent flowers and seed heads. Others hold onto them for months, or even right through the winter. Removing spent flowers will keep your plants looking their best, and it often stimulates reblooming. It also prevents plants from expending their energy on seed production. After bloom, some plants should be shorn rather than deadheaded. This is true for creeping phlox, nepeta, hardy geraniums, daisies, pinks and lavender.

7. Staking
Many tall or weak-stemmed plants need support when they reach blooming size. Delphiniums and hybrid lilies are two prime candidates. But other, shorter plants can also benefit from some kind of support. Supports should be as invisible as possible. For individual stems, you can use bamboo canes. For entire plants you can use wire support rings. For loose and airy plants, try using a few thin branches. For best results, put the supports into position in early spring. That way the plants will hide the supports as they grow.

8. Dividing
If your perennials are happy, most of them will need to be divided every few years. They may become too large for the space; the center or oldest part of the plant may die out leaving a bare middle; or the growth may become so dense that the plant is no longer blooming well.

Use a shovel to remove the entire plant from the garden and place the root ball on a tarp. Then you can either pry the plant into pieces using two forks, tease the pieces of the plant apart into different sections, or use a shovel or knife to cut the plant into several pieces. Plants should not be divided when they are in bloom or in full growth. In all but a few cases, this is a job for early spring or late fall.

Perennial Tips for the Ages
  • When planting a new perennial garden, prepare the soil well at the outset. That may be your only opportunity to loosen the soil, remove rocks, and add organic matter.
  • If you start plants by seed, put your first-year seedlings in a "nursery bed" rather than directly into your flower garden. They will not bloom or have much of a presence until their second year anyway, and a nursery bed will allow you to keep a better eye on their performance.
  • Most perennials should be divided in early spring when new growth is only a few inches high. If you miss your chance in the spring, wait until fall. Irises are the one major exception to this rule: they should be transplanted in early summer, right after they have bloomed.
  • Keep newly transplanted perennials well watered for the first few weeks. Water deeply to saturate the entire root ball and establish good contact between the roots and the surrounding soil.
  • Most perennials prefer a pH of about 6.5, although, some prefer more alkaline or acidic soil. If you have trouble with a particular plant, check its pH requirements and the pH level of the soil in your flower garden.
  • If your plants look stressed during the growing season, or if you see disease or insect damage, feed your plants with a quick-release organic fertilizer (try a blend of seaweed and fish emulsion).
  • All plants die eventually, and some will die sooner than others, no matter what you do about it. If a plant performs poorly, try moving it to a different location. If it still is not happy, give it away or send it to the compost pile.
  • When designing a perennial garden, think about how you'll get access to your plants to stake, deadhead, or divide them. Flat rocks can be used as stepping stones within the garden. A walkway created at the back of a border will be hidden during the growing season, but will make the bed accessible for spring and fall chores.

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