Perennials

16 February 2005

Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580   fax: 434.872.4578

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February is a good time to plan for your spring perennial garden. As you consider which plants to buy think about your site, what your plants will require to thrive, how they will grow (fast, tall, wide), when they will bloom, and what resources will be available for your plants in the way of water, light, soil, and nutrients.

There are advantages to perennials, the most obvious being that they do not have to be set out every year like annuals. Some perennials, such as delphiniums, have to be replaced every few years. Another advantage is that with careful planning, a perennial flowerbed will change colors as one type of plant finishes and another variety begins to bloom. Also, since perennials have a limited blooming period of about 2 to 3 weeks, deadheading, or removal of old blooms, is not as frequently necessary to keep them blooming. However, they do require pruning and maintenance to keep them attractive. Their relatively short bloom period is a disadvantage, but by combining early, mid-season, and late-blooming perennials, a continuous colorful show can be displayed.

You need to consider many of the same aspects of site selection for perennials as you do for annuals; sunlight (full sun to heavy shade), slope of the site (which affects temperature and drainage), soil type, and the role the selected plants will play in the garden. This is especially important with perennials, as they usually are left in the site for several years. An ideal location would provide a background such as a wall or hedge against which perennials will stand out while in bloom. In island beds, perennials can provide their own background if tall plants are placed in the center and short ones toward the edges.

Preparing the soil is extremely important for perennials. Some annuals can grow and flower in poorly prepared soil, but few perennials survive more than one year if the soil is not properly prepared. Preparation is best done in the fall but do not let that stop you from doing some preparation in the spring. First, have the soil tested. The results will indicate how much fertilizer needs to be added in the spring, and the pH level - which should be adjusted if needed. Check and adjust drainage. To do this, dig a hole about 10 inches deep and fill with water. The next day, fill with water again and see how long it remains (should not exceed 8 hours). (If drainage is poor, plan to plant in raised beds.) The next step is to dig the bed. Add 4 to 6 inches organic matter (OM) to heavy clay to improve soil texture. Dig to a depth of 12 or 18 inches and leave "rough" in fall or early spring. (Note: 2 to 3" of OM should be applied if bed can only be turned 6 to 8" deep.) Finally, in spring, add fertilizer, spade again, and rake the surface smooth.

Since herbaceous perennials grow back from the roots every year, it is important to encourage healthy, deep roots. Proper watering promotes good root development. Make sure that all the roots are reached when watering. Do not rely on summer rainfall to keep flowerbeds watered. Plan to irrigate them from the beginning. When watering, moisten the entire bed thoroughly but do not water so heavily that the soil becomes soggy. After watering, allow the soil to dry moderately before watering again.

Perennials should be mulched during the winter months to protect them from the heaving that results from repeated freezing and thawing of the soil. However, you must be careful with winter mulching, as it can do more harm than good. Be careful not to pile mulch heavily over the crowns, as this would encourage rotting. Evergreen branches give ample protection but allow air circulation. Apply mulch around the plants only after the soil temperature has decreased after several killing frosts. If winter mulch is applied too early, the warmth from the protected soil will cause new growth to start. Severe damage to the plant can result from new growth being frozen back. Remove winter mulch as soon as growth starts in the spring. If you do not, new growth will develop abnormally with long, gangly, pale stems.

Most perennials left in the same place for more than 3 years are likely to be overgrown, overcrowded, have dead or unsightly centers, and need basic fertilizer and soil amendments. The center of the clump will grow poorly, if at all, and the flowers will be sparse. The clump will deplete the fertility of the soil as the plant crowds itself. To divide mature clumps of perennials, select only vigorous side shoots from the outer part of the clump. Discard the center of the clump. Divide the plant into clumps of three to five shoots each. Be careful not to over-divide; too small a clump will not give much color the first year after replanting. Divide perennials when the plants are dormant, just before a new season of growth, or in the fall so they can become established before the ground freezes. Stagger plant divisions so the whole garden will not be redone at the same time; good rotation will yield a display of flowers each year. Do not put all the divisions back into the same space that contained the original plant. That would place too many plants in a given area. Give extra plants to friends, plant them elsewhere in the yard, or discard them.

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For more information about landscape topics contact your local Virginia Cooperative Extension office. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.