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Divide and conquer in the garden: Dividing perennials

June 23, 2010

From University of Illinois Extension Garden packet

URBANA, Ill.—Now is the time to start thinking about what will need to be divided in our gardens.

“In general, most perennials need to be divided every three to five years,” said Martha Smith, a University of Illinois Extension horticulture educator. “The ideal time to divide them is in the spring just as active growth starts. At this time, you can do the least amount of irreparable damage. All systems are go. The plants are coming out of their dormancy and are prime for growing. Some may argue that there are specific plants that respond to dividing better at later times during the growing season. Yes, that is true, but spring is still OK.”

Usually, a plant will show signs of needing division, she added. They become overgrown with dead, unsightly centers, or their vigorous growth has sent them to places in your garden where they are not welcome. Plants may have fewer flowers and are in overall poor health due to the plant depleting the fertility of the soil as it crowds itself. Let the plant be the gauge and not a calendar. Some perennials may need to be divided every two years, whereas others may not need any attention for seven years.

“To divide mature clumps of perennials, lift the clump and separate the outer active-growing portion from the center,” she said. “If the center has stopped producing new growth, discard it and replant the active-growth areas. Be careful not to overdivide.

“Leave a proportionate amount of roots to crown,” Smith added. “Too few roots will not be able to support a large crown. Too small a crown will not give you much of a visual display. Too large a clump may mean you’ll be dividing this plant again soon.”

Don’t worry about being rough. Some perennials need a backhoe and a pickaxe. After lifting these brutes, go at them with a saw, two pitchforks back to back, or a sharp, flat-edged shovel. On the softer side, some perennials lift very easily, and you can actually separate them with your hands.

“Replant the portion you are keeping and discard or give away the rest,” Smith said. “Prepare the planting area by adding compost or aged manure. Remember, a perennial can be expected to live in that spot for several years, so take a few minutes and amend the planting area. You will be glad you did.”

Smith said you should stagger your plant divisions so the whole garden will not be redone at the same time. A good rotation plan will yield a nice display of flowers each year.

From the June 23-29, 2010 issue

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