Sooner or later, most perennials need to be divided. Perennial plants typically increase in size by producing shoots and roots on the perimeter of the plant. After 3 or 4 years, the clump may die out in the center, overrun its space, or produce fewer flowers. When these things happen, it's time to divide.

The dividing process is fairly simple. You will need a few tools, which may include: shovel, garden fork, knife, scissors and rubbing alcohol.

To prepare the plants and soil for digging, water the bed a few days in advance. (This is only necessary if your soil is very dry). Remove any dead foliage from the plant.

First, dig around the plant so that it can be lifted in a clump. Then split the plant with a fork, inserting it deep into the clump. If the roots are tightly compacted or very woody and won't break apart, you can divide them with a sharp knife. This may be damaging to the roots so cut them as little as possible.

Once divided, the clumps are ready for replanting. It is a good idea to have a new planting site in mind and prepared before you begin. This will make the process go both quickly and smoothly. Water and care for them as you would any newly planted perennial.

Perennials with rhizomes, such as iris, can be lifted after flowering. Cut the rhizomes into sections. Each section should have one fan of leaves. Replant the sections placing the rhizomes just barely below the surface of the soil.

General rules of thumb to follow when dividing:

1. Spring and summer flowering perennials are best divided in the fall. Late summer and fall blooming perennials are best divided in the spring.

2. Try to avoid dividing perennials on very hot days. The heat of the sun can be stressful to the plants.

3. Use clean equipment. If knives or scissors are needed, it is a good practice to disinfect them with alcohol often to avoid spreading disease.

4. Don't be afraid to "dig in" and divide your perennials. They are tougher than you think and the benefits of dividing are worth the effort!

Divide in Early Spring or Fall: Campanula, Centaurea, Lamium, Astilbe, Eupatorium, Trollius, Alchemilla, Pulmonaria, Salvia, Sedum, Brunnera, Stokesia, Coreopsis, Chelone, Veronica, Achillea

Divide in Spring: Asters, Chrysanthamums, Rudbeckia, Boltonia, Mondarda, Echinacea, Helenium, Gailardia, Grasses, Helebores, Stachys, Sedum, Plumbago, Tricyrtis

Divide in Late Summer or Fall: Amsonia, Daylily, Geranium, Hosta, Iris, Peony, Fillipendula, Cimicifuga

Divide when Dormant or After Bloom: Dicentra, Allium, Primula, Dodecathion, Tradescantia, Mertensia

Difficult Perennials to Divide: Baptisia, Dictamnus, Echinops, Dianthus, Poppy, Milkweed