Dividing perennials has many benefits, it keeps your plants healthy, minimizes risk of diseases and insect infestations, keeps them beautiful and avoids overcrowding neighboring plants, and creates more plants to give to friends and family or plant elsewhere.
April is a great time to divide perennials in New England. The ground is getting warmer and the temperatures are mild, allowing your plants to recover quickly once replanted.
You'll know when a plant is ready to be divided when you can see that you can make several clumps from the base of one plant. Sometimes you may just divide a plant into two, other times you can divide a plant into four or more divisions. Consider watering the plants you intend to divide for a few days prior to dividing (unless you've had a lot of rainfall). This will make the soil easier to work with.
When you're ready to divide, you'll probably want to have a large hand spade or a shovel, gloves, a watering can or hose, and a knife to cut through tight root systems. Insert a large hand spade or a shovel into the soil around the perimeter of then plant to loosen the soil and the roots of the plant clump.
Wedge your shovel under the root ball and gently loosen and then lift the ball up and out of the ground, trying to keep the root system intact. Gently shake or brush excess soil from around the root ball.
Using your hands or a knife, separate or cut apart individual crowns. Each new plant clump will need to have leaves and roots in order to grow.
Replant the divisions right away to avoid drying out the roots. Plant them at the same depth as the original plant and provide lots or water. Add mulch over the soil to help maintain moisture while your new divisions recover, take root and get established.
If you're giving away your divisions rather than planting them, soak the roots in water and give them to their new owners as soon as possible for planting.