Keep plants healthy and productive
Dividing is a matter of digging up plants, prying them apart with hands, spades, or knives and replanting them in new holes. Many plants form clumps as they mature. These plants start to go stagnant, lose vigor and produce fewer blooms. If left too long, the plant may stop blooming altogether.
Create more plants
The other reason to divide plants is to create more of them. Many landscapers recommend repeating the same plant in different areas of the landscape and division is a great way to produce those extra plants.
Different plants produce varying amounts of divisions
Hostas, grasses, and daylily can give 4-6 new plants from a 3-year clump; while heuchera may only result in 2-3. Each new planting division should have a minimum of 2-3 viable plants for best results.
When to divide
As a general rule, it is best to divide plants that bloom in the summer and fall in the spring and plants that bloom in the spring during the fall. Since blooming plants are using much of the energy a plant produces, it's is never a good idea to divide a plant while it is flowering.
If a plant is doing well, you don't need to divide it, unless of course you want more plants. Even when the recommendation is the divide every 3 years, if the plant is producing flowers or is not dying back in the middle, it is okay to wait another year.
Tips for successful division
Soften the ground where you plan to dig up a plant for division by watering a few days in advance. This makes it easier to remove and divide the plant. Don't over water or you end up with a muddy clump that is hard to work with.
Prepare the new planting area, so the new divisions can be immediately replanted.
Trim up the divisions to reduce above ground foliage. This will help direct the plant's energy to the roots and help establish the new plants.
Make sure each division has viable growing sections. Don't make the divisions too small. Fewer, larger divisions are better.
Discard the center of the plant. This area is generally the least healthy or has already lost vitality.
Plants to divide in the spring (1-3 years)
Divide every 3-5 years
Divide every 5-10 years