Even though perennials bloom year after year, that doesn't mean that you can plant them and forget them. The best way to keep them healthy and to keep them from crowding out other plants is to divide them before summer growth begins.
For perennials that bloom in spring to early summer, fall is the best time to divide:
- Bleeding hearts
- Lilies of the valley
For perennials that bloom in summer or fall, divide in spring:
- Ornamental grasses
- Siberian irises
- Purple coneflowers
- Obedient plants
- Many groundcovers
Thin perennials when they start getting overgrown. When crowded, the plants produce fewer blooms and are more likely to become diseased.
Thinning is an easy, inexpensive way to generate more plants and to prevent one species from crowding out others.
The basic tools you need for this task are:
- Clean plastic pots
- Potting soil
- Pair of shears
- Strong, sharp knife
- Sharp spade
- Plastic bags
A few tips to keep in mind as you divide perennials:
- Resist the temptation to use soil from your garden when planting new divisions in containers; the soil becomes hard and compact, and plants suffer from a lack of oxygen to their roots. Instead, use potting soil.
- Divide perennials late in the day or when the sky is overcast; keeping new divisions out of direct sunlight spares them from extra stress during replanting.
- If you live in a warmer climate (Zones 7 to 10) and your perennials have already started growing, you can still divide the crowded plants, but they may not recover as quickly as dormant plants.