Dividing perennials is easily the best way to increase your plant stock, control a plant’s size and rejuvenate an old plant. A few years after you've planted a perennial you'll probably notice that it begins outgrowing its allotted spot. Dividing large perennials into smaller plants will solve the problem of over crowding in the garden while giving you new plants to add to other gardens. If you don't have space for any new plants, give some away to your friends. Most perennials should be divided every three to five years. However, perennials which are rapid growers, such as black eyed Susan and daisy may need to be divided more often while peony may never need dividing.
Dividing perennials is generally best when they are not blooming. This means dividing spring and summer blooming perennials in the fall, and fall bloomers in spring. This will allow all they plants energy to go towards root development.
When dividing your perennial in the fall, do so six weeks before the ground freezes so the plants can reestablish themselves. In the spring, allow enough time for roots to settle in before hot weather. Spring division is ideally done in the early spring as soon as the growing tips of the plant have emerged. Spring divided perennials often bloom a little later than usual. Avoid dividing your perennials during hot weather or during droughts.
Perennials which flower poorly have dead centers or unhealthy foliage that may need to be divided.
If you’re planning on dividing your perennials remember to water them a day or so in advance. Also, its always a good idea to prepare the area in which you’ll be planting your new plants. This will cut down on the time your plants will be out of the ground.
To divide your perennials, dig out plants with a shovel. Try to preserve as much as the root system as possible. Dig about 6” away from the plant an all sides and carefully lift the plant out of the ground. If that’s not possible, you can divide the plant right in the ground to.
Shake or hose off loose soil and remove dead leaves and stems. Perennials have several different types of root systems. Each of these needs to be treated a bit differently.
Spreading root systems have many slender shallow roots that originate from many locations with no distinct pattern. Plants with spreading root systems include black eyed Susan, lambs’s ear, purple cornflowers and thyme. Many of these spreaders will encroach on other plants if not occasionally divided. They can usually can be pulled apart by hand, or cut apart with shears or knife.
Clumping root systems originate from a central clump with multiple growing points. Many have thick fleshy roots. Perennials which grow in clumps include sedum ‘autumn joy‘, hosta, daylily and some ornamental grasses. These are easy to divide. Simply cut through the clump with a shovel or a garden edger preserving buds within each new plant.
Rhizomes are stems that grow horizontally at or above the soil level. Bearded irises are the most common perennial with this type of root system. Rhizome sections which are more than a year old should be discarded, as should rhizomes with disease and insect damage. For each new iris division preserve a few inches of rhizome and one fan of leaves which can be cut back to about 6“. Replant with the top of the rhizome just showing above soil level.
Tubers, such as dahlias, should be cut apart with a sharp knife. Every division must have a piece of the original stem and a growth bud attached. After division they can either be replanted or stored for spring planting.
Some plants, if not divided for a long period will develop tough, dense root systems and may need to be divided with a saw or ax.
Some perennials do not respond well to division. These include butterfly weed, euphorbias, oriental poppies, baby’s breath, false indigo, candytuft, lavender, rosemary, and columbine.
Plant the divided sections immediately in the garden or in containers. Never allow divisions to dry out. Trim all broken roots with a sharp knife or pruners before replanting and replant divisions at their original depth. Firm the soil around the plant's roots to eliminate air pockets and water well after planting.