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The general rule of thumb is that it is best to divide spring and early summer perennials in the fall, and late season bloomers in the early spring. Spring blooming perennials can also be divided immediately after flowering. Dividing a perennial outside of its flowering season means that all of the energy of the plant can go into root and leaf growth. Most perennials are not temperamental and, if properly divided, will fare well whether they are divided in spring or fall.
Reasons for Dividing Perennials
To control the size of the plants. Rapidly spreading perennials can be kept under control by dividing yearly or in alternate years. Artemisia ‘Valerie Finnis’ is a good example of a beautiful addition to a perennial border that will, however, need taming and management from the gardener. Some ornamental grasses simply get so large and tough to divide that it is best to split them frequently when they are still small and manageable. Most perennials can be left several years before they are divided.
To increase the number of plants in your garden. When you are planting edging for your perennial bed or designing with medium to large sized swaths of perennials, a cost efficient way to fill your border is to buy fewer plants than you actually need and fill in the empty gaps with inexpensive annuals or bulbs until your perennials increase in size and can be divided.
To help rejuvenate the plant. Listed below are some common problems that occur when a plant is left alone for too long:
Reduced flowering occurs and the flowers get smaller on the plant. When the root system becomes too congested the roots become less efficient in absorbing nutrients from the soil and plant growth is compromised.
The center of the plant dies back leaving a hole with all the growth on the periphery.
The plant starts to flop over, open up from the center, or lose its shape.
The plant becomes less vigorous.
Techniques for Dividing Perennials
If your soil is dry, water your bed a few days beforehand to make the soil easier to work with.
It is always best to divide perennials on a cloudy day. Never leave divisions exposed in the hot sun. Keep them in the shade until replanted. Avoid letting them dry out.
Before dividing, cut the foliage down to 6 to 8 inches. This will cut down on moisture loss and you will be able to see what you are doing. If you are dividing early in the season this is not applicable.
Dig around the plant with a spade or a fork, leaving ample space for a good sized root ball, and lift the clump out of the ground.There are several possible methods for dividing perennials:
- Lay the plant on its side and cut in half using a sharp knife or a garden spade. Then repeat, cutting in quarters.
- Pull the plant apart into pieces. This will work well for perennials that grow in loose clumps. The popular lamb’s ears (Stachys byzantina) is a good example of a perennial that can be pulled apart by hand.
- For tough and fibrous rooted plants use a hand saw or the double fork method to cut them apart. For the double fork method take two spading forks (borrow one from your neighbor), place them back to back in the center of the perennial with the tines of the forks intersecting and pull them back and forth in a scissor motion prying the roots apart as you go.
Cut divisions so that they are the size of a quart or a gallon perennial. Each division should have 3 to 5 vigorous shoots and a good root system. The ultimate size of your cutting depends on the size of your garden and how quickly you would like the new divisions to grow. If you have a large garden, larger divisions will keep everything in scale. For smaller spaces, small clusters of new plants will do the trick.
Throw away old or diseased roots. Remember that you will not be able to salvage the entire perennial. The goal is to end up with between two to four healthy divisions per plant.
Replant perennial at the same level that they were growing before. Coral bells, Solomon seal and peonies should be planted slightly below soil level.
Different Types of Root Systems
Spreading root systems have slender matted roots. These can usually be pulled apart by hand or cut apart with a knife. Asters, bee balm, and lamb’s ears are several examples.
Clumping root systems originate from a central cluster with multiple growing points. Hostas, daylilies and many ornamental grasses are included in this group. These perennials can be cut with a knife or spade or pried apart with two spading forks.
Rhizomes grow at or below ground level. (Bearded irises have rhizomes, for instance.) These should be divided in mid- to late summer, about one month after the irises finish flowering. The older, shriveled rhizomes can be removed with a knife. Keep young, firm rhizomes that have a fan of leaves and several healthy roots. The foliage should be trimmed back by ⅓ so that the division is not top heavy.
Specific Seasonal Care
- A number of fleshy root perennials such as poppies, peonies and irises are best divided in late summer and early fall.
- Fall divisions should occur no later than the middle of October. Divisions will need approximately one month to get established. The roots of perennials will continue to grow until the ground freezes (even after the tops have died).
- Do not fertilize plants at this time of year. Fertilizer encourages new growth that will only be killed off by the frost. Replant your divisions, water them well and continuing watering weekly for two to three weeks or until the ground freezes. The soil should be amended with compost when you plant.
- Cut back foliage so that it is in proportion to the root system. Also, clean away dead leaves and weeds from the divisions as they can harbor diseases.
- It is important in colder climates to mulch your divisions. Many plants are susceptible to frost heaves that occur when the soil temperature constantly changes during the winter months. Mulch will act as an insulator and keep the temperature constant. Shredded leaf mulch is one of the best mulches to use. Alternatively use composted bark, conifer bows, pine needles or straw. Mulch around late November once the ground begins to freeze and take if off in the spring in late March/early April.
- Divide perennials in the spring just as the new shoots start to show (when they are 2 to 4 inches high).
- If you are dividing plants that flower early in spring, the general rule is to wait until they have finished flowering and then divide at least one month before the summer heat starts. Amend the soil with compost and water them well.
- Spring divided perennials usually bloom slightly later than usual.