Dividing Perennials

Make more plants by dividing perennials

The following advice is based on my experience gardening in zone 5. Please note that techniques will change slightly from zone to zone.

Perennials are plants that come back year after year in your garden. When planting perennials, make sure you check how hardy they are on the plant tag. Some plants are not hardy to colder zones, but yet are sold in these areas as annuals even though technically they are perennial.

How to divide perennials

Though perennials can live a very long time in your garden, without being replanted, they will need dividing about every 3-4 years to remain vigorous. This is a great way to get more plants to give to friends, sell or to spread around in your own garden. Some perennials such as hosta and peonies can live for many years and never need dividing, while others, such as yarrow, need dividing often to keep from flopping over.

There are also some plants that don't like to be disturbed, so if you don't think it needs dividing, there is no need to do so.

Here are some reasons to divide a perennial:

  1. The center of the plant has begun to die out.
  2. It's become too big for the space.
  3. It has stopped blooming or blooms very little.
  4. The plant begins to, uncharacteristically, flop over or become top heavy.
  5. To get more plants.

In general, spring blooming perennials should be divided in the fall and summer to autumn bloomers should be divided in the spring. This allows the plant to begin growing a strong root system before it has to bloom again, although to be honest, I've divided at many times of the year with great results. These times are also good because temperatures are usually lower and moisture is usually plentiful in the spring and autumn months.

General guidelines on how to divide:

  1. Choose an overcast or cloudy day. Sun will only stress the plants. Create some temporary shade, if necessary, by using shade cloth.
  2. Choose a time when rain is in the forecast so you don't have to hand water as much. The roots will need a lot of water to help get established again. You may need to water daily for the first week if temperatures become too warm. After that first week, watering twice a week should be sufficient.
  3. Water the plants thoroughly the day before dividing. This softens the root ball to make it easier for dividing and also helps prevent shock after dividing.
  4. Dig up as many of the roots as possible to help prevent shock.
  5. Use a shovel or fork to dig up your plants. I like using a fork to lift the plant after I have used the shovel to cut the majority of the root ball.
  6. Have the new holes ready so that the freshly divided plants can go right back into the soil, therefore helping to prevent further stress.
  7. Amend the soil with compost and/or aged manure before replanting. This will be easier on the plant than traditional fertilizer and the compost makes it easier for the roots to grow in the soil. It's also important to only use compost or manure in the fall as any chemical fertilizers will force too much tender growth to form before the winter months set it, which could harm the plant in the long run. Always stop fertilizing six weeks before your average first frost date.
  8. When dividing look for natural divisions or crowns on the plant and cut these apart with a knife or spade. Discard any dead looking parts that may be in the center of really old plants. Leave 2-3 crowns for each new plant. This size will get established much faster than smaller divisions.
  9. Replant at the same depth as the original plant. If you are unsure, planting higher is better than planting deeper. The plant will likely settle a bit anyway, so I always plant a little higher than the original plant.

Benefits of Dividing in the Spring
The plants are generally easier to handle at this time and you are less likely to damage surrounding plants. Divide when you first start to see new growth. This could be as early as February or March or as late as April or May depending on your growing zone. The plant will have plenty of time to recover before hot weather sets in or before cold temperatures start in the fall. This is also a good time because the plant has rested over the winter months and generally has more energy stored in its roots.

Benefits of dividing in the fall
Fall tends to be a less busy time in my garden, so it's often a good time to divide plants and fill in any bare spots at this time. This is also a good time because it's very easy to find the plant in question and dig up the entire root ball. Since temperatures start cooling and rains generally increase, it's a good time for the roots to get re-established before winter. Just allow a good 4-6 weeks before the ground freezes in northern zones, to make sure they will survive the winter. This could be as late as September or October, depending on your growing zone. When in doubt, start dividing when night time temps are consistently in the low 50s.

Problems you may encounter after dividing

  • Plant looks droopy and wilted. - Check to see if the root ball is dry. If so, water thoroughly. If the root ball seems moist, it may just be in shock. You can cut off up to a third of the foliage to help it recover. Don't worry too much about this as some plants tend to wilt easily after dividing. As long as it's getting enough water, it should perk up and start to look more normal after about a week.
  • Leaves begin to dry up suddenly. - If you moved the new divisions from a shady location to a sunny location, the leaves may be burnt from the extra sun exposure. Provide some extra shade immediately, water well and then slowly increase the sun exposure a couple of hours a day over the next week or two. That should help reduce the stress. If the plant is transplanted in the spring, it should replace the old leaves with new ones shortly. If the plant is transplanted in the fall, it may not replace these leaves and may in fact look a little raggedy for the remainder of the season. And for heaven's sake, don't apply any chemical fertilizers while a plant is stressed; this only makes the problem worse.
  • Plant wilts during the day, but looks normal in the evening or at night. - This is common with some plants and is nothing to be alarmed about. This usually happens during severe hot weather and can happen with established plants as well as newly transplanted ones. Wilting is a defense mechanism and helps the plant cope with hot weather. If the soil and the root ball are moist, leave it be, no need to water. If the soil does indeed feel dry, go ahead and water it. This should stop once temperatures cool down. You can provide some temporary shade during the hottest part of the day if you think that will benefit the plant.

Following is a table that I found on the University of Minnesota's Sustainable Urban Landscaping web site on when to divide certain perennials. There is some great information here. http://www.sustland.umn.edu/implement/Perennial

You can also view lots of great pictures on how to do just that by visiting this web site at Fine Gardening. http://www.finegardening.com/how-to/articles/dividing-perennials.aspx

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