A number of perennial plants grow in an ever-widening clump. However, after several seasons of growing, the plants will begin to die out in the center—and it will look more like a ring than a clump. To keep the plants vigorous and blooming, divide the plants, and you'll be rewarded with healthier, longer-lived plants. As a bonus, you'll get even more plants out of the division.
Deciding When to Divide
The right time to divide perennials depends on the type of plant and how quickly it's growing. You don't have to wait until your perennial plants begin looking like doughnuts—in fact, it's better if you don't.
Keep an eye out for clumps of plants that have grown two to three times their original size within two to five years. Any overgrown clump—or any clump that has simply exceeded the space allotted—is a candidate for division.
Spring is usually the best time for division because the plants are actively growing. At this time, the leaves are not so developed that the root system can't take a little disturbance and still feed the top of the plant. However, just as different plants can go different lengths of time before being divided, some plants, such as peonies, do better when divided in the early fall.
Preparing for Division
Although dividing perennials is good for the plants in the long run, it's still a shock to their system. Give the plant a good soaking, preferably the day before you intend to divide, to help with your success. Disturbing the root system of any plant interrupts its ability to feed and hydrate itself, and ensuring that the roots are well-saturated before disturbing them reduces the trauma.
If you find you must divide a plant with a great deal of top growth, cut back the leaves by about one-third to lessen the amount of work the roots will need to do to maintain the foliage.
Determining a Location for New Plants
Prepare a hole for the new plants before you divide, as this will plant's time out of the ground and the stress on the root system. When digging the hole, give the new division plenty of room to expand. Remember, the divisions will all be smaller and require shallower holes than the original plant.
Digging up the Plant to be Divided
In most cases, it is easiest to divide a perennial plant by first digging and lifting the entire plant. Using a shovel or flat-edged spade, slice completely around the outer perimeter of the plant, a few inches away from the foliage. Slice down several inches deep—at least 6 inches for most plants and more for extremely large, well-rooted plants. The idea is to dig up as much of the root ball as possible.
Try to keep the soil intact around the root ball. This is an additional advantage of watering the soil around the plant, as wet soil adheres better than dry soil.
Once you have sliced completely around the plant, you will see the plant beginning to lift out of the hole. Try lifting the plant out of the hole with the shovel, but it may be too heavy to lift this way. If so, use the shovel as a lever and lift the plant manually. Place the plant on level ground nearby.
Using Pitch Forks to Divide Perennial Plants
To divide the perennial, use two pitch forks to pry and split the plant apart. Perennial plants with fleshy roots are easily pried apart with forks.
Insert the forks into the center of the lifted plant so that the backs of the forks are touching each other and the tines are crossing. Press down so that the forks go through the plant. You will probably hear some cracking as the plant splits.
However, some plants are so dense that this method will not work. Exercise caution, as garden tool handles can break and send you tumbling.
Begin the Dividing Split
Once your garden forks are securely anchored in the center of your perennial, simply pull the handles in opposite directions, away from the center of the plant. Again, you will hear cracking. The roots will not break cleanly, but the plant will recover. Sometimes a densely rooted plant will resist, and it will take two people to pull the forks apart and split the plant.
Keep pulling on the handles until the plant has completely split into two plants. If the resulting plants are a good size for replanting—meaning they're not so large that you'll have to divide again next year or don't fit into the space you've allocated—you are done dividing and ready to replant.
If you have an extremely large plant, you may have to divide it several times before you have new plants of an appropriate size.
Dividing Densely Rooted Perennial Plants
Most perennial plants can be divided with the simple two fork method, but occasionally you will encounter a perennial that has either been growing too long in one spot or that simply has a really thick root system. Ornamental grasses tend to develop such heavy root systems that just digging them out of their hole is a challenge. Trying to divide these with the two fork method almost guarantees a broken fork and a very sore back.
Densely rooted plants require cutting the root system open. This can be done with a sharp saw, by two people and a bow saw, or even with an ax. However, these methods should all be undertaken with extreme caution, as you can hurt yourself a lot more than you can hurt the plant.
Dividing Plants Without Digging up the Original
On the other side of the coin, some plants such as geraniums and Jacob's Ladder are simply quick spreaders, so their root systems are quite easy to dig and separate. In this case, you do not need to lift the entire plant out of the ground. Instead, chop a portion near the end with a shovel and lift it out of the soil. You will still want to pre-water your plant and take as much of the root ball and soil as possible, but you do not need to dig and disturb the whole plant to reduce its size
Maintaining Recently Divided Perennials
Once you have replanted the divisions, treat them like new seedlings.
- Divide on an overcast day, or at least not in during the hottest part of the day.
- Don't leave the exposed root ball sitting about any longer than necessary. Hot sun and breezes will quickly dry the roots.
- Keep them well-watered until new growth appears.
- Provide some shade if they appear to be wilting during the afternoon. A floating row cover will protect them from the hot sun.