Share the Love: Dividing Perennials

One of my favorite things about gardening is watching the landscape change and mature over the years. As plants start to fill in, they sometimes become overcrowded and need to be thinned or moved. Many perennials respond well to being divided, and then you can spread them to other areas of your yard, or share the joy with friends, family, and neighbors. Fall is the perfect time of year to divide many plants. Today we'll talk about what kinds of plants can be divided, and then we'll go over different methods for splitting and transplanting them.

To divide or not to divide?

Not all plants divide equally well. Generally speaking, the plants that are easiest to split are ones that increase in size with the help of bulbs, rhizomes, runners, tuberous roots, and stolons. Most perennials with a dense clump of roots at the crown can also be divided, but this can cause a little more stress to the plant, so closer after care is recommended. Perennials with a single main root should not be divided. Below we have listed some common perennials that should not be divided

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Divide and Conquer

Roots (and other underground plant structures) come in all shapes, sizes, and habits, and different types need to be handled in different ways. We'll divide them into three main categories: spreading mats, clumps, and tubers and bulbs. Not sure what kind you're dealing with? Dig up the whole clump to take a look. Once you knock a little soil off the roots, it should become clear what you're dealing with.

Spreading Mats

Plants in this category spread by sending out new roots and shoots growing in all directions with new shoots rising up from these roots at any point. A lot of these plants are fast growers, like black-eyed-Susans and and bee balm, which are valued for their ability to spread out and fill in quickly. The best way to divide these plants is to dig up the whole clump and gently pull on a section until it comes loose. Sometimes this just isn't practical (especially when you have large patches of a plant). These plants can also just be cut into sections with a shovel or spade, just be aware that this will mean you are separating some of the stems from their roots. You will see some (usually small) dead spots when this happens, but with a big enough chunk it will bounce back and fill in just fine. The plants below are just some of the species that work this way:

Black-Eyed Susan spreads to quickly, you'll definitely want to divide it to keep it in check. Photo by Maria Gulley

Photo by Maria Gulley

Clump-Forming Roots

Clump-forming perennials gradually get larger by adding on more and more to their main chunk of roots each year. Ornamental grasses are great examples of this. To divide these plants, you simply need to cut off a section with both roots and shoots. I would say you want something at least the size of an adult's fist. However, this can be easier said than done. Some of these plants have such dense roots that you may need to dig up the entire plant and use a saw to cut off a piece. This list covers some of the most common examples, but also keep in mind that any of the perennials on the above list can also be divided this way, although you may lose a few stalks here and there.

Karl Foerster Grass can be revitalized by occasional division. Photo by Maria Gulley

Karl Foerster Grass can be revitalized by occasional division.
Photo by Maria Gulley

Tubers and Bulbs

This final category refers not only to structures technically called tubers and bulbs, but also just thick, fleshy underground structures in general. Here we're talking about tulips, irises, daylilies - the list could go on and on. This is kind of a catch-all group for several slightly different division techniques, but the unifying factor is that there will be large, obvious underground plant pieces that are important to successful division. With plants like this, it's usually easy to pull or cut away a piece of the root system that will be viable as a new plant.

Irises have horizontal rhizomes that can be cut and divided. Photo by Maria Gulley

Irises have horizontal rhizomes that can be cut and divided.
Photo by Maria Gulley

Bulbs like tulips and daffodils should not be cut, but new bulbs budding off of a parent bulb can be broken off and start a new plant. Bulbs are actually modified leaves with a short, dense stem and roots attached to the base (that rough spot on the bottom). It is important that you get some of that stem and roots when breaking off a new bulb.

Plants with thick, horizontal underground roots/stems like irises and peonies can be cut into segments and planted again horizontally at the same depth you dug them from. You will want to get at least three eyes (spots where stems will grow up) to be sure that the divided plant will survive.

The third kind of tuber and bulb we'll talk about is the kind where you have several tubers attached to a single crown. Daylilies are a good example of this. These kinds of plants require a least a little bit of the original crown to go with them when divided. Sometimes, like with daylilies, the best option is to pull the roots apart, pick a crown, and tease it and its roots out of the clump. Other times you may have to make a cut somewhere, just make sure that whatever part you cut off has at least one growing point for new stems.

Here are some examples of plants that can be divided in these ways:

African Lily forms dense colonies of bulbs that are easily separated for dividing. Photo by Maria Gulley

Photo by Maria Gulley

•    Anemone (Anemone species and hybrids)
•    Canna (Canna species and hybrids)
•    African Lily (Crocosmia x curtonus)
•    Crocus (Crocus species and hybrids)
•    Winter Aconite (Eranthis hyemalis)
•    Daylily (Hemerocallis species and hybrids)
•    Hosta (Hosta species and hybrids)
•    Iris (Iris species and hybrids)
•    Daffodil (Narcissus species and hybrids)
•    Garden Peony (Paeonia lactiflora)
•    Tulip (Tulipa species and hybrids

Timing and Care

We'll end with a few final notes on how to make your plant division as successful as possible. First, we'll talk about timing. Given intensive enough care, most plants can probably survive being transplanted at any time. But to keep the plants as healthy and happy as possible, it's best to transplant during mild temperatures, so here in central Indiana that typically looks like mid-March through mid-May in the spring, and mid-September through October in the fall. It's also best to move plants when they are not blooming or about to bloom. During blooming plants divert a lot of energy into producing flowers and developing seeds. Division and transplanting requires a lot of root growth for recovery, so by splitting a plant in the middle of its bloom season you're putting a lot of unnecessary stress on it. So fall bloomers should be transplanted in the spring, spring bloomers should be transplanted in the fall, and summer bloomers can go either way.

If you are dividing in the spring, it's easier if you do it right as the plant starts to flush out new growth but before it's fully grown in. Not only does this make the job easier for you with less plant to work around, but it also means the divided plants will look better all year since they have been in the same shape all year. For example, hosta leaves spread to make an even mound. If you split them after they are grown in for the year, you will have several weeks (or even the rest of the season) where one side looks bare and scraggly, but if you move them before the leaves have unfurled you will get nice, even shapes on all of your divisions. If you are dividing in the fall, it can be helpful to cut back the shoots for the year first. Again, this makes it easier on you with less plant to manage, but it also makes it easier on the roots if they have fewer shoots to support after the roots are cut.

No matter what time you divide, it's important to plant your new starts as soon as possible, and make sure you water heavily in the weeks follow. If you can't plant the divided plants right away, put them in a cool, shaded spot either in pots with soil or with some soil mounded over them, and keep them watered daily until they can be planted. Both the original plant and the new starts should be watered two or three times a week for at least three weeks after dividing. This helps the plant through the initial stress of losing so many roots, and it helps the roots quickly establish in the soil.

Got any other questions on dividing plants? Feel free to send them our way! There are so many variables that go into timing and splitting specific plants that it's hard to cover everything in one blog post.

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