Divide to Multiply

The first question any gardener should ask is "Do I have any plants that can be divided?" as not all plants can or should be. Herbaceous perennials are the plants we tend to divide. Shrubs like roses, hydrangea or even lavender and rosemary are multiplied via cuttings and that is a topic for another day. Also, certain plants, while herbaceous perennials, just don't allow division, such as Oriental Poppies. In time, through some trial and error, you'll learn which plants cannot be divided.

Here in NYC, I like to divide in mid-March to early April and then again in late October to early November. I don't like to stress the plants too much by doing it on sunny, warm days. Choose days that are cool and cloudy with rain on the way or do it right after a good rain. The rain and lack of sun will help the plants settle in with a minimum of stress. If you're on top of things, technically you can divide any time of year except when the ground is frozen. But why tax the plant when its putting energy into flowering or when it most needs its roots to pull up moisture? Do it early, do it late, but avoid it in summer. A general rule is to divide summer/fall bloomers in early spring and spring/summer bloomers in mid-autumn.

How do we identify if a plant can be divided? We have to look carefully at the plant. A series of questions may help determine its divide-ability:

• Is the plant an herbaceous perennial?

• Has the plant been in your garden for over 2 years?

• Has the perennial gotten quite large in that time?

• Does it look like it is crowding itself or has it died back in its center?

• Are their little clusters of leaves and stems growing on the outside of the main clump?

• Does the plant look overgrown yet seem to be under-performing with less flowering?

If you can answer yes to most of those questions, you can divide the plant.

At this point, take your shovel and slice into the dirt around the plants perimeter, keeping the shovel a few inches from the plant. On your last slice, lever the plant and its soil-bound roots up from its hole. Take the clump in your hands. Shake off some of the soil, remove dead leaves and stems and divide!

Sounds simple, right? It is mostly. Except that there are different kinds of root and stem systems. So each requires a different kind of attention. If you just pulled every perennial you wanted to divide in half and replanted it, you'd probably have some success with that. But its useful to know which have spreading root systems, which are clump forming, and which are rhizomatous.

Plants with spreading roots are common to our gardens, such as Chrysanthemum, Aster, and Yarrow. I find these the easiest to multiply through division. After digging up the plant, you will see stem-like roots shooting in all directions. Some of these roots will have a stem and leaves with fine roots growing from a node. You can separate this new plant from its parent.

Sometimes these spreading roots are what we call stolons. Stolons are near-soil-surface stems that run horizontally. New roots and stems form at the nodes of the stolon. Cut the stolon that has some roots and developing stem and leaves from its parent plant and boom!, new plant. Above ground stolons are sometimes simply called runners. A good example of a plant with runners is strawberry.

Rhizomatous plants, like Iris, are divided with attention to its tuber-like rhizomes. An Iris sends roots out from the underside of the rhizome. As the plant grows, its rhizome gets bigger, branching in a manner that looks like fresh ginger at the grocery store. The Iris has a leafing node, usually at the end of a branch of the rhizome. If you have a rhizome with multiple leaf nodes and roots along the branching rhizome, it can be divided. Break or cut the rhizome up so that each leafing node has some rhizome and some roots. Also, discard any rotted rhizome.

Clumping roots require that you split the plant into parts. Sometimes you will do this simply by pulling it apart( as in the case of sedum or some chrysanthemums), sometimes you have to cut the plant (as in the case of a large hosta or some yarrow) into parts with a knife or sharp pruner after shaking the soil free.

Ultimately you want a division to have enough roots to establish the new plant, and either young leaves or leaf buds. In general, you want to plant these divisions as you would any new potted perennial. Keep it well watered until it appears healthy and growing.

This process will become easier as you pick up on the similarities between different perennials in your garden. Of course, I am available for a hands-on how-to. Just click on the Garden Coaching link at the top right of the page. Good luck Ellen!

Below are some photos of three plants I divided this spring: Aster, Yarrow, and Chrysanthemum.
These methods will work for many plant divisions and the work is in identifying which method you need for the perennial you want to divide. I will add more photos and descriptions as I divide more plants this season.

Aster:

Fall blooming Asters tend to be clump growers. Some spread by runners or stolons, but this one here does not. I divide it every 3 or 4 years or simply as I need to control its size.

Dig up the aster and remove from its hole or do it in place, digging out the division afterward.

Find the clump's center and try to push your shovel through it. You may need the force of your weight on the shovel for it to cut through. The clumps can be surprisingly dense and tough.

Once through, the hard work is done.

You now have two where you once had one. Plant them as you would any perennial and give em a drink.

Yarrow:

The yarrow I have grows like mad. Every year I need to chop it up and give some away. Yarrow has a web of roots under the soil and a somewhat horizontal, woody stem at soil level from which the fleshy parts (leaves and roots) of the plant grows.

Be careful then when digging yarrow up as its possible to break its fleshy parts from the woody parts (although you can often just replant the roots with success). Shake the soil from the roots.


I choose to use my by-pass pruners (Felco no. 2) to cut the main woody stem. You can also use a sharp knife or even break it if you must.

Here is the woody stem cut by the pruners. You can see the roots and leaves growing from it. As long as the leaves above also have roots below, your division should grow easily.

Now I have two. And I could have had more if I wanted to cut the plant into smaller sections of leaves and roots. Plant and water in.

Chrysanthemum:

Chrysanthemums don't have the woody central stem of yarrow, but has fleshy, near soil surface stems or stolons that extend out from the "mother" plant. When you dig up a mum, its a good candidate for cutting or simply pulling it apart and replanting as you need. Make sure there are roots to go with your leafy stems.

Chrysanthemums also make little "new" plants on the exterior of the main plant. You can simply pull these out. Toss the ones you don't want, but plant any leaves that have attached roots to create new plants elsewhere in the garden.

In two years these will become full sized perennials.