Dare to divide
“The great beauty of an herbaceous plant is the never-ending element of surprise that comes with its renewal each spring.”
— Monty Don, “The Complete Gardener,” 2009
I know exactly what Monty Don is talking about.
After the rains this week, I’ve been in the garden marveling at the new growth my perennial flowers and herbs are putting out in anticipation of the official beginning of spring. I think we’re all ready for it to come!
I wrote last week about spending a lot of happy time propagating seeds in the greenhouse, but I’ve also been outside dividing herbaceous perennials. This is a satisfying way to add more plants to my garden without having to spend money on buying them (I already spent too much on seeds).
Many herbaceous perennials benefit from dividing them every three or four years. There are different ways to divide plants, depending on how they grow roots and reproduce asexually (other than by producing seeds).
The simplest, most common method is through root division. This is the way I divided some of the daylilies that had grown into dense clumps in my garden. I dug around the whole cluster so I could lift up as much of the root ball as possible, and then I cut through the roots using a sharp knife and untangled individual new shoots from the mass with plenty of healthy roots attached.
Daylilies have fleshy roots, whereas asters such as my Frikart’s aster (Aster x frikartii ‘Monch’) have a fibrous root system that I cut through with a sharp spade and lift out sections for transplanting. Many other perennials have fibrous root systems, including my coreopsis, achillea and bunch grasses.
Some of my herbaceous perennials, such as Shasta daisy and hostas die back in the middle and produce healthy offspring along their outer edges. To rejuvenate these plants, I dig up the older central portion of the plant and compost it, and then gently lift out the healthy new sections, tease apart the roots and transplant where desired.
Other herbaceous perennials in my garden (strawberries and ajuga, for example) grow long stolons, or runners, that produce buds on the tips. Once the buds root, they can be lifted up from the soil carefully with a small spade for transplanting, or young buds can be cut from the stolon, rooted in containers and planted out later.
Whereas stolons are plant stems that grow on the surface of the soil, rhizomes are modified plant stems that grow horizontally underground. Some of the most aggressive herbaceous plants in my garden are rhizomatous, such as mints, trailing blackberry and Virginia creeper. (If the plant’s name has trailing, creeping or the Latin reptans in it, it’s probably rhizomatous).
However, lots of other plants with rhizomes are more well-mannered, including canna lilies and my Western sword ferns, irises and calla lilies.
Typically, it’s best to divide ferns and late summer/fall-blooming rhizomatous plants in the spring, and spring-blooming rhizomatous plants in the fall. The same guidelines for division apply to plants that produce tubers, bulbs and corms. However, I’ve had success dividing in early spring if I accept the fact that new divisions probably won’t bloom until next year. It’s also a good time to remove old rhizomes without foliage because they will not rebloom.
To divide my bearded irises, first I decide which rhizomes I want to remove and follow them back to the parent stem, digging carefully to unearth the horizontal rhizomes and the roots. I use a sharp knife to cut the rhizomes as close to the parent stem as possible, and then I lift them with a shovel, making sure to keep some roots attached. After rinsing away the soil, I cut off pieces of the rhizome that have a growing tip and roots attached.
Tuberous plants like my dahlias and Jerusalem artichokes are similarly divided, making sure each cut piece has at least one growth bud, called an eye, and a piece of the old stem attached to it. I replant tubers with the eye an inch or two below the soil surface.
Bulbous plants, like my daffodils and lilies, produce bulblets, bulbils or scales. Each of these can be detached gently from the parent plant and replanted. Some new plants will bloom the following year; others will need two or more years before they are mature enough to produce flowers.
Corms, such as gladioli and crocuses, are productive for only one year, but they grow replacements in the form of one large corm and several miniature cormels. The new, large corm will bloom the following year, but it will take several years for the cormels to flower.
Whichever method of dividing herbaceous perennials is used, it’s important to replant new divisions as soon as possible so they don’t dry out. The planting hole should be dug deep enough to accommodate the length of the roots, and the base of the plant’s stem should be planted level with the soil line. The exception is bearded iris rhizomes, which should be planted so the top half of the horizontal rhizome is exposed to air and sunlight.
I add compost and a tablespoon of bone meal (high in phosphorous) to each planting hole and then backfill the hole with soil, pressing firmly so air pockets aren’t left around the roots that could fill up with water from early spring rains and cause rotting.
Monty Don tells an interesting story about gardener and writer William Robinson, an Irishman who Dons says led a revolution against the “tyranny of bedding plants.” Annuals, particularly those that originated outside of England, were the foundation of every fashionable English garden during the Victorian period.
Robinson advocated for more natural-looking gardens with hardy perennials and shrubs. His books “The Wild Garden” (1870) and “The English Flower Garden” (1883) spurred widespread popularity of the cottage garden, which, in turn, led to today’s focus on gardens that mimic natural environments. Here’s to you, Mr. Robinson!
Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at Rnowak39@gmail.com. For more about gardening, check out her podcasts at https://mailtribune.com/podcasts/the-literary-gardener and her blogs at www.literarygardener.com.