One of the greatest joys of herb gardening for me is sharing my perennial plants with other gardeners. (I love to receive plants, too.) But buying plants to give away can be expensive, and raising plants from cuttings or seeds takes time. Thus, I take comfort in knowing that I can satisfy my urge to share my herbs without spending a cent and at the cost of only a few minutes of my time and a little elbow grease. I do this by dividing them.
Vegetative (asexual) methods are the only way to propagate certain herbs, such as French tarragon, which doesn’t set seed, or those cultivars of lavenders, oreganos, and thymes that don’t come true from seed. These methods ensure that offspring are genetically identical to their parents, that (barring a rare mutation) any start of tarragon that I give you will be identical to the mother plant and to its mother plant, and so on back through dozens of generations.
Division is only one method of vegetative propagation; others include taking stem, root, or leaf cuttings (see “Growing Herbs from Stem Cuttings”, The Herb Companion, February/March 1993), tissue culture, and layering (see “Layering: Propagation without Separation”, August/September 1990). Division is my favorite method because it’s quick: it takes just minutes versus the weeks or months required to obtain plants from cuttings. Division requires no fancy equipment–no lights, heating cables, cold frames, or propagating chambers. The new plants (called divisions) are much bigger than rooted cuttings, and they come equipped with all the parts (roots, stems, leaves) they need to start growing in their new location.
Although my motive for dividing perennial herbs is to obtain more plants to give away or to fill my own beds, there is another good reason to divide them. When I first set them in the ground, I give my plants plenty of room; small ones often look puny and lonesome in a sea of bare soil. With sun, water, and warmth, though, they soon expand outward, quickly using up their allotted space. The more rampant spreaders, such as yarrow, begin trespassing on the territory of their neighbors. Eventually, the enlarged plants must pay a price. Typically, the younger roots at the edge of the plant get their nutrients from the soil in the new space, but the older central roots have exhausted the soil they’re in; they weaken and die, as does the original top. What’s left is a ring of healthy plant material around a dead center; what’s needed is to move the good parts to fresh soil and to dig out and discard the old, worn-out inside part.
How Often to Divide Herbs
Every book I’ve read recommends dividing most perennial herbs about every three years, but I do it more often, even annually if the plants are growing well. If I put it off, the plants get woodier and much harder to dig. Another benefit is a more frequent change of exhausted soil for fresh, which makes sense to me.
When to Divide Herbs
Here in Massachusetts, I start thinking about dividing my herbs in April, though late snowstorms can postpone the actual digging for days or weeks. I remove the winter mulch of pine needles gradually, in two or three steps, so as not to expose the plants to the harsh realities of the weather too abruptly. As the tops begin to make a little growth, I see that the bee balm is beginning to tangle with the lemon balm, the lady’s-mantle is mingling with the orris, and the fern-leaved tansy is moving out of its enclosure of small stones. As the clump of chives begins to send up dark green succulent fingers, I envision it cut into four or six sections, each a mass of lavender flowers come June.
Some people divide their herbs in the fall, but I nearly always do it in spring. I’m all fired up to get into the garden after a winter of enforced idleness. In fall, I’m usually too busy harvesting herbs and vegetables to think of dividing plants, and if I do finally get around to preparing a new bed, I worry that the newly divided plants may not have enough time to become established before the ground freezes.
I’m tempted to start digging immediately, but I tell myself to wait a bit: it won’t hurt the plants to divide them now but a couple of weeks of warm weather will cause them to put on more growth, making it easier to distinguish the plants I want to divide from those I want to leave undisturbed. Butterfly weed (Asclepias tuberosa) emerges particularly late, but I have it well marked with wire-legged plant labels.
Waiting also will make it easier to tell whether plants are really dead or just slow in showing life. I may try snapping a twig or scratching it to see if it’s green inside. A twig that snaps and is brown and dry all the way through is almost certainly dead. Before giving up a plant for lost, though, I’ll brush away the soil from the crown (the junction of the stem and roots) and look for evidence of buds. If I see any, I’ll cover them up and wait a little longer.
Choosing Herbs to Divide
As I walk about the garden in April, I’m on the lookout for candidates for division. Besides perennials that are outgrowing their allotted space, I look for those that are crowded and are not blooming as well as they used to–orris, for example–and for clumps that are dying out in the middle such as yarrow and bee balm. I check those that spread by stolons, such as mints and silver-king artemisia; those that consist of a mass of tiny bulbs, such as chives and garlic chives; and those that form small satellite plants, such as lady’s-mantle. I note whether the French tarragon is big enough to divide, and think about where I could establish a ground cover of caraway thyme or sweet woodruff.
How to Divide Herbs
Some gardeners divide herbs by digging up the entire plant, shaking or washing off the soil, and cutting or pulling the root mass into sections, each with several buds or living top growth. This is a good way to maximize the number of divisions, but I rarely do this. I find that small divisions take too long to make much of a show, and they may suffer transplant shock, especially later in spring when the sun is more intense and the soil has dried out more.
I prefer to cut plants into chunks that are 4 to 6 inches across. Preparing them takes a fraction of the time it takes to pick apart plants as described above. And my divisions don’t experience much transplant shock because I disturb as few of the roots as possible when digging them. Only those on the outside of the division are at all damaged, leaving all those on the inside of the chunk to carry on with their activities.
I work some compost and perhaps a bit of fertilizer into the area where I plan to put each division. I dig a hole about the size of the division into the now-fluffed-up soil and set the division into it, firming the soil around it and watering it in. In spring, the soil is usually moist so I don’t bother shading them, and I never cut the tops back, even when dividing plants in the fall. If the tops should begin to wilt, I’d cover them with a bushel basket for a few days. It’s a good idea to mulch all first-year plants after the ground freezes in the fall to minimize the freeze/thaw cycles that can heave them right out of the ground.
I take a heavy kitchen knife, along with my pruning shears and a shovel, out to the garden and use whichever tool does the job. A knife will just bounce off an overgrown clump of fibrous catmint roots, for example–a good reason not to let your plants get really overgrown before dividing them.
A garden spade with a square, flat blade cuts nice, neat pieces, but my long-handled round-bottomed shovel is more comfortable to use even though the pieces it makes are more irregular. It’s a mistake to lift a clump out of the hole and then try to cut it up by jumping on the shovel. If the clump wobbles, you can lose your balance and fall, and you can injure your back if the plant is woody enough to repel the shovel.
An Alternative Method
If a plant is in a good location but is just too big, it’s tempting to leave one part and cut away the rest. The roots of the part that is left are not disturbed at all so there is no transplant shock. However, I prefer to dig everything up, reserve one sturdy chunk to put back in the original location, and find new homes for any other vigorous pieces. I compost weak or dead parts (or throw them in the trash if they seem diseased) and amend the soil with compost and maybe a handful of 5-10-5 commercial fertilizer. I omit the fertilizer if the plant to be divided is especially robust.
Betsy Strauch’s time is divided among several editorial functions, including assistant editorship of The Herb Companion.