Have you ever tried to get all the bindweed or goutweed—or mint—out of your herb bed, only to discover new plants flourishing a few weeks later? All that hoeing and digging you did just chopped up the roots and rhizomes, and each bit turned into a new plant. Aggravating as this tendency can be when you’re trying to get rid of an unwanted plant, it can be turned to your advantage to multiply desirable herbs.
Herbs that have fleshy roots or that tend to produce suckers are good candidates for this kind of propagation. They include bayberry, sassafras, horehound, bee balm, butterfly weed, purple coneflower, violets, salvias, sea holly, perennial mullein, Oriental poppies, and sea lavender. Root cuttings can give you more plants than division, and the technique is easier than stem cuttings, at least for many herbs. Vegetative propagation using root cuttings is an especially useful method for increasing prized cultivars or hybrids that don’t come true from seed.
Comfrey is a fine choice for your first attempt at this technique, as the root pieces readily form roots and tops. For the greatest likelihood of success, take root cuttings early this spring before the plant has put out a lot of new top growth or or early next fall after flowering is done. When the plant is putting its energy into flower production, stem buds form less readily in the root tissue. Even so, most of the root cuttings of comfrey that I took late last spring when the plant was several feet tall showed new top growth within five weeks.
Taking cuttings from large roots takes less dexterity and fussing than with stem cuttings. You can dig up an established comfrey plant (or other herb) and cut off roots with pruning shears or a sharp knife, or leave the plant in the ground and just dig up soil next to it that contains some of the larger roots.
You are after the fleshy roots 1/4 to 1/2 inch thick, not the fine feeder roots. Cut these thick roots into 2-inch lengths and gather several together with a rubber band to keep them oriented in the same way that they grew. You can cut the tops of the pieces straight across and the bottoms diagonally to help you recognize which end is which, but the plant needs no such reminders. The end of the root piece that was originally uppermost will sprout a stem and leaves, and the opposite end will sprout roots, no matter how it is planted, but orienting the piece correctly enables it to expend the least amount of energy in sprouting.
Into A Mix
Weeds obviously root perfectly well in ordinary garden soil, but the ideal rooting medium for more challenging plants combines good drainage with good moisture-holding ability—qualities that may not be present in your garden soil. Equal parts of peat and sand, vermiculite and perlite, or peat and perlite are commonly used soilless potting media that drain well yet hold moisture. I have tried them all, with varying degrees of success.
If you want to root a lot of cuttings at once, select a flat that is a little deeper than they are tall. If you are rooting only a few large cuttings, you may wish to place them in individual pots. This will also save one step in transplanting after they root.
You may place roots in the container vertically, diagonally, or horizontally. If you lay them horizontally, you won’t need to worry about which end is which, and you can use a shallower container than you’d need if you oriented the roots diagonally or vertically. On the other hand, horizontal cuttings occupy more surface area, which may be a consideration if you have very limited space for this project and want lots of new plants.
Fill the container to within 1/2 to 3/4 inch of the top with dampened rooting medium. Place the cuttings in neat rows in whichever orientation you choose. You don’t need to poke holes for the vertical and diagonal pieces unless they are quite thin, nor do you need to use a rooting hormone. Fill the rest of the container with dampened medium, covering the root pieces.
Time To Grow
If you take cuttings in the fall, you may place the filled flat in a cold frame or cool porch for the winter. Don’t expect to see any top growth until spring. Alternatively, you may bring the flat indoors and keep it in a warm spot or on a heating cable. Top growth may begin in a few weeks, and you will then need to transplant the cuttings to pots and keep them indoors until spring. Keeping them outside is a lot less bother.
Flats of cuttings taken in spring may be placed in the shade and kept moist but not wet. They should begin to show top growth in three weeks to two months.
A Comfrey Experiment
I tried using three rooting mixes (peat/sand, vermiculite/perlite, and peat/perlite) and two orientations (horizontal and vertical), with six cuttings of comfrey root in each category. I placed filled flats under a rhododendron where they would get some morning sun. I checked them every few days and watered them only if the medium seemed dry.
At the end of five weeks, top growth was evident on all of the peat/sand cuttings, all six horizontal and five of the six vertical vermiculite/perlite cuttings, but only four horizontal and two vertical peat/perlite cuttings. But top growth told only part of the story.
When I examined the roots on the cuttings, I discovered five well-grown root systems on the horizontal peat/ sand cuttings and one cutting with no roots at all, even though it had a top. All the vertical peat/sand cuttings had well-developed roots, but two were growing in the wrong direction (I must have planted them upside down). The roots, like the tops, of the horizontal vermiculite/perlite cuttings were all well developed, but only three of the vertical pieces had both tops and roots; two had tops and no roots, and the remaining one had a single root and no top. Only one horizontal peat/perlite cutting had well-developed roots; the others showed signs of overwatering. The vertical cuttings in this flat had rotted roots or no roots at all.
I transplanted one of the better-looking plants from each category into a 41/2-inch square pot filled with a potting mix containing about equal volumes of peat, perlite, vermiculite, sand, compost, and soil; watered it; and placed it under the rhododendron. After a month of warm weather, all of the plants had developed healthy tops, and each had a nice cube of soil nearly filled with fine roots. The time from cutting to transplant ready to go into the garden was just over two months, no matter what the cutting’s orientation in the flat or which potting mix it had rooted in.
Lessons On Root Cuttings
I learned two lessons from this experiment. One was the likelihood of overwatering. I hadn’t figured on the contribution of natural rainfall. Even though I watered only when the medium seemed dry, the peat/perlite mix could be overwatered by rainfall alone.
The other lesson came as a surprise. Although more cuttings developed roots and tops in the heavy peat/sand medium than in the other two mixes, it was very difficult to remove rooted cuttings from the flat without breaking off many of the small roots. I would avoid using this formulation again in flats, although it would work fine when rooting cuttings in individual pots where they could remain for some time after forming roots. For rooting cuttings in flats, I would choose the vermiculite/perlite mix. The orientation of the cuttings makes little difference.
Don’t unintentionally multiply your herbs (or weeds) by tilling in the root zones of plants that increase readily by root cuttings—unless you want to create a bed of the stuff. To prevent a sea of unwanted herbs, carefully dig up the soil with a shovel between the plant and the area to be tilled and pick out every bit of root before bringing in the tiller.
Betsy Strauch is assistant editor of The Herb Companion. She and her husband, Joe, are rooted in Lenox, Massachusetts.