Roots on Roots

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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.

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