Researchers in British Columbia analyzed the NPK (nitrogen, phosphorus, and potassium) ratio of comfrey leaves by air-drying them and analyzing the powdered leaf tissues. They found that the leaves have an impressive proportion of 1.8-0.5-5.3. To compare, kelp meal has an NPK ratio of 1.0-0.5-2.5, and homemade compost ranges from 0.5-0.5-0.5 to 4-4-4 (depending on what ingredients you use). Comfrey is also rich in calcium and many other valuable plant nutrients it mines from deep in the subsoil.
Harnessing the Power
Mulch. Freshly cut comfrey leaves make good mulch because they're high in nitrogen, so they don't pull nitrogen from the soil while decomposing, as high-carbon mulches like straw and leaves do. And comfrey's high potassium content makes it especially beneficial for flowers, vegetables (such as tomatoes, peppers, and cucumbers), berries, and fruit trees. Michelle DeFord, owner of Crimson Sage Nursery, in Colton, Oregon, has mulched her stock herb plants with comfrey for 25 years because it boosts seed yields. For home gardeners, this means increased flower and fruit production. But using comfrey to mulch root crops (like carrots) or leafy greens (like lettuce and spinach) may encourage them to go to seed prematurely.
Soil amendment. Use freshly cut comfrey leaves (but not the flowering stems in this case—they can root) as fertilizer in planting holes. The leaves break down rapidly and provide nutrients right at the roots.
Compost activator. Comfrey is especially useful if you have lots of dry brown material and the pile is slow to heat up. Just layer the fresh comfrey leaves and stems in as you add other material to your pile.leaves and stems in as you add other material to your pile.
Liquid fertilizer. One of the best ways to tap your fertilizer factory is to brew comfrey tea. Fill a barrel or trash can about halfway with fresh comfrey, add water, cover it, and let it steep for 3 to 6 weeks. Comfrey tea smells foul, so brew it away from sensitive noses (yours or your neighbors). The tea may be used full strength or diluted to about half strength—to the color of weak tea. Use it whenever you water your plants. It's great for watering stressed plants to help get them back on track, reports Jerome Osentowski, director of the Central Rocky Mountain Permaculture Institute, in Basalt, Colorado.
You can also make liquid fertilizer concentrate by packing fresh-cut comfrey tops into an old bucket, weighing them down with a big rock or a plastic bag of water, covering tightly, and waiting a few weeks for them to decompose into a lovely thick black goo. Some gardeners put a hole in the bottom of the bucket and collect the concentrate in another container as it drips out. Dilute this comfrey concentrate about 15 to 1 with water, and use as you would comfrey tea. You can seal this concentrate in plastic jugs until you are ready to use it.
Pest prevention and control. Scientists at Moscow State University in Russia observed that powdery mildew spores that landed on wheat seedlings sprayed with comfrey tea did not germinate, and the wheat seedlings did not become infected. The researchers concluded that the comfrey tea sprays had activated natural defense mechanisms in the wheat seedlings, making them more resistant to disease. This summer, researchers at the Rodale Institute, near Kutztown, Pennsylvania, are conducting controlled experiments testing comfrey tea as a preventive for powdery mildew on sage grown in the greenhouse.
To use comfrey tea or diluted comfrey extract as a foliar drench or spray, add a few drops of liquid soap (it helps the spray stick to leaves) and apply it to your plants. You can use a watering can with a fine rose, but you'll get better coverage with a garden sprayer. Be sure to strain yourliquid very carefully (let it drip through a large coffee filter) before you put it in your sprayer, or you'll clog up the nozzle before you even get started. When you spray your plants, don't coat just the tops of the leaves; reach under and spray the bottoms, too, at least until the liquid starts to run off.
Easy to Grow
If you're now ready to put comfrey to work in your garden, wait until you find out how little it expects from you. Russian comfrey is a hardy perennial (USDA Plant Hardiness Zones 3 to 9) with large, hairy leaves; long, fleshy roots; and clusters of small cream, pink, or blue flowers. Unharvested plants grow to about 3 feet tall and wide. Comfrey spreads rather enthusiastically both by self-seeding and sprouting from even small sections of severed root. You can prevent this by planting only sterile cultivars such as 'Bocking 14' and not digging or cultivating around your comfrey.
Comfrey grows best in full sun or partial shade. It thrives in clay soil with plenty of moisture but tolerates a wide range of conditions. Once established, it is difficult to get rid of, so choose a site where it can stay. Six plants is enough for most gardeners, which means allowing a planting space of about 6 by 10 feet or 3 by 20 feet. Don't plant comfrey in any area you cultivate, as breaking off bits of root will create oodles of new plants. Remove any perennial weeds in the bed. Plant root cuttings or plants about 3 feet apart either in spring or fall, and keep the soil moist until plants are well established. Don't harvest the first year, and cut off any flower stalks that form, as your plants need to establish a good root system.
If you have a small yard or you're concerned about comfrey taking over your garden, grow it in large trash cans. Just cut drainage holes in the bottom of each can, fill with a soil and compost mix, and plant.
Comfrey produces huge quantities of leaves during the growing season (4 to 5 pounds per plant per cutting) and will happily soak up any nitrogen-rich fertilizer it's given, though it grows just fine without extra feeding. Eileen Weinsteiger, regenerative garden design specialist at the Rodale Institute, maintains a thick layer of grass clippings around the plants, which keeps her patch producing more topgrowth than she can use.
Although 'Bocking 14' Russian comfrey is sterile, individual plants will expand, so divide them every few years if your patch is getting crowded. Don't even dig them up; just slice through each one with a sharp spade while its in the ground. Replant the sections you remove or share them with friends, but don't put the roots in your compost pile, or you'll have comfrey plants popping up everywhere next year.
Comfrey is ready to harvest when it is about 2 feet tall or starts to form flower stalks. Depending on your climate, you will probably get four or more harvests a year. Cut off the whole plant about 2 inches above the ground with pruners or a sickle. Be sure to wear gloves and long sleeves when harvesting comfrey, as some people find it irritates their skin. After harvesting, give your comfrey a good watering and renew the mulch layer
Jean Nick gardens organically on 11 acres overlooking the Delaware River in eastern Pennsylvania with her partner, Tom, and her two children.