Comfrey – Symphytum officinale

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Comfrey is an important herb in organic gardening. It is used as a fertilizer and as an herbal medicine. The most commonly used species is Russian comfrey Symphytum × uplandicum , which is a cross or hybrid of Symphytum officinale (Common Comfrey) and Symphytum asperum (Rough Comfrey).

Comfrey (Symphytum officinale L.) is a perennial herb of the family Boraginaceae with a black, turnip-like root and large, hairy broad leaves that bears small bell-shaped flowers of various colours, typically cream or purplish, which may be striped. It is native to Europe, growing in damp, grassy places, and is locally frequent throughout Ireland and Britain on river banks and ditches. More common is the hybrid between S. officinale and S. asperum, Symphytum × uplandicum, known as Russian Comfrey, which is widespread in the British Isles, and which interbreeds with S. officinale. Compared to S. officinale, S. × uplandicum is generally more bristly and has flowers which tend to be more blue or violet.

Comfrey has long been recognised by both organic gardeners and herbalists for its great usefulness and versatility; of particular interest is the ‘Bocking 14’ cultivar of Russian Comfrey. This strain was developed during the 1950s by Lawrence D Hills, the founder of the Henry Doubleday Research Association (the organic gardening organisation itself named after the Quaker pioneer who first introduced Russian Comfrey into Britain in the 1910s) following trials at Bocking, near Braintree, the original home of the organization.

The comfrey bed should be well prepared by weeding thoroughly, and dressing with manure if available. Offsets should be planted 0.6 – 1m apart with the growing points just below the surface, while root segments should be buried about 5 cm deep. Keep the bed well watered until the young plants are established. Comfrey should not be harvested in its first season as it needs to become established. Any flowering stems should be removed as these will weaken the plant in its first year.

Comfrey is a fast growing plant, producing huge amounts of leaf during the growing season, and hence is very nitrogen hungry. Although it will continue to grow no matter what, it will benefit from the addition of animal manure applied as a mulch, and can also be mulched with other nitrogen rich materials such as lawn mowings, and is one of the few plants that will tolerate the application of fresh urine diluted 50:50 with water, although this should not be regularly added as it may increase salt levels in the soil and have adverse effects on soil life such as worms. Mature comfrey plants can be harvested up to four or five times a year. They are ready for cutting when about 60 cm high, and, depending on seasonal conditions, this is usually in mid-Spring. Comfrey will rapidly regrow, and will be ready for further cutting about 5 weeks later. It is said that the best time to cut comfrey is shortly before flowering, for this is when it is at its most potent in terms of the nutrients that it offers. Comfrey can continue growing into mid-Autumn, but it is not advisable to continue taking cuttings after early Autumn in order to allow the plants to build up winter reserves. As the leaves die back and break down in winter, nutrient minerals are transported back to the roots for use the following spring.

Comfrey should be harvested by using either shears, a sickle, or a scythe to cut the plant about 5 cm above the ground, taking care handling it because the leaves and stems are covered in hairs that can irritate the skin. It is advisable to wear gloves when handling comfrey. Despite being sterile, Bocking 14 Russian Comfrey will steadily increase in size. It is therefore advisable to split it up every few years. It is however difficult to remove comfrey once established as it is very deep rooting, and any fragments left in the soil will regrow. Rotovation can be successful, but may take several seasons. The best way to eradicate comfrey is to very carefully dig it out, removing as much of the root as possible. This is best done in hot, dry summer weather, wherein the dry conditions will help to kill off any remaining root stumps. Comfrey is generally trouble free once established, although weaker or stressed plants can suffer from comfrey rust or mildew. Both are fungal diseases, although they rarely seriously reduce plant growth and thus do not generally require control. However infected plants should not be used for propagation purposes.

Comfrey is a particularly valuable source of fertility to the organic gardener. It is very deep rooted and acts as a dynamic accumulator, mining a host of nutrients from the soil. These are then made available through its fast-growing leaves (up to 4-5 pounds per plant per cut) which, lacking fibres, quickly break down to a thick black liquid. There is also no risk of nitrogen robbery when comfrey is dug into the soil as the C:N ratio of the leaves is lower than that of well-rotted compost. Comfrey is an excellent source of potassium, an essential plant nutrient needed for flower, seed and fruit production. Its leaves contain 2-3 times more potassium than farmyard manure, mined from deep in the subsoil, tapping into reserves that would not normally be available to plants.

There are various ways in which comfrey can be used as a fertilizer. These include:

  • Comfrey as a compost activator – include comfrey in the compost heap to add nitrogen and help to heat the heap. Comfrey should not be added in quantity as it will quickly break down into a dark sludgy liquid that needs to be balanced with more fibrous, carbon-rich material.
  • Comfrey liquid fertilizer – can be produced by either rotting leaves down in rainwater for 4–5 weeks to produce a ready-to-use ‘comfrey tea’, or by stacking dry leaves under a weight in a container with a hole in the base. When the leaves decompose a thick black comfrey concentrate is collected. This must be diluted at 15:1 before use.
  • Comfrey as a mulch or side dressing – a two-inch layer of comfrey leaves placed around a crop will slowly break down and release plant nutrients; it is especially useful for crops that need extra potassium, such as fruit bearers but also reported to do well for potatoes. Comfrey can be slightly wilted before application optionally but either way, avoid using flowering stems as these can root.
  • Comfrey potting mixture – originally devised to utilize peat, now environmental awareness has led to a leaf mold-based alternative being adopted instead; two year old, well decayed leaf mold should be used, this will absorb the nutrient-rich liquid released by the decaying comfrey. In a black plastic sack alternate 7–10 cm (3-4 inch) layers of leaf mold and chopped comfrey leaves. Add a little dolomitic limestone to slightly raise pH. Leave for between 2–5 months depending on the season, checking that it does not dry out or become too wet. The mixture is ready when the comfrey leaves have rotted and are no longer visible. Use as a general potting compost, although it is too strong for seedlings.

Contemporary herbalists view comfrey as an ambivalent and controversial herb that may offer therapeutic benefits.

One of the most common uses of Comfrey extract is as a skin treatment. The plant contains the small organic molecule allantoin, which is thought to stimulate cell growth and repair while simultaneously depressing inflammation. Scientists and physicians agree that the use of Comfrey should be restricted to topical use, and should never be ingested, as it contains dangerous amounts of hepatotoxic pyrrolizidine alkaloids (PAs). Use of comfrey can, because of these PAs, lead to veno-occlusive disease (VOD). VOD can in turn lead to liver failure, and comfrey has been implicated in at least one death. In 2001, the United States Food and Drug Administration issued a warning against internal usage of herbal products containing comfrey, and eventually banned Comfrey products intended for internal use. In addition to restrictions on oral use, scientists and medical professionals recommend applying Comfrey extracts no longer than 10 days in a row, and no more than 4–6 weeks a year.

One of the country names for comfrey was ‘knitbone’, a reminder of its traditional use in healing bone fractures. Modern science confirms that comfrey can influence the course of bone ailments.

The allantoin contained in the plant is thought to help replace and thus repair cells in the body through its proliferant properties. Comfrey was used in an attempt to treat a wide variety of ailments ranging from bronchial problems, broken bones, sprains, arthritis, gastric and varicose ulcers, severe burns, acne and other skin conditions. It was reputed to have bone and teeth building properties in children, and have value in treating “many female disorders”. Constituents of comfrey also include mucilage, steroidal saponins, tannins, pyrrolizidine alkaloids, inulin, and proteins.

The flowers of Russian comfrey

Excessive doses of symphytine, one of the PAs in comfrey, may cause cancer in rats. This was shown by injection of the pure alkaloid. The whole plant has also been shown to induce precancerous changes in rats.

Most recently, in a placebo-controlled study comfrey was found to decrease back pain when used topically. However, it is not clear if these results reached statistical significance.

All information provided is for informational purposes only. Please seek professional advice before medicinal use of any plants.