Comfrey (also comphrey) is a common name for plants in the genus Symphytum. Comfrey species are important herbs in organic gardening. It is used as a fertilizer and in herbalism, although ingesting it may cause liver damage. 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.
The Russian comfrey "Bocking 14" cultivar 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 nineteenth century) following trials at Bocking, near Braintree.
The comfrey bed should be well prepared by weeding thoroughly, and dressing with manure if available. Offsets should be planted 0.6–1 m (2 ft 0 in–3 ft 3 in) apart with the growing points just below the surface, while root segments should be buried about 5 cm (2.0 in) 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 nitrogen hungry. Although it is a tenacious grower, 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 clippings. It 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 (24 in) 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. After the growing season, leaving comfrey beds fallow may deliver higher yields in future harvests, as the plant builds up energy reserves in its roots.
Comfrey should be harvested by using either shears, a sickle, or a scythe to cut the plant about 2 inches 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 source of fertilizer 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 1.8–2.3 kilograms (4.0–5.1 lb) per plant per cut) which, lacking fibres, quickly break down to a thick black liquid. There is also no risk of nitrogen loss 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:
Blocking 14 is sterile, and therefore will not set seed (one of its advantages over other cultivars as it will not spread out of control), thus is propagated from root cuttings. The gardener can produce his or her own "offsets" from mature, strongly growing plants by driving a spade horizontally through the leaf clumps about 7 cm (2.8 in) below the soil surface. This removes the crown, which can then be split into pieces. The original plant will quickly recover, and each piece can be replanted with the growing points just below the soil surface, and will quickly grow into new plants. When choosing plants to divide, ensure that they are strong healthy specimens with no signs of rust or mildew. When dividing comfrey plants, take care not to spread root fragments around, or dispose of on the compost heap, as each can re-root, and comfrey can be stubborn. Offsets can also be purchased by mail order from specialist nurseries in order to initially build up a stock of plants.
Folk medicine names for comfrey include knitbone, boneset and the derivation of its Latin name Symphytum (from the Greek symphis, meaning growing together of bones, and phyton, a plant), referring to its ancient uses.
Comfrey contains mixed phytochemicals in varying amounts, including allantoin, mucilage, saponins, tannins, pyrrolizidine alkaloids, inulin, and proteins, among others. Liver toxicity is associated with consuming this plant or its extracts. In modern herbalism, comfrey is most commonly used topically.
In 2001, the United States Food and Drug Administration issued a ban of comfrey products marketed for internal use, and a warning label for those intended for external use. Comfrey should not be used during pregnancy and lactation, in infants, and in people with liver, kidney, or vascular diseases.