T.M. Teynor1, D.H. Putnam2, J.D. Doll3, K.A. Kelling3, E.A. Oelke2, D.J. Undersander3, and E.S. Oplinger3
1Center for Alternative Plant and Animal Products, University of Minnesota, St. Paul, MN 55108.
2Department of Agronomy and Plant Genetics, Minnesota Extension Service University of Minnesota, St. Paul, MN 55108.
3Departments of Agronomy and Soil Science, College of Agricultural and Life Sciences and Cooperative Extension Service, University of Wisconsin-Madison, W153706. Feb. 1992.
Comfrey has been cultivated since about 400 BC as a healing herb. The word comfrey, derived from the Latin word for "grow together", reflects the early uses of this plant. Greeks and Romans used comfrey to stop heavy bleeding, treat bronchial problems, and heal wounds and broken bones. Poultices were made for external wounds and tea was consumed for internal ailments.
Comfrey (Symphytum spp.) is native to Europe and Asia. Although comfrey has been used as a food crop, and as a forage crop, in the past 20 years scientific studies reported that comfrey may be carcinogenic, since it appeared to cause liver damage and cancerous tumors in rats. Comfrey-pepsin capsules, which are sold as a digestive aid in herbal and health-food stores in the USA, have been analyzed and found to contain pyrrolizidine alkaloids. These alkaloids cause liver damage in people and are a potential carcinogen. Huxtable et al. (1986) cited cases of hepatic veno-occlusive disease that were produced by using these capsules. These reports have temporarily restricted development of comfrey as a food crop.
Three plant species in the genus Symphytum are relevant to the crop known as comfrey. Wild or common comfrey, Symphytum officinale L., is native to England and extends throughout most of Europe into Central Asia and Western Siberia. Prickly or rough comfrey [S. asperum Lepechin (S. asperrimum Donn)], named for its bristly or hairy leaves, was brought to Britain from Russia about 1800. Quaker, Russian, or blue comfrey [S. × uplandicum Nyman (S. peregrinum Lebed.)] originated as a natural hybrid of S. officinale L. and S. asperum Lepechin. This hybrid was called Russian or Caucasian comfrey in reference to its country of origin. Cuttings of this hybrid were shipped to Canada in 1954 and it was named Quaker comfrey, after the religion of Henry Doubleday, the British researcher responsible for promoting comfrey as a food and forage. The majority of comfrey grown in the United States can be traced to this introduction.
Prickly comfrey was evaluated for its value as a forage by the USDA and numerous state experiment stations more than 80 years ago. Comtrey yielded less than some common forage crops and its high water content of 85 to 90%, in comparison to 75 to 80% for alfalfa, made forage preservation difficult. The extensive hairs on comfrey leaves restricts its use as a forage. Fresh leaves are eaten by pigs, sheep, and poultry, but are frequently unpalatable to cattle and rabbits. Cattle and rabbits will eat the wilted forage. Horses, goats, chinchillas, and caged birds are also fed this forage. In a grazing trial in St. Paul, MN, comfrey was judged to be poorly palatable in comparison with several other plant species. This is probably due to the presence of hairs which wilting alleviates.
Wild comfrey was brought to America by English immigrants for medicinal uses. The allantoin content of comfrey, especially in the root, has resulted in its use in folk medicine for healing wounds, sores, burns, swollen tissue, and broken bones. Allantoin, found in milk of nursing mothers and the fetal allantois, appeared to affect the rate of cell multiplication. Wounds and burns seemed to heal faster when allantoin was applied due to a possible increase in number of white blood cells. Comfrey has been reported to promote healthy skin with its mucilage content that moisturizes and soothes, while the allantoin promotes cell proliferation.
The allantoin applied to external wounds is either a 0.4% solution or a 2% ointment. An effective allantoin formulation is difficult to prepare from comfrey due to the low and variable content of this substance. Hart (1976) reported that dried comfrey leaves contain 0.1 to 1.6% allantoin while dried roots have 0.4 to 1.5%. Since fresh leaves are 85% water, they could not contain more than 0.2% allantoin. It would require anywhere from 8 oz to 8 lb of dried comfrey leaves per quart of water to produce a 0.4% solution that would be effective.
Comfrey produces large amounts of foliage from late May until hard frosts in October or November. This crop has been used as a salad green and potherb because it was considered a good source of protein and a rare plant-derived source of vitamin B12. Vitamin B12 is produced usually by soil bacteria and fungi or in the small intestines of some animals. Humans usually obtain this vitamin from eggs, dairy products, and meat. However, a study on the nutritional value of comfrey conducted in Australia in 1983 found that you would need to eat more than 4 lb/day of fresh comfrey to obtain the minimum daily requirement of B12. Eating such large amounts of comfrey, a poor source of vitamin B12, is inadvisable due to the potential health hazards.
Protein content of comfrey dry matter (15 to 30%) is about as high as legumes. Robinson (1983) reported specific amino acid and mineral content of comfrey. Hart (1976) mentioned that comfrey has lower amounts of eight amino acids that are essential for humans than turnip greens or spinach, but more than cabbage. Comfrey, like most green vegetables, is deficient in methionine and is also low in phenylalanine. Three ounces of dried turnip greens or spinach, in comparison to 20 oz of dried comfrey, supply adults with the total daily requirement of all essential amino acids, except for methionine. Comfrey also tends to have high ash content.
Comfrey is a herbaceous perennial plant with short, thick, tuberous roots, a deep and expansive root system: Comfrey begins growth in early-April and by early May compact clusters of young leaves are visible in the crown of the old plant. Within a few weeks, the leaf blades with long petioles have grown to over 12 in. high. Basal leaves are large, lance-shaped, stalked, and coarsely hairy. The stem elongates rapidly and reaches a height of over 3 ft. Upper leaves do not have long petioles and are attached closely to the stem.
Flowering starts in late May or early June and continues until fall. Leaves on flowering, erect stems are sessile or decurrent, and decrease in size up the stem. The bell-shaped flowers with pedicels are in terminal cymes or one-sided clusters. Flowers of common comfrey are usually creamy yellow, but white, red, or purple types have been found in Europe. Prickly comfrey has pink and blue flowers while Quaker comfrey has blue, purple, or red-purple flowers. Seed production is rare, and crops are usually established from root cuttings and crown divisions. Vegetative growth does not cease with the start of flowering, and the plant will add new stems continuously during the growing season. The plant will grow rapidly after harvest and flower again. Comfrey crowns and roots are very winterhardy in northern Midwestern environments.
Comfrey produces the highest yields in full sunlight and under cooler conditions. Unlike annual crops, the leaves do not readily wilt during extended periods of drought due to its deep root system. This crop is also very frost resistant.
Comfrey is adaptable to many soils, but prefers moist, fertile soils. Thin soils over rock will give a poor crop, but on light sands and loams, this crop will be productive if adequate nutrients are present.
Comfrey is propagated from root cuttings, crown divisions, and transplants. Production during the first year is greatest for a crop started from transplants and lowest for one using root cuttings. After the first year, comfrey yields the same regardless of propagation method. Root cuttings are the least expensive method of propagation, and consequently, are used most often. Root cuttings are usually 1 1/2 to 6 in. long and 1/4 to 3/4 in. in diameter. Smaller cuttings will also generate plants, but plants from longer cuttings emerge and establish more quickly. Wilted cuttings should be soaked in cold water until they become firm before planting. Root cuttings develop buds about 3 to 6 weeks after planting, while crown divisions emerge in about 10 days. Prickly comfrey is propagated by seed in the Soviet Union.
Comfrey plantings should last indefinitely (more than 20 yrs) if proper weed control and soil fertility are maintained.
Till the soil so that it is free of perennial and annual weeds, and is level prior to planting. When an area is to be replanted with a different crop, repeated tillage is usually used to remove the comfrey plants. Deep moldboard plowing should be done in September or October, and then followed by tillage with a field cultivator, which will expose roots to the drying and freezing conditions of winter. Herbicides could be used in removing the old plants. Glyphosate was sprayed in June at Rosemount, MN and killed stems and leaves. However, plants grew again from the roots to produce a full stand by September.
The best time to plant is in April, or as early as the soil can be tilled, but the crop can be planted throughout the growing season. A small crop can be anticipated the first year if it is planted early in the season. However, the most efficient use of the land usually occurs when comfrey is planted after the harvest of small grain or other early season crops. Root cuttings should be planted before September, while transplants or crown divisions can be planted as late as early October. Planting late in season has the advantage that the land can be summer fallowed to kill weeds. Plants must be established and grow before winter in order to produce a high yield the following year.
Root cuttings should be laid flat and covered with soil. The cuttings should be planted at a depth of 2 to 4 in. according to soil texture and expected soil moisture. A 4-in. depth is commonly used, but 2 in. is adequate with irrigation. Very small cuttings should not be planted as deeply as the longer cuttings. Young transplants should be planted upright with the crowns about 2 in. deep. Comfrey is planted in a checkerboard arrangement in rows that are 3 to 4 ft apart to permit cross cultivation for effective weed control. Closer row spacing, such as 30 in., may produce higher yields but the cost of cuttings will be greater.
Comfrey is a high-protein forage that, unlike legumes, obtains all of its nitrogen from the soil. Older plantings with leaves showing a lighter green color will usually require broadcasting or sidedressing of nitrogen ferrilizer. Recommended rates vary from 40 to 100 lb N/acre depending on soil organic matter. Barnyard manwe is an excellent nutrient sowce for comfrey. Productive comfrey, like silage corn, removes relatively large amounts of potassium, phosphorus, and calciwm from the soil. Comfrey productivity is not very sensitive to soil pH, but highest yields occur on soils with a pH of 6.0 to 7.0. Soils testing in the medium range should receive about 25 lb P2O5 and 120 lb K2O/acre.
All commercial plantings in the United States appear to be derived from cuttings of the Bocking Mixture cultivar. This cultivar was imported from England into Canada in 1954 and subsequently arrived in the USA. This cultivar is a mixture of several clones that differ slightly in plant vigor and general morphology.
1. Mechanical: Comfrey is an excellent weed competitor due to its rapid and dense growth. Weeds may become established between comfrey plants under a multiple-cut harvesting regime. As a result, two cultivations per year are often required. Rototilling between plants is an effective method for destroying weeds.
2. Chemical: Comfrey has usually been grown without herbicides. No herbicides are labeled for use on this crop in the Upper Midwest.
Diseases have not been a serious problem with comfrey in the United States. Comfrey rust fungus (Melampsorella symphyti) overwinters in roots and reduces yield of old plantings in Great Britain. This disease problem has not spread to the United States due to plant quarantine regulations on the importation of roots or plants.
Insects have not been reported to be a problem with comfrey in the United States.
When comfrey is wilted or ensiled after cutting, cattle and rabbits will eat it, since this practice collapses the leaf hairs. Leaf hairs of comfrey are apparently not a problem when cattle are fed green chop that is harvested daily.
Drying comfrey for use as a medicinal herb or clean hay is not easy for mechanized farms since at least three days of dry weather are required to cure it in a windrow, and the leaves may get dirty when laying on the soil. The medicinal herb or clean hay can be made by drying leaves by spreading them out in open areas. The comfrey may turn a dark color, but it is still acceptable to livestock as a forage.
Comfrey must be cut and allowed to wilt for a minimum of 24 hrs when used as silage. Additives such as molasses or grain are sometimes helpful, and mixing up to 25% comfrey with small grain or corn forage serves as an economical method to make high quality silage.
More than half of the annual yield in Minnesota trials was produced in the first two cuttings during June and July (Table 1). Chinese researchers found that the greatest annual yields were obtained from three cuttings, which start at full bloom in mid-June. Comfrey yields are frequently reported on a fresh-weight basis, which exaggerates the yield potential of the crop due to its high moisture content. Comfrey forage averaged about 89% moisture in Minnesota trials (Table 1). Moisture content of comfrey forage is higher than in some legume and grass forages. For example, alfalfa cut in early bloom has 80% moisture and winter rye has 75 to 85% moisture when harvested between the tillering and boot stages.
Annual dry-weight yields in Minnesota ranged from about l 1/2 to 6 t/acre for four or five cuttings. However, on a fresh-weight basis, the same yields ranged from 13 to 61 t/acre. The low yields in 1979 and 1980 from the 1979 planting, compared to the higher yield obtained in 1980 from the 1979 planting that was not harvested in 1979, indicates that the crop plants need time to accumulate root reserves before intensive harvesting begins.
Quaker comfrey may produce forage yields almost as high as those of alfalfa or orchardgrass when planted at the optimal spacing and rate of fertility. Hart (1976) compiled a table of comparative yields for comfrey and other forages that showed comfrey is not better than other forages (Table 2). Robinson observed that comfrey was high in crude protein (21 to 31%), which increased from the first to last harvest. Research trials conducted by USDA scientists (Hart et al., 1981) found crude protein contents that ranged from 13 to 17% for comfrey and 16 to 17% for alfalfa. However, they also observed that except at the highest nitrogen rate, its digestibility was usually somewhat lower than those for alfalfa, orchardgrass, and other forages.
Table 1. Moisture percentages and dry-matter yields of comfrey at Rosemount, Minnesota, 19791981.
|April 28, 1975||92||87||88|
|October 14, 1977||90||88||88|
|April 23, 1979||89||87||88|
|April 23, 1979||--||88||90|
|Yield (lb. dry matter/acre)|
|April 28, 1975||5,340||8,650||9,485|
|October 14, 1977||12,140||7,950||9,485|
|April 23, 1979||2,720||6,240||9,480|
|April 23, 1979||-||10,210||8,110|
Table 2. Dry-matter yields of comfrey and other forage crops in different areas of the world.
|Location||Years of Data||Crop||Yield (Ton/acre)|
|Red Clover + Timothy||4.1|
Comfrey has some disadvantages compared with other forage crops. Comfrey reguires the addition of nitrogen fertilizer to produce a high yield and protein content, while alfalfa produces high yields and protein content without addition of nitrogen fertilizer. Alfalfa and other forages can be established more cheaply than comfrey, since it is usually planted as root cuttings, especially at the close spacing needed for maximum yield. Comfrey, like other perennials, may be difficult to eradicate in order to plant other crops. The high moisture content and unpalatability for some livestock species make utilization of comfrey as a feed difficult. The presence of toxic alkaloids is also a problem. Advantages of comfrey are that it is very winterhardy in northern environments and could stabilize soil on erodible lands. It also produces fresh forage at a time of the year (spring, fall) when forage may be short from other sources.
Controversy about the safety of comfrey for internal use continues. Occasional use of comfrey for variety in one's diet and medicinal purposes will probably continue. Nonetheless, comfrey should no longer be considered a crop that can be consumed by humans or animals with complete safety.
It should be noted that consumption of comfrey is usually at lower levels than in toxicity research. Studies need to be conducted that involve the normal or low intake of comfrey for a proper evaluation of the health hazard to people or farm animals. Comfrey can be used externally as a medicinal herb for the allantoin content and as a crop for composting, mulching, or green manuring.