Comfrey (Symphytum officinale) is a perennial flower that grows in clumps naturally along riverbanks and in grasslands. It also can be a nice addition to a wildflower garden and for container plantings. The plant grows slightly taller than it is wide. It features large, pointed, dark green leaves up to 8 inches long that have a coarse, hairy texture. The lower leaves tend to be larger than the upper ones on the plant. Tiny bell-shaped flowers bloom in the late spring in clusters off drooping stems and often attract bees and other pollinators. Comfrey has a vigorous growth rate and can be planted at any point when the soil is not frozen.
|Botanical Name||Symphytum officinale|
|Common Names||Comfrey, common comfrey, true comfrey, boneset, knitbone, knitback, comfrey consound|
|Plant Type||Herbaceous, perennial|
|Mature Size||1–3 ft. tall, 0.75–2.5 ft. wide|
|Sun Exposure||Full, partial|
|Soil Type||Loamy, well-drained|
|Soil pH||Acidic, neutral|
|Flower Color||Purple, pink, cream, white|
|Hardiness Zones||4–8 (USDA)|
|Native Area||Europe, Asia|
|Toxicity||Toxic to people and animals|
Comfrey is highly adaptive to a variety of growing conditions and requires very little maintenance. Mature plants grow an extensive root system, including a deep taproot. This allows them to efficiently obtain nutrients and moisture from the soil. But it also makes comfrey plants difficult to eradicate if you ever want to remove them. Any small portion of the root left in the soil after you dig up a plant likely will grow a new plant. So if you want to limit the plant's spread, it's often best to grow it in a container or raised garden bed instead of the ground.
If you promptly remove the spent blooms, this can prevent the plant from spreading its seeds. Cutting back the stems after the plant flowers also can result in a rebloom. As with all rapid growers, comfrey needs a lot of nitrogen to look its best and flower well. So making sure the soil has enough organic matter mixed in is essential. Otherwise, comfrey mostly takes care of itself except for requiring water during prolonged dry spells.
Comfrey can grow in full sun to partial shade, meaning it needs at least three hours of direct sunlight on most days. In the warmer parts of its growing zones, plant it where it will get shade from the strong afternoon sun.
The plant can tolerate a wide range of soil conditions, including clay soil and somewhat sandy soil. But it prefers an organically rich, loamy soil that has good drainage. A slightly acidic to neutral soil pH is ideal, but it can tolerate slightly alkaline soil as well.
Comfrey plants like an even amount of soil moisture. They have some drought tolerance once they're established but prefer at least a moderate level of moisture. Be sure to keep the soil of young plants consistently moist but not soggy. Then, water mature plants whenever the top inch or two of soil begins to dry out.
Temperature and Humidity
Comfrey is hardy both to the extremely cold and hot temperatures within its growing zones. It will die back in the late fall once frost and freezing temperatures have arrived. But the roots will remain, and the plant will come up again in the spring. Humidity typically isn’t an issue for comfrey as long as adequate soil moisture is maintained.
The best feeding regimen for comfrey is to provide regular organic amendments to the soil, such as a layer of compost applied each spring. Comfrey's very long roots are good at finding deep nutrients in the soil, so additional feeding is not necessary.
Is Comfrey Toxic?
The leaves, flowers, and roots of comfrey have been used in herbal medicine. The plant was popular to treat pain from sprains, bruises, fractures, and arthritis. However, medical professionals limit it's medicinal use due to comfrey’s toxicity. All parts of comfrey contain pyrrolizidine alkaloids, which are toxic both to people and animals when ingested and used topically. But brief skin contact with the plant should not cause any issues, such as irritation or rash, unless you have an individual allergy to comfrey.
Symptoms of Poisoning
Comfrey often doesn’t cause any short-term symptoms of toxicity. But ingesting its toxic chemicals or absorbing them through the skin has been linked to serious health issues, including liver damage, cancer, and lung damage. Comfrey also can harmfully interact with certain medications. Discuss any kind of herbal use with a doctor first, and contact a medical professional as soon as possible if you suspect poisoning.
Comfrey can be grown from seed, but it requires a winter chilling period to germinate. It's also not unusual to sow the seeds and not see any germination for two years. So it's more common to propagate the plant from root cuttings. Trim off 2- to 6-inch lengths of the root, and plant them horizontally roughly a few inches deep. Plant more shallow in clay soil and deeper in sandy soil. Keep the soil consistently moist (but not soggy) until you see growth.
Comfrey plants generally don't have any serious issues with pests or diseases. One disease, comfrey rust, can overwinter in the roots and weaken the plant's growth and flowering. However, it is not common in most areas. Slugs and snails also might damage the foliage, but deer tend to leave the plants alone.
There are multiple related species that also use the common name comfrey, including:
- Symphytum caucasicum: This plant is commonly referred to as Caucasian comfrey or blue comfrey for its flowers that start pink but then transition to a bright blue.
- Symphytum grandiflorum: Known commonly as large-flowered comfrey, this plant features showy cream to white blooms.
- Symphytum x uplandicum: Also known as Russian comfrey, this hybrid can grow up to 6 feet tall and sports violet flowers.
Kruse LH, Stegemann T, Jensen-Kroll J, Engelhardt A, et al. Reduction of Pyrrolizidine Alkaloid Levels in Comfrey (Symphytum officinale) Hairy Roots by RNAi Silencing of Homospermidine Synthase. Planta Med, 85(14-15):1177-1186, 2019. doi:10.1055/a-0998-5125
Salehi B, Sharopov F, Boyunegmez Tumer T, Ozleyen A, Rodríguez-Pérez C, M. Ezzat S, et al. Symphytum Species: A Comprehensive Review on Chemical Composition, Food Applications and Phytopharmacology. Molecules, 24(12):2272, 2019. doi:10.3390/molecules24122272