Ledum palustre (Rhododendron tomentosum Harmaja, wild rosemary / marsh Labrador tea)

Description
Ledum palustre, currently classified as Rhododendron tomentosum Harmaja, is a small evergreen shrub of the family Ericaceae, typical of bogs, peatlands and wet heaths in Northern Europe, Siberia and parts of North America. In English it is often called wild rosemary, bog rosemary or marsh Labrador tea, although it is not botanically related to culinary rosemary.

The plant forms low, dense shrubs usually 30–100 cm tall, with woody, erect branches that are distinctly tomentose (covered with short hairs), especially on the younger parts. The leaves are narrow, linear-lanceolate, leathery, dark glossy green on the upper surface and rusty-brown and felted underneath. When crushed, they release a strong resinous–camphoraceous odour, due to their high content of essential oil.

The flowers, white to pinkish-white, are grouped in terminal clusters and appear in late spring to early summer. The entire plant is rich in oxygenated sesquiterpenes (notably ledol and palustrol) and other volatile compounds that provide aromatic, insect-repellent and also toxic properties, particularly in the concentrated essential oil. Traditionally, in some Nordic cultures, the leaves were used in folk medicine for rheumatic and respiratory complaints and as a natural insect repellent, but internal use is now considered risky and must be approached with great caution.

Botanical classification

• Common names: ledum, Labrador tea, wild rosemary, marsh rosemary

• Botanical name: Rhododendron tomentosum Harmaja (syn. Ledum palustre)

• Botanical family: Ericaceae

• Native range: cold regions of northern Europe, Asia and North America (boreal and subarctic areas)

• Habit: evergreen, highly branched shrub

• Height: 50–120 cm

• Lifespan: perennial

Cultivation and growth conditions

Climate

• Prefers cold or cool-temperate climates.

• Very tolerant of prolonged frosts and harsh winters.

• In hot, dry climates it suffers and tends to decline.

Exposure

• Prefers full sun in cold regions.

• Tolerates light partial shade in milder or mountain climates.

Soil

• Requires acid, peaty soils, typical of bogs and peatlands.

• Ideal soil is:

  • very moist,

  • rich in organic matter,

  • but with good surface drainage.

• Does not tolerate calcareous or alkaline soils.

• Grows best in nutrient-poor substrates with pH around 4.0–5.5.

Irrigation

• Naturally adapted to constantly moist environments.

• In cultivation, keep the soil fresh and moist with regular watering.

• Avoid stagnant, oxygen-poor water that can cause root asphyxia.

Temperature

• Very hardy: withstands temperatures well below –20 °C.

• Sensitive to prolonged heat above about 28–30 °C, especially if combined with dry air.

Fertilization

• Does not need heavy fertilization.

• Avoid rich or frequent feeding.

• If required, use low-dose fertilizers for acid-loving plants.

Crop care

• Maintain soil acidity with additions of peat, pine needles or bark mulch.

• Check that the soil remains evenly moist.

• Carry out light pruning only to maintain shape or remove damaged branches.

• Monitor for root fungi if the soil stays waterlogged for long periods.

Harvest

• Aromatic leaves can be harvested during the growing season, preferably in summer.

• Flowers appear from late spring to early summer.

Propagation

• Mainly by semi-woody cuttings in summer.

• By seed: possible, but with slow and often irregular germination.

• Requires an acidic, very light and constantly moist substrate for rooting or germination.

Indicative nutritional values per 100 g (dried leaves)

(Ledum palustre is not a conventional food; nutritional values are of purely theoretical interest. Internal use is limited and safety, not nutrition, is the key concern.)

• Energy: about 250–300 kcal

• Water: 5–10 g

• Total carbohydrates: 40–50 g

• Total fibre: 20–30 g (mainly insoluble fibre)

• Protein: 5–10 g

• Total fat: 5–8 g

  • SFA (saturated fatty acids; excess from all dietary sources is associated with less favourable blood lipids): minor fraction of total lipids

  • MUFA (monounsaturated fatty acids): trace amounts

  • PUFA (polyunsaturated fatty acids, including n-6 and n-3 series): trace amounts

• Minerals: small amounts of calcium, magnesium, potassium, manganese and others

• Phytochemicals: high content of essential oil rich in sesquiterpenes (ledol, palustrol, etc.)

Because only very small amounts of plant material were traditionally used per infusion or decoction (and modern internal use is strongly reduced for safety reasons), its caloric and nutritional contribution is essentially negligible compared with its phytotoxicological profile.

Key constituents

The chemically most important fraction of Rhododendron tomentosum is the essential oil, concentrated mainly in leaves and young shoots.

• Oxygenated sesquiterpenes (major essential oil components):

  • Palustrol

  • Ledol (a key toxic constituent limiting internal use)

  • Lepalone, lepalol, cyclocolorenone and related derivatives

• Monoterpenes and related compounds:

  • γ-terpineol, p-cymene, β-myrcene and others, depending on chemotype

• Other terpenes and volatiles with documented antimicrobial, insect-repellent, antifungal and antioxidant activities in vitro

• Polyphenols (flavonoids, tannins) in leaves and young shoots

• Minor organic acids, phenolic compounds and bitter substances

Several chemotypes of R. tomentosum essential oil are described, for example types rich in palustrol + ledol, or mixtures with significant γ-terpineol and p-cymene. These differences impact both the odour profile and the toxicity and biological activities (insect-repellent, antimicrobial, antifungal, radical scavenging).

Production process

Herbal raw material (leaves and flowering tops)

• Harvesting: flowering tops and leaves are usually collected at flowering or just before, when essential oil content is highest.

• Selection: only healthy plants from unpolluted habitats, ideally from controlled wild collection or cultivation.

• Drying: gentle drying in a shaded, well-ventilated, moderately warm environment, to preserve volatile constituents and limit oxidation.

• Cutting and grading: reduction to herbal-cut or fragments for extraction, removing excess woody stems.

• Storage: in airtight containers, protected from light, moisture and heat.

Essential oil production

• Steam distillation of flowering tops:

  • separation of essential oil from the aqueous phase;

  • decantation, filtration and, if required, standardisation.

• The oil is then used as a technical, herbal or cosmetic ingredient, at strictly controlled concentrations for safety reasons.

Physical properties

• Dried plant: fragments of linear, leathery leaves (green-brown above, rust-brown tomentose beneath), with whitish-brown floral remnants.

• Essential oil: colourless to pale yellow, sometimes slightly greenish, with a strong camphoraceous–resinous odour, intense and persistent.

• Density and refractive index: typical of sesquiterpene-rich essential oils.

• Volatility: high; the oil is sensitive to oxidation and photodegradation.

Sensory and technological properties

• Odour: strongly balsamic, resinous, reminiscent of pine, thyme, turpentine with a faint camphor-like sweetness.

• Taste: bitter, aromatic and potentially irritating; for this reason, unsupervised internal use is not recommended.

• Technological functionality:

  • at low concentrations, the essential oil can be used as an aromatic note in fragrance blends and as a natural insect repellent;

  • in cosmetic formulations it may contribute to the fragrance and a perceived toning effect on the skin, but the presence of potentially neurotoxic and irritating molecules requires very low dosage and careful safety evaluation.

Food applications

Historically, in some northern regions, leaves of Ledum palustre were used to prepare “marsh tea” or “Labrador tea” and as a flavouring for traditional beers and drinks. However:

• The presence of ledol and other toxic sesquiterpenes makes food use high-risk if not strictly controlled.

• Intoxications with nausea, dizziness, headache and more severe neurological symptoms have been reported after excessive ingestion.

• Modern safety standards generally discourage internal use or limit it to strictly controlled traditional preparations, in very small quantities and under expert supervision.

From a contemporary food safety perspective, Rhododendron tomentosum should not be treated as a routine food ingredient, and has no stable role in modern food industry.

Nutrition and health

From a nutritional standpoint, Ledum palustre offers no benefit that would justify its use as food, while it presents clear safety issues linked to its essential oil constituents.

• In vitro and experimental studies report antimicrobial, antioxidant, antifungal and possibly anti-inflammatory activities for extracts and essential oil.

• At the same time, the oil shows significant toxicity, and the sesquiterpene ledol is recognised as a major neurotoxic component, which severely restricts internal use.

• Ethnomedical uses (for rheumatism, colds, coughs, insect bites) must be interpreted in the historical context of a very different risk–benefit perception and cannot be directly transferred to modern phytotherapy without robust safety assessment.

Nowadays, practical use is mainly topical (e.g. in insect-repellent products or post-bite preparations), where systemic exposure is lower and the dose can be more tightly controlled.

Allergens and intolerances

• Rhododendron tomentosum is not among the major regulated food allergens.

• However, essential oils in general carry a risk of skin sensitisation and irritation, especially in predisposed individuals or on damaged skin.

• Ingestion may cause, in addition to systemic toxic effects (nausea, dizziness, neurological disturbances), individual hypersensitivity reactions.

In cosmetics, formulations containing Ledum-derived ingredients should be carefully patch-tested at development stage and used at very low concentrations, particularly in leave-on products.

Storage and shelf-life

• Dried herb (leaves and tops)

  • Store in airtight containers, away from light, humidity and heat.

  • Typical shelf-life: 2–3 years, with gradual loss of aroma and changes in essential oil profile.

  • Prolonged exposure to air promotes oxidation and modifies volatile composition.

• Essential oil

  • Store in dark glass bottles, tightly closed, filled as full as practical, ideally cool.

  • Indicative shelf-life: 1–2 years, after which oxidation and degradation may increase and generate more irritant by-products.

  • Development of harsh, resinous off-odours or marked darkening indicates an oil that is no longer suitable.

Safety and regulatory

• Rhododendron tomentosum is regarded as a potentially toxic species, mainly due to essential oil rich in ledol and palustrol.

• In several European countries it is not included among herbs allowed for general food use, and internal use is restricted or discouraged.

• Essential oil and extracts fall into the category of natural substances with potential risk, requiring:

  • specific toxicological assessment;

  • clear concentration limits in cosmetic and technical formulations;

  • compliance with legislation on hazardous substances and appropriate classification and labelling.

• In cosmetics and technical repellents, derivatives such as LEDUM PALUSTRE LEAF OIL may be registered as cosmetic ingredients or active substances in certain markets, subject to individual safety requirements.

Labelling

For herbal or technical products containing Ledum:

• Clear botanical designation: e.g. “Ledum palustre (Rhododendron tomentosum), leaves/flowering tops”.

• For essential oils: names such as “Ledum palustre / Rhododendron tomentosum essential oil”.

• Appropriate warnings, such as:

  • “Do not use undiluted on skin”

  • “Do not ingest”

  • “Keep out of reach of children”

  • Specific information on dilutions and usage conditions.

In cosmetics, ingredients appear under their INCI names, for example:

• Ledum Palustre Leaf Oil

• Ledum Palustre Extract

listed in descending order of concentration.

Troubleshooting

• Overly pungent or irritating odour in finished product

  • Possible cause: excessive essential oil concentration or oxidised oil.

  • Action: reduce dose, verify freshness and quality, consider adding suitable antioxidants.

• Skin irritation or redness after cosmetic use

  • Cause: individual sensitivity or too high a level in the formulation.

  • Action: reformulate at lower dosage, favour rinse-off products, or remove the ingredient entirely for sensitive-skin lines.

• Loss of insect-repellent effect

  • Cause: degradation of essential oil due to poor storage.

  • Action: replace with fresh batches, improve packaging (dark bottles, reduced oxygen headspace).

• Instability in emulsions

  • Cause: interactions between essential oil components and emulsifiers or preservatives.

  • Action: test different emulsion bases, adjust pH, optimise the preservative system.

Sustainability and supply chain

Rhododendron tomentosum is characteristic of peatlands and boreal wetlands, ecosystems that are sensitive and often threatened by drainage, climate change and human activities.

• Wild harvesting must be strictly regulated, especially in areas where the species is vulnerable or declining.

• To reduce environmental impact, preference should be given to:

  • Cultivated sources or certified sustainable wild collection;

  • Controlling extraction volumes to real market needs;

  • Ensuring full traceability along the supply chain.

• Distillation and extraction generate effluents that must be treated according to environmental regulations, with monitoring of parameters such as BOD/COD.

• Solid plant residues (spent herb) can be used in composting or other circular-economy pathways, reducing biomass waste.

Overall, valorisation of Ledum palustre must go hand in hand with protection of peatland ecosystems, which are crucial for biodiversity and carbon storage.

Main INCI functions (cosmetics)

In cosmetics, Ledum-derived ingredients are used mainly as aromatic and occasionally as supporting tonic or insect-repellent components:

• Ledum Palustre Leaf Oil

• Ledum Palustre Extract

The principal functions include:

• Perfuming / fragrance: contribution to the product’s olfactory profile with balsamic, resinous notes.

• Masking: helping to cover or modulate unpleasant odours from other raw materials.

• Skin conditioning (in carefully formulated products): perceived toning or refreshing effect, often in products with insect-repellent positioning.

Because of the essential oil’s potential toxicity, cosmetic use always requires thorough safety evaluation and compliance with recommended maximum concentrations.

Conclusion

Ledum palustre (Rhododendron tomentosum Harmaja) is a boreal shrub with a high content of essential oil, strongly aromatic and traditionally used in some cultures as a medicinal plant and natural insect repellent. Its distinctive volatile constituents (ledol, palustrol and other sesquiterpenes) confer antimicrobial, antioxidant, insect-repellent activities but also significant toxicity, particularly via the central nervous system, which limits internal use and demands caution even for external applications.

In modern practice, Rhododendron tomentosum is best regarded as a technical and cosmetic plant (for essential oil in repellents and fragrances) rather than a food ingredient. Proper valorisation requires a traceable, sustainable and toxicologically controlled supply chain, with targeted applications where the benefits (for example, repellency or distinctive fragrance) clearly outweigh the risks.

Studies

The essential oil of Ledum palustre has an abundant content of polyphenols with potential analgesics, anti-inflammatory, antimicrobial, antiviral, antifungal activities demonstrated in vivo and in vitro studies. In addition, recent scientific research has reported the promising antidiabetic, antioxidant and anticancer properties of this plant (1).

The topical combination of mountain arnica and Ledum palustre may be effective in reducing postoperative ecchymosis and oedema after oculofacial surgery (2).

The anti-inflammatory activity of two extracts of the aerial parts of the Ledum palustre has been established with a significant improvement of the edema (50-73%) for the essential oil obtained by hydrodistillation and (52-80%) for oil obtained by supercritical fluid extraction. These results were similar to those obtained with piroxicam (70%) and ketoprofen (55%) (3).

The essential oil of Ledum palustre, which gives a coniferous aroma to the herbal teas is a precious source of ascorbic acid that acts as a tonic, improving digestion and relaxing activity. However, this drink should not be drunk more than once a day due to the toxicity of ledol and grajanotoxin (4).

In the branches of the ledum is present the ursolic acid, a known triterpene (5) used by athletes, in cosmetics and for the cure of obesity.

Ledum palustre studies

Mini-glossary

• Ledol: an oxygenated sesquiterpene present in Rhododendron tomentosum essential oil; considered a key contributor to its neurotoxic potential.

• Palustrol: another major oxygenated sesquiterpene in Ledum oil; contributes to aromatic profile, antimicrobial and antifungal effects in vitro.

• Chemotype: a chemical variant within the same plant species, characterised by different dominant constituents in the essential oil (e.g. palustrol+ledol vs γ-terpineol+p-cymene chemotypes).

• SFA (saturated fatty acids): fatty acids without double bonds; high overall intake is associated with less favourable blood lipid profiles.

• MUFA (monounsaturated fatty acids): fatty acids with one double bond, generally beneficial when they replace SFA in the diet.

• PUFA (polyunsaturated fatty acids): fatty acids with two or more double bonds, including n-6 and n-3 families that influence inflammation and cardiovascular health.

• GMP/HACCP (good manufacturing practice / hazard analysis and critical control points): systems used to ensure quality, hygiene and safety in the production of foods, supplements, cosmetics and botanical raw materials.

• BOD/COD (biochemical oxygen demand / chemical oxygen demand): measures of the organic load of wastewater, used to assess the environmental impact of extraction and processing operations.

References_________________________________________

(1) Dampc A, Luczkiweicz M. Rhododendron tomentosum (Ledum palustre). A review of traditional use based on current research. Fitoterapia. 2013;85:130–143. doi: 10.1016/j.fitote.2013.01.013.

(2)   Kang JY, Tran KD, Seiff SR, Mack WP, Lee WW. Assessing the Effectiveness of Arnica montana and Rhododendron tomentosum (Ledum palustre) in the Reduction of  Ecchymosis and Edema After Oculofacial Surgery: Preliminary Results. Ophthal Plast Reconstr Surg. 2017 Jan/Feb;33(1):47-52. doi: 10.1097/IOP.0000000000000645.

(3)   Baananou S, Bagdonaite E, Marongiu B, Piras A, Porcedda S, Falconieri D, Boughattas NA. Supercritical CO₂ extract and essential oil of aerial part of Ledum palustre L.--Chemical composition and anti-inflammatory activity.  Nat Prod Res. 2015;29(11):999-1005. doi: 10.1080/14786419.2014.965167.

(4) Dampc A, Luczkiewicz M. Labrador tea--the aromatic beverage and spice: a review of origin, processing and safety J Sci Food Agric. 2015 Jun;95(8):1577-83. doi: 10.1002/jsfa.6889.

(5) Dufour D, Pichette A, Mshvildadze V, Bradette-Hébert ME, Lavoie S, Longtin A, Laprise C, Legault J. Antioxidant, anti-inflammatory and anticancer activities of methanolic extracts from Ledum groenlandicum Retzius. J Ethnopharmacol. 2007 Apr 20;111(1):22-8. Epub 2006 Oct 26.