SALICACEAE

The Salicaceae or willow family consists of 4 genera and about 500 species of dioecious and deciduous trees and shrubs distributed almost worldwide.

Salix (willow) and Populus (poplar) are the main medicinal genera.

Populus spp.

There are a number of Populus species that have been traditionally used in medicine. They appear to have been used for their barks and/or their buds, but roots and leaves have also been used. Most of the current scientific information available is based on a much smaller number of species and is represented, in use, by P. tremuloides and P. alba bark, and P. x gileadensis buds. A degree of overlap must exist and the various species may be interchangeable. There is however an enormous scope for the research and clinical assessment of the whole of this genus.

Other species of Populus not described here, which are traditionally used in medicine include:

Populus x canadensis Canadian poplar
P. tremula aspen
P. yunnanensis Yunnan poplar
P. tremuloides quaking aspen
P. deltoides necklace poplar
P. nigra Lombardy polar
P. trichocarpa Western balsam poplar

Populus alba

White poplar

Description

A tall, spreading deciduous tree to 25 m high which suckers profusely. Bark smooth or shallowly fissured and grey. Shoots white-tomentose, buds white but not sticky or aromatic. Leaves, petiole 1.5-5 cm long, white tomentose; lamina on vegetative shoots 3-10 x 2-9.5 cm, more or less deltoid with 3-5 lobulate or toothed primary lobes; on adult shoots lamina is smaller, ovate to oblong, lobed or strongly toothed, always white and loosely tomentose beneath and glabrous green and shining on the upper surface; base truncate, rounded or sub-cordate, apex obtuse or rounded. Catkins, female, more or less pendant, 2-8 cm long, bracts membranous, shining and brown in upper part; cup shaped disc. Capsules containing dense, white cottony hairs which give the appearance of snow under the trees. Flowers in spring before leaves appear.

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Family Salicaceae

Habitat and cultivation

Native to Britain, South Central Europe and North Africa and naturalised elsewhere. Easily grown from cuttings, suckers or poles. Planted along roadsides, used as shelter belts and along streams. Frost resistant, drought tender.

Parts used

Dried bark, preferably collected in spring as the sap begins to rise.

Active constituents

1) Phenolic glycosides including salicin, populin and benzoyl salicoside (populoside)1–3

2) Tannins

Catechol and a small amount of α-tococopherol have been isolated from P. tremuloides45

Actions

1) Antirheumatic

2) Antiseptic

3) Astringent

4) Cholagogue

5) Diuretic

6) Anodyne

7) Anti-inflammatory

Scientific information

The bark of all the Populus species are considered to be chemically similar and all contain salicortin, the precursor of salicin, as a main constituent.3 The phenolic glycosides listed are derivatives of salicylic acid, a synthetic version of which is sold worldwide as aspirin (See Salix alba).

Pharmacological information on whole extracts of the above species is lacking and chemical investigations were carried out many years ago. There is very little current information, other than tests done on P. tremula, the European aspen.

In vitro—P. tremula is antioxidant6 and antibacterial against Streptococcus pneumoniae and Haemophilus influenzae.7

A commercial preparation containing Solidago virgaurea, Fraxinus excelsior (common ash) and P. tremula exhibits strong anti-oxidant and anti-inflammatory activity.8–11

In vivo—The 3 herb combination above was shown in clinical trials to be an effective anti-inflammatory and analgesic for a variety of arthritides, the benefit apparently comparable to that of NSAIDs.12,13

In Canada traditional healers use the bark of P. tremuloides to treat digestive and gynaecological disorders.14

The properties of these species show many features in common with salicylate-containing willow bark (See Salix alba).

Medicinal uses

Respiratory tract

Gastro-intestinal tract

Urinary tract

Musculoskeletal

The combined actions improve waste excretion and treat pain and inflammation.

Pharmacy

Three times daily  
Decoction of dried herb –   1–4 g
Fluid Extract (25%) –   1-4 ml

Potter's upper limit is 5 g and 5 ml.

Precautions and/or safety

The commercial preparation containing P. tremula was safe with few side-effects.12,13,15 There have been recorded cases of contact allergy to the salicyl alcohol and salicylaldehyde in P. tremula bark.16,17

Historical uses

See the end of P x gileadensis.

Populus species used traditionally for their leaf buds include P. x canadensis, P. x gileadensis, P. nigra, P. trichocarpa and P. yunnanensis. The balsam poplar P. tacamahaca, which appears to be identical to P. balsamifera18 is also used.

Populus x gileadensis

Balm of Gilead poplar

Description

Deciduous tree to about 60 m high, with a spreading habit and extensive suckers. Bark grey, fissured. Buds very viscid and glabrous; young leaves balsam scented. Petioles 3.5-7 cm long and glandular viscid. Leaves 6-23 x 5.5-20 cm, broad ovate, hairy on veins above and beneath, green above, greenish white beneath. Margins ciliate, crenate-serrate with gland-tipped teeth, without translucent border. Leaf base cordate or sub-cordate, sometimes rounded with 2 glands; apex cuspidate or acuminate. Flowers catkins female, pendulous 13-19 cm long at anthesis. Bracts 4-5 mm long excluding long filiform lobes, greenish white. Cup-shaped disc 1.5-3 mm deep, long, broad, ovoid, containing abundant long, white, silky hairs.

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Family Salicaceae

Habitat and cultivation

The origin of this species is unknown but it is thought to be a hybrid of the North American P. deltoides and P. tacamahaca and is only known as a female clone. Propagated by cuttings or suckers, forming large clumps. Planted for shelter in many countries. Drought and frost resistant.

Parts used

The leaf buds harvested in winter.

Active constituents

1) Flavonoids the nature and content of which varies for the particular species, but includes chrysin. They are major constituents in P. nigra and P. candicans but minor in P. tacamahaca which has mainly dihydrochalcones19,20 P. nigra

2) Phenolic esters of caffeic and isoferulic acids21,22

3) Volatile oil (up to 0.5%) including mainly sesquiterpenes also monoterpenes and hemiterpenes23 P. tacamahaca

4) Aromatic acids and esters of cinnamic and coumaric acids but little or no caffeic and isoferulic acids or esters22

5) Volatile oil—terpenoids (11-13%) mainly bisabolol and humulene18

Also oleoresin and in P. balsamifera prostaglandins E, F, A and B and arachidonic acid the content of which varies with the time of year.24 (Arachidonic acid is believed to be a precursor for plant-based prostaglandins.) Phenolic glycosides populin, salicin and benzoyl salicoside as found in the bark, have also been reported.20

P. x candicans is less well characterised than the other two but is believed to be chemically similar to P. tacamahaca. It appears likely that misidentification of P. x gileadensis occurs and buds of the above species may be used interchangeably.25

Actions

1) Expectorant

2) Antiseptic

3) Anti-inflammatory

4) Counter-irritant

Scientific information

There has been a great deal more investigation of the chemistry of the bud exudates than the bark, however once again this information is old and no longer easily available. No pharmacological information is available. German Commission E has approved the use of buds for external problems like superficial skin injuries, haemorrhoids, frostbite and sunburn.

Propolis derived from bees visiting poplar trees has many constituents in common with the bud exudate.26

According to the BHC the lipophilic fraction of P. nigra buds has strong antibacterial, antifungal, anti-inflammatory and “capillary” activities. The caffeic acid derivatives and volatile oil fractions are antimicrobial and the latter is also expectorant.

Medicinal uses

Respiratory tract

Mainly used internally in this area to treat both acute and chronic problems:

Externally

In ointment form for:

Also an infusion can be gargled to treat laryngitis.

Pharmacy

Three times daily  
Infusion of dried herb –   4 g
Tincture 1:5 (45%) –   4–8 ml
Fluid Extract (90%) –   4–8 ml

Topical use in ointment 20-30%

Precautions and/or safety

Rarely allergy, systemic or contact—see above.

Historical uses

As a cleansing agent; for sciatica and strangury. Headaches. Juice of leaves applied locally for ear ache and the leaves used for epilepsy. The buds for “dull sight”; as a beauty aid and for external inflammations; water collected from the “hollow places of the tree for warts; pushes; wheals and breakings out of the body”. The herb was used by Culpeper to dry up milk flow after weaning. Also used for gonorrhoea and gleet and as a substitute for Peruvian bark (Cinchona—quinine) in intermittent fevers, for coughs and ’flu. Bark used externally as styptic; with Salix spp. as ash for arthritic joints.

Salix spp.

There are 300-500 species of willow world-wide, including those below, containing salicin and used medicinally.

S. alba willow, white willow
S. daphnoides violet willow
S. fragilis crack willow
S. pentandra bay willow
S. purpurea purple osier

Salix alba

Willow, white willow

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Family Salicaceae

Description

A deciduous tree up to 26 m tall with deeply fissured grey bark and ascending branches. Leaves alternate, shortly petiolate to 11 cm long, lanceolate with silky whitish hairs on both sides. Flowers appear at the same time as the leaves, growing in dense cylindrical catkins. Male catkins up to 5 cm long with 2 stamens and yellow anthers; female catkins up to 4 cm long. Blooms in spring.

Odour—slight; taste—astringent and bitter.

Habitat and cultivation

Native to Britain, Europe, Asia and North America growing in moist grassland, marshes and along streams. Widely naturalised elsewhere. Easily grown from cuttings in spring. Frost resistant, drought tender.

Parts used

The bark collected from young branches during the growing period. Salicin is highest in spring and summer, lowest in winter. This genus hybridizes easily.

Active constituents

1) Phenolic glycosides27 (max. 11%) including salicylates as salicortin (an ester of salicin) and salicin. In S. alba the main form is salicortin up to a maximum of 1% also a low level of free salicin, 2'-acetylsalicortin and triandrin. S. purpurea (up to 8.5%) including salireposide and tremulacin. S. daphnoides (up to 8.4%) including triandrin, salicylic acid, picein and tremulacin.28 S. fragilis (up to 10.2%)

2) Flavonoids (max. 4%) including prunin (naringenin 7-glucoside) and salipurposide in S. purpurea and S. daphnoides27

3) Tannins (8-20%) mainly of the condensed type

Also a lignan—sisymbrifolin (S. alba),29 flavan-3-ols and oligomeric procyanidins (S. purpurea),30 anthocyanins (variety of species)31 and polysaccharides including arabinan (S. alba).32

Actions

1) Anti-inflammatory

2) Analgesic

3) Antipyretic

4) Astringent

5) Antirheumatic

Scientific information

The analgesic and antipyretic effects of Salix were known to the ancient Egyptians and Greeks and were commonly used through the ages from the early physicians like Hippocrates and Dioscorides to the present day. Modern re-evaluation of the herb is attributed to Edward Stone who in 1758 reported to his peers his successful use of the bark to treat the pain and fever of malaria.33,34 “Salicine”, the name given to the active constituent, also found in wintergreen oil and meadowsweet, was isolated in 1828. When limited supplies of bark failed to satisfy medical need a totally synthetic version was produced which, with refinement to acetylsalicylic acid or “aspirin”, was produced in 1853. Acetylsalicylic acid did not however enter into common medical use until the end of the 19th century. The mechanism of its anti-inflammatory action through inhibition of prostaglandin synthesis was not elucidated until 1971 and its use as an anti-coagulant was not established until 1988.35 This form of salicylate replaced Salix use in conventional medicine.

Willow bark is still widely used in Germany, along with Harpagophytum in the treatment of joint inflammation.36 It has been approved by German Commission E as an antirheumatic agent and to treat fever and headaches. Although analgesic, anti-inflammatory, antipyretic and lack of ulcerogenic effects have been demonstrated in animal models these effects in humans are largely deduced from empirical use.37

Anti-inflammatory

It has been assumed that the action of Salix is primarily due to its salicylic acid content. However although salicin is converted to salicylic acid in vivo its effective level is probably too low to be responsible for the type of anti-inflammatory effects observed with acetylsalicylic acid. It has been proposed that other constituents like the flavonoids or tannins must contribute or that the herb's mechanism of action is quite different to that of acetylsalicylic acid.28–40 The mode of action of Salix is in fact considered to be much wider than that of standard NSAIDs41 and it has been shown that it is much more effective as an anti-inflammatory than either salicin or salicylic acid at equivalent doses.37

In vitro—Apart from the strong anti-oxidant activity of Salix (related to the high level of phenolics),42 and the effect this would have on inflammation, other mechanisms that may account for its anti-inflammatory action include:-

However the complete mode of Salix's activity is still not understood.

In vivo—Commercial willow bark extracts have been tested in clinical trials albeit of short duration. In the treatment of low back pain these extracts have been shown to:-

For the treatment of osteoarthritis two trials have been reported. In one the extract gave moderate improvement, measured both objectively and subjectively, in patients with osteoarthritis of hip and knee.49 In contrast another trial found no significant reduction in pain levels when standardised extracts were used to treat either osteoarthritis or rheumatoid arthritis for a period of 6 weeks.50 Again only short term studies have been conducted.

Reviews to-date conclude that the evidence of an anti-inflammatory effect in the treatment of osteoarthritis and/or low back pain is not consistent and this may relate to variations in doses used.36 However there is moderate evidence in support of short-term improvement in pain level by these standardised preparations.37,51

Other

In vitro—The bark and all its main fractions inhibit the growth of lung and colon cancer cells.52S. alba protects fibroblasts from UVA light53 and inhibits binding to specific 5-hydroxytryptamine receptors, a potential aid in the treatment of migraines.54

In vivo—A combination of Tanacetum parthenium and Salix in tablet form effectively reduced the severity, duration and frequency of migraine attacks.54

Salix extracts standardised on salicin (240 mg) inhibited platelet aggregation compared to placebo but this effect was about one fifth that of acetylsalicylic acid given at a dose of 100 mg.55 (That would make Salix one-tenth as strong as aspirin in this effect.)

Medicinal uses

Respiratory tract

Nervous system

Musculoskeletal

The analgesic and anti-inflammatory activities come together in the treatment of:

Pharmacy

Three times daily  
Decoction of dried herb –   1–2 g
Tincture 1:5 (25%) –   5–8 ml
Fluid Extract (25%) –   1–2 ml

Commercial preparations are likely to contain a mixture of species28 and are standardised on salicin content. The daily doses used in trials were based on either 120 mg or 240 mg of salicin where 120 mg of salicin is equivalent to 393 mg of dried willow bark.

Pharmacokinetics

Studies have identified salicylic acid (86%) and its metabolites, gentisic (4%) and salicyluric acid (10%), in plasma after ingestion of willow bark tablets.56 Salicin is metabolised before and/or during absorption. Salicylic acid is the first metabolite detected in plasma; it reaches a peak in less than 2 hours and appears to be absorbed through the mucosa of the stomach or upper digestive tract.57 Its half life is around 2.5 hours, excretion is mainly as salicyluric acid in urine with 95% of urinary excretion being completed by 24 hours.57 Peak serum levels of salicylic acid are very much lower than for synthetic salicylates (1.4 mg/L after 240 mg salicin compared to 35-50 mg/L after 500 mg of acetylsalicylic acid).49,57

Precautions and/or safety

In general clinical trials using standardised extracts found side effects were minimal being equivalent to, or lower than, that for placebo. The most frequently reported reactions were allergic reactions—pruritus and exanthema—and gastrointestinal reactions, all of which were easily resolved.47Salix was considered safe and well tolerated.48,49

Serious side effects recorded are an anaphylactic response to a commercial preparation, which contained willow bark as one of its ingredients, in an aspirin-sensitive person58 and one case of a severe allergic reaction to standardised Salix amongst those patients taking part in the above clinical trials.48

Interactions

There is the potential for Salix to increase the effect of warfarin59 although this interaction has not been reported to-date.

Historical uses

To staunch bleeding; to prevent vomiting. “It helpeth also to stay thin, hot, sharp distillations from the head upon the lungs, causing a consumption.” The leaves to treat windy colic; both leaves and seeds used for “subduing lust”. Urinary retention; spots and discolouration of the skin. Tonic for digestive tract and post acute illness; worms; chronic diarrhoea; dysentery. “Redness and dimness of sight; films that grow over the eyes.” Leaves and/or bark for dandruff and scurf (topically).