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Plants in focus

Manuka (Leptospermum scoparium)

 


manuka image, Leptospermum scoparium

Multi-useful manuka

Much of the regenerating bush where I live is manuka (Leptospermum scoparium): a vital member of this community. Probably New Zealand's most common native tree species, it occurs in many diverse sites and soil types. Perhaps it is a rather dull green, and not that attractive unless in flower; however, I'm a fan of this plant, and think it deserves much greater appreciation. This tree yields therapeutic oil, tremendously valuable honey, terrific hard red wood, has ornamental value and can grow almost anywhere.

Manuka grows to about 4 m tall, though some specimens can be larger. It has small, somewhat prickly, oil-filled leathery leaves that emit a pleasant myrtle-like aroma when crushed. In the wild, its pretty five-petalled flowers are usually white, though can be tinged pink. En mass, their blossom coats hillsides of regenerating bush, and is a magnet for bees for miles around. The wonderful photo of manuka above, taken by Peter Wilson in his garden in Cornwall, shows the multitudes of deep pink flowers on an ornamental variety.

  • It will grow from the coast to ~1600 m in New Zealand.
  • It will grow in dense clay and acidic soils.
  • It grows in low-nutrient soils.
  • Tolerates drought once established.
  • Tolerates some water-logging.
  • Is wind hardy and withstands coastal spray.
  • Survives frosts.
  • Can tolerate soils contaminated with heavy metals, ultramafic soils.
  • It can even grow right next to sulphur-rich thermal pools.

Although not a popular garden plant in its native New Zealand, it is much more popular in the UK and California, where varieties with deep pink to reddish-purple flowers are sought after, e.g. Burgundy Queen, Red Damask, Red Falls, Crimson Glory and the Wiri series of ornamental manukas.

Its plentiful seeds can remain dormant for several years, and then rapidly germinate with a little sunlight, moisture and warmth. It can grow in the most inhospitable spots. However, it is a fairly short-lived tree: in New Zealand, within a few years, it makes way for larger native tree species.

Manuka is fairly pest and disease free though, in New Zealand, it is often covered by a black sooty mould. This fungus is a friend rather than foe as it lives on the honey dew and parasitises a scale insect that attacks this tree, sometimes killing it. If the scale insect (Eriococcus orariensis) isn't present, then no fungus develops. Manuka can also suffer from leaf-roller caterpillar and borer.

Within New Zealand, manuka provides a terrific habitat for native trees to germinate and grow. It will grow on the steepest slopes and unpromising cleared land. It decreases erosion, makes a good wind break, and is utilised as shelter for stock. A bonus is that its roots form plentiful fungal mycorrhizal associations, thus making more nutrients available and helping to improve soil structure and biodiversity.

Manuka leaves were famously brewed by Captain Cook, with rimu (Dacrydium cupressinum), to make an ale to staved off scurvy. The leaves have also been brewed to make a tea, hence its common English name of tea tree. (Though this species, although similar, should not be confused with Australian tea tree, Melaleuca alternifolia).

Maori have utilised the leaves to reduce fever and for urinary problems, and, steeped in hot water, their vapour is inhaled to reduce the symptoms of colds and flu. The crushed bark was also used as a sedative and to reduce diarrhoea, and a salve made from the leaves and bark was applied to aching joints and muscles.

Although Australian tea tree (M. alternifolia) is more famous for its medicinal properties, manuka oil also has excellent antiseptic, antifungal, antiviral and antibacterial activities, including against Staphylococcus aureus. It is also used to treat fungal infections such as athletes foot and difficult-to eradicate nail infections, and also for acne. Its antiviral properties have been recently demonstrated against Herpes simplex. It has insecticidal properties too and has been used to eliminate internal parasites. A drop or two in shampoo can reduce dandruff, and a diluted preparation can reduce skin infections, as well as being used to relieve the pain of arthritis. Manuka oil also has more antioxidant activity than Australian tea tree.

The best manuka oil is richest in the compound leptospermone. However, like all essential oils, dilute before application, and probably best not to ingest it.

And then there's wonderful manuka honey. Appreciated for its rich flavour, it is a highly prized global product. All honey contains hydrogen peroxide, which has antibacterial properties but, in addition, manuka honey contains UMF (Unique Manuka Factor), which is graded according to strength from 5+ to 20+. This wonderful compound, discovered by Professor Molan of Massey University, New Zealand, has powerful healing properties. This has been further found to contain methylglyoxal, which is why honey is sometimes labelled MGO manuka honey.

Manuka honey's properties are so significant that it is now impregnated into bandages and applied directly to difficult-to-heal wounds such as abscesses. It has been repeatedly shown to significantly improve wound healing, often when traditional pharmaceuticals have failed. It has also been used on burns with remarkable results.

Manuka honey has even been shown to combat antibiotic-resistant Methicillin-resistant S. aureus (MRSA). Indeed, Professor Molan says "We haven't found anything it doesn't work on among infectious organisms". It speeds up the rate of healing and seems to "work on any wound at any stage".

So, just this single insignificant plant has so many uses benefits and properties. And yet these have only been discovered because of this plant's abundance on New Zealand's deforested land. What then resides within all the other millions of unexplored plant species around the world?

For more research information on the wondrous benefits of manuka honey visit: the Waikato Research Unit