Common name: Manuka

Manuka is an evergreen shrub or small tree native to New Zealand and neighbouring Australia and whose flowers are the source of the world-renowned honey of the same name. Although widely distributed in New Zealand, its natural range in Australia is limited to the cool-climate southeast corner of the continent, including the island of Tasmania.

It may only grow to be a shrub in alpine areas, though elsewhere, it is typically a small tree 2 to 4 meters (7 to 13 ft ) tall with a wide-spreading, rounded crown. Exceptional specimens may be up to 7 meters (23 ft) tall. The trunk is single-stemmed, occasionally forked, up to 15 cm (6 in) in diameter, and has grey-brown bark that on mature trees is fissured and shredding.

Leaves are up to 1.5 cm (0.6 in) long, oval and pointed or elongated-oval and sharply pointed, dull green above, paler underneath, stalkless and alternately arranged on long, thin, wiry branches. When crushed, they give off a sharp, gingery aroma.

Flowers are five-petaled, white, occasionally pink or red, near stalkless and arise singularly at the leaf axis. They bloom in great profusion from spring through summer and then, if fertilised, develop into woody, button-shaped seed capsules with valves that open when the capsule matures, releasing the seed.

Use

Manuka honey is world-renowned for its antibacterial (germ-killing) properties and has long been used medicinally in wound care and healing. Although most honeys contain Hydrogen peroxide, a known antibiotic, Manuka honey also uniquely contains Methylglyoxal (MGO), a potent antibacterial. And the higher the concentration of MGO in the honey, the more potent it is.

Besides wound healing, it is effective against Golden staph (Staphylococcus aureus), sore throat, gingivitis, stomach upset (indigestion) and skin conditions such as eczema and dermatitis. And according to a recent study, manuka honey greatly outperformed xylitol chewing gum in preventing the growth of dental plaque, a biofilm that forms on the surface of teeth.

Manuka honey is mainly packaged in jars and marketed as a single-floral honey. It is also used as an ingredient in throat drops (lozenges), capsules, gels, creams, breast pads, soaps and cosmetics.

Manuka honey is thixotropic, which means it is gel-like when still but becomes fluid when stirred or shaken. At one time, this was considered a drawback because it makes extracting the honey from the honeycomb difficult, but Manuka is now highly sought after for its bioactivity.

It is a medium to dark amber, orange-tinted honey with a distinct, slightly astringent, somewhat objectionable flavour, reminding of burnt sugar with a menthol or medicinal aftertaste. It is probably best enjoyed as an in-comb honey.

The flowering stems are cut for floral arrangements and harvested when around half of the flowers have fully opened. The buds tend not to open in the vase, so stems with a high bud-to-flower ratio should be avoided, as should the stems with the flowers falling off when shaken. If they are to be shipped, the stems should be kept cool at between 1 and 4°C (34 and 39°F). 

On vasing, leaves that would be submerged are stripped off and the stems recut by 2 cm (0.8 in), then stood in water with a preservative added. With proper care and handling, Leptospermum species have a vase life of about 10 to 12 days.

Climate

Grows naturally in humid subtropical and warm temperate climates, generally areas with annual lows of 6 to 13 °C, annual highs of 14 to 25 °C, annual rainfall of 600 to 2500 mm and a dry season of 5 months or less.

Manuka is introduced and cultivated successfully as a flowering ornamental in California and upland parts of Hawaii and India.

Growing

New plants can be grown from seed, though starting plants from cuttings is more common and gives predictable results.

Performs well on a range of clay, loam and sand soils, from free- to slow-draining or seasonally flooded and with an acid to neutral nature, generally with a pH of 4.5 to 7.0. Flowering is at its best and fullest on sites with full sun exposure. 

Problem features

Manuka is a persistent weed that is difficult to eradicate once established. The Hawaii Pacific Weed Risk Assessment project (HPWRA) assessed it as a high weed risk species for Hawaii.

Books

• Crane, E., Walker, P. & Day, R. 1984, Directory of important world honey sources, International Bee Research Association, London

• Editors of Sunset Magazine 2012, The New Western Garden Book: The Ultimate Gardening Guide, 9th edition, Sunset Publishing Corporation, California

• F. R. Beuhne 1922, Honey flora of Victoria, Melbourne Albert J. Mullett, Govt. printer

• Jones, R. 2001, Caring for cut flowers, 2nd ed, Landlinks Press, Victoria, Australia

• Leech, M. 2013, Bee Friendly: A planting guide for European honeybees and Australian native pollinators, Rural Industries Research and Development Corporation (RIRDC), Canberra, Australian Capital Territory

• Little, E. L. & Skolmen, R. G. 1989, Common forest trees of Hawaii (native and introduced), Agricultuural Handbook No. 679, Forest Service, U.S. Department of Agriculture, Washington, D.C.

• Macmillan, H. F. 1943, Tropical planting and gardening : with special reference to Ceylon, 5th ed, Macmillan Publishing, London

• Perry, F. & Hay, R. 1982, A field guide to tropical and subtropical plants, Van Nostrand Reinhold Company, New York

• Randall, R. P. 2002, A global compendium of weeds, R.G. and F.J. Richardson Press, Melbourne

• Randall, R. P. 2007, The introduced flora of Australia and its weed status, Cooperative Research Centre for Australian Weed Management, Glen Osmond, South Australia