I think Manuka (Leptospermum scoparium) leaf tea may have the most pleasing flavor the native New Zealand flora has to offer. Scientists have long known the plant contains the chemical 1 8 cineole (Ref 1), found in the leaves and bark. Recently scientists have found 1 8 cineole from Rosemary and other herbs boosts speed and accuracy at mental tasks (2). They also seem to report as a good thing that it is "negatively correlated with changes in contentment levels" I'm not sure what that means, perhaps it is some kind of mood stabilizer.
Many of the experiments involved inhalation of the chemical through essential oil of herbs but oral administration of essential oil and "extracts" was also effective (3, 4). I'm assuming just drinking the tea is a way of getting the benefits of 1 8 cineole but have not been able to find any experts committing on this. Essential oils can be extracted simply by pressing, crushing plants, I don't see how this would make any chemicals available not available through boiling, though they would be more concentrated. I expect essential oils are usually used in experiments because they can be bottled to prevent evaporation then stored for years and used when convenient. I find Manuka leaves loose most of their flavor in a matter of days once picked, presumably because the essential oils have broken down or escaped (presumably this would pose a serious problem for anyone trying to transport and sell the leaves for tea, significant seasonal fluctuations in quantities of 1 8 cineole in the leaves another issue(5)).
It is interesting that the BBC TV series 'Trust me I'm a Doctor' found in an experiment coffee makes you think you are performing better mentally but actually doesn't help, by contrast essential oil of Sage containing 1 8 cineole does improve mental performance (4*). So the humble, largely overlooked cup of Manuka tea may actually be a better workplace drink than coffee (I don't see anything wrong with coffee as recreational drug on the basis of their findings. At least coffee doesn't actually make you perform far worse while making you think you're doing wonderfully as alcohol and cannabis do, though glancing into certain "cool" and "classy" cafes I think this may need further research).
The similar, related native plant Kanuka (Kunzea ericoides) is also used as a tea (7) and also contains 1 8 cineole (1). I find the flavor facinatingly unique but overpowering, like essential oil straight.
1 8 cineole is also found in the leaves of a number of other plants used to make very pleasant teas such as Broad Leaved Paperbark (Melaleuca quinquenervia)(9)*, Myrtle (Myrtus communis) (10), once popular with French women, and Bay Laurel (Laurus nobilis) (11), perhaps this is why the Delphic Oracle of Ancient Greece used Bay as an aid in making prophecies.
Often confused with one another, I mainly tell Manuka and Kanuka apart by their leaf shape. Manuka (above) has broader more curved oval leaves, Kanuka (right) straighter, narrower leaves.
1. Brooker S G, Cain B F, Cambie R C. A New Zealand Phytochemical Register-Part 1. Transactions of the Royal Society of New Zealand Vol 1 No7 May 31 1963
*This BBC experiment tested Coffee against Sage in a one off single experiment, most people probably use coffee to sustain effort over a long period throughout the day, the effectiveness of this strategy was not tested in this experiment.
5. http://webcache.googleusercontent.com/search?q=cache:toKutzBZxIwJ:www.mdpi.com/1420- 3049/16/2/1181/pdf+&cd=8&hl=en&ct=clnk&gl=nz
6. Singh R J Editor. Genetic Resources, Chromosome Engineering, and Crop Improvement: Medicinal Plants, Volume 6. 2011
7. Crowe A. A Field Guide to the Native Edible Plants of New Zealand. 1981.
9. Cribb AB JW. Wild Food in Australia.
* The authors experimented with this tea and liked it, I don't know if it has become widely used (proven safe) as a result of their recommendations, it appears no incidents have resulted from their publication, I like it a lot & use it a few time a month.
11. Facciola S Cornucopia II. 1998.