Artemisia annua Seeds | Sweet Wormwood
LATIN NAME: Artemisia annua
COMMON NAME: Sweet Wormwood, Qinghao
Plant Profile
Two species have been confused under the same herb name for most of their history, and the distinction still matters when a specimen is pressed. The plant grown for artemisinin is a soft annual with finely divided, feathery leaves and small nodding heads of yellow-green florets in loose open sprays. Its former rival has broader leaf segments and a different head shape, and is now classified as Artemisia carvifolia.
Science & Biotechnology
Chinese pharmacy had used the herb for intermittent fevers for centuries, and the written clue that mattered appears in a handbook of prescriptions for emergencies compiled by Ge Hong, which instructs that the plant should be squeezed and taken cold rather than boiled. When a Chinese research programme was set up in 1967 to find new antimalarials, that instruction led directly to a low-temperature extraction and then to the isolation of the active principle. Tu Youyou received the Nobel Prize in Physiology or Medicine in 2015 for the work.
Uses & Benefits
The leaves and the florets bear biseriate glandular trichomes, small two-tiered hairs with a cuticular sac on top, and it is inside those sacs that artemisinin accumulates. Artemisinin is a sesquiterpene lactone carrying an endoperoxide bridge, the peroxide grouping that makes the molecule behave in the body as it does. Levels of the compound build through the vegetative season and are highest in the leaves and young flower buds, falling away as the plant sets seed, which is why the timing of a harvest matters more here than in almost any other herb.
Artemisinin is difficult to administer on its own, so the pharmaceutical industry converts it into derivatives such as artesunate and artemether, which are then paired with a slower-acting partner drug. Those pairings, the artemisinin-based combination therapies, are the standard treatment for uncomplicated falciparum malaria around the world. Added to the supply chain in recent years is semi-synthetic artemisinin, produced by fermenting engineered yeast that has been given the plant’s own biosynthetic pathway and fed artemisinic acid.
The essential oil of the plant is dominated by artemisia ketone, with camphor and 1,8-cineole, and it is that volatile fraction, not artemisinin, that perfumes a room when dried stems are hung up. Artemisinin itself is almost odourless and intensely bitter. Growers sometimes assume that a strongly scented plant must be a potent one; the two chemistries run on separate tracks, and only laboratory analysis can tell you what a particular stand contains.
As a garden plant sweet wormwood is tall, fast and generous, and florists buy it by the bundle for its chartreuse haze. Stems cut when the flowers are just opening dry well and hold their colour in wreaths and bunches. The tops also give a soft yellow-green to wool and silk when simmered with a mordant, and the dried leaf has been tucked into linen chests as a bitter-scented moth deterrent.
The dried leaf makes a bitter tisane, and in parts of Asia it is drunk as a cooling summer infusion rather than as medicine. Anyone growing it should be clear that the drug content of a garden stand varies enormously with strain, soil and stage of growth, so a home preparation cannot be treated as a measured one. It is a good plant to grow, smell and look at, and a poor one to dose from.
Growing from Seed
The seed is tiny and dark and needs light, so scatter it on the surface of a fine, firm tilth or a seed pan of loam and leaf mould in early spring, press it in with the flat of the hand, and water with a fine rose. Once the seedlings are a few centimetres high, thin them to about half a metre apart if you want a full, bushy plant rather than a drawn-up single stem.
In rich ground a plant will grow two metres or more, and a crowded stand will go over in a summer storm. Thin hard, then push a single cane alongside each stem and tie it in two places. Water deeply at long intervals rather than daily, and avoid feeding with nitrogen once the plants begin to extend, because soft growth is what makes them flop. After the first month they tolerate dry soil well, and a little stress seems to concentrate the aroma rather than spoil it. Leave a few stems to ripen seed and the species will come back in the same spot without further help.
Our Take
Sweet Annie is the easiest tall herb I grow and the one I am most careful about, since a stand allowed to seed will follow you around the garden. Grow it beside other bitter aromatics for a hot dry border: Artemisia absinthium and Artemisia vulgaris both take the same treatment, and if you want another medicinal herb with a serious laboratory literature behind it, Salvia miltiorrhiza is worth the space.
Related Reading
Gardeners who grow Sweet Wormwood often pair it with these related subjects: