History
Tansy has a long history of use. The first historical records of its cultivation for medicinal purposes involved the ancient Greeks. In the 8th century AD it was grown in the herb gardens of Charlemagne and by Benedictine monks of the Swiss monastery of Saint Gall. Tansy was used to treat intestinal worms, rheumatism, digestive problems, fevers, sores, and to bring out measles.
During the Middle Ages and later, high doses were used to induce abortions. On the other hand, tansy was also used to help women conceive and to prevent miscarriages. In the 15th century, Christians began serving tansy with Lenten meals to commemorate the Passover bitter herbs eaten by the Israelites. Tansy was thought to have the added Lenten benefits of controlling flatulence brought on by days of eating fish and pulses and of preventing the intestinal worms believed to be caused by eating fish during Lent.
Tansy was used as a face wash and was reported to lighten and purify the skin. In the 19th century, Irish folklore suggested that bathing in a solution of tansy and salts would cure joint pain.
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Source: https://www.wildlifetrusts.org/wildlife-explorer/wildflowers/tansy
Fun fact?
Tansy leaves were traditionally eaten at Easter to help kill off the worms that the diet of fish at the time caused. They were quite bitter, so were mixed with eggs, milk and flour in a kind of pancake or omelette.
About
Tansy is an aromatic plant of disturbed ground, rough grasslands, riverbanks and roadside verges. It has fern-like foliage and yellow flower heads that appear in clusters from July to October. It is a composite flower, so the flower heads are made up of lots of tiny flowers called ‘disc florets’, but no outer ‘ray florets’. It has a long history of use for medicinal purposes and was cultivated by the Ancient Greeks. Now, many forms are available as decorative garden varieties, attracting a wide range of insects.
How to identify
The leaves of Tansy alternate up the length of its reddish stem. Each leaf is divided into many leaflets, which are finely toothed, giving them a fern-like appearance. The yellow flower heads (consisting of lots of tiny flowers) look like small, rounded buttons and cluster together.
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Why You Should (and Shouldn’t) Grow Tansy
Source: https://www.growveg.co.uk/guides/why-you-should-and-shouldnt-grow-tansy/
Ann Marie Hendry 
Just when you think you can’t fall hopelessly in love with another flower, along struts tansy, and the fireworks go off again. Its ferny foliage captivates, its scent allures, and its quirky button-shaped flowers enchant. But only fools rush in, especially when they’ve been burned by unsuitable floral beaus before – I’m thinking of that aggressive mint, the over-ardent aquilegia, and the rambunctious periwinkle that I’ve dallied with in the past.
So, with plenty of painful experience to rein me in, I’m approaching a romance with tansy with utmost caution. There are few things more heartbreaking than ripping the roots of an adored plant from the soil (and having to do it over and over again for weeks, months, years…possibly forever) because it loves your garden a little too much.
Reasons Not to Grow Tansy in Your Garden
When you’re first getting to know a handsome stranger it’s tempting to only notice his good points, but if you’re planning on entering into a long-term relationship it’s worth being realistic. Everyone has their dark side!
And tansy’s dark side is darker than most – it’s literally a killer. Despite historically being commonly used as a flavouring, bitter-tasting tansy contains a toxic essential oil that can cause liver and brain damage and even kill humans and other animals. On a less lethal level, it can also prompt an allergic reaction in some individuals when touching the leaves.
If that wasn’t enough to set the alarm bells ringing, tansy is also the type to make itself at home in your garden – a little too well! It both reseeds readily and spreads by underground rhizomes, so a flirtation with a few tansy plants could soon turn into a more permanent arrangement than you originally envisaged. In some parts of North America tansy is so invasive it’s actually listed as a noxious weed and is not permitted.
And Now Some Great Reasons to Grow Tansy
OK, so tansy isn’t perfect, but who is? A few flaws could almost be considered endearing…couldn’t they? All right, I admit – poisoning people to death is a pretty major flaw. So it’s got to have more to back it up than just a pretty golden face.
Fortunately, it does. That garden-grabbing nature means it’s happy in most soils and it doesn’t need feeding, so even those of us who are less than nurturing in our plant relationships can rest assured that we won’t ever neglect our beloved. Tansy can tolerate a little shade, making it useful for filling space in those less-loved parts of the garden, and once established it can cope with drought too. It will even help improve the soil because it accumulates potassium.
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