In Keats's 'Ode on Melancholy', he writes
neither twist Wolf's-bane, tight-rooted, for its poisonous wine;
Why did Keats choose to describe Wolf's-bane as "tight-rooted"?
These lines are a poetic description of the way in which a 19th-century apothecary might extract a solution of the active compound (the alkaloid aconitine) from the roots of wolfsbane and other species in the genus Aconitum: that is, by compressing the roots tightly in a screw press by twisting the screw. So “tight-rooted” describes the extraction process rather than any distinctive feature of the roots of wolfsbane.
Whatever the exact herbal reference, the anguished image of twisting out its poisonous wine (which starts in the first line of the ode) points to the preparation of these pharmaceutical juices. (This is one of many such vinous references in his poetry and letters at this time.) Robert Thornton in his popular account of the medicinal plants gives a descriptive account, ‘Bruise the leaves of wolf’s bane and enclosing them in a hempen bag, compress them strongly till they yield their juice’. As we have seen, the press was a part of traditional equipment of the apothecary and Keats converts twisting vector of the screw into a more literary image; that of the expression of the poisonous wine by wringing hands.
Gareth Evans (2002). ‘Poison Wine—John Keats and the Botanic Pharmacy’. The Keats-Shelley Review 16:1, p. 52.
Keats had been apprenticed to apothecary Thomas Hammond from 1810 to 1814, so he must have been familiar with the early-19th-century pharmacopoeia and the methods used to prepare it.
In this interpretation, it is not the roots that are twisted, but the screw, the rhetorical figure being hypallage. Keats was fond of hypallage: other examples include “tale of pleasing woe” (= pleasing tale of woe) in ‘To Lord Byron’, “in some melodious plot” (= melodious in some plot) in ‘To a Nightingale’, “flock in woolly fold” (= woolly flock in fold) in ‘The Eve of St. Agnes’, “shady sadness of a vale” (= sadness of a shady vale) in ‘Hyperion’, etc.
Although the quote from Robert Thornton refers to compressing the leaves of wolfsbane, aconitine is most strongly expressed in the roots of the plant, so that someone who wished to extract the poison would want to start with the roots. Here’s a reference showing that this was well known in Keats’ time:
Aconitum, Aconite, Wolfsbane, or Monks-hood […] Of the bad qualities of these plants we sometimes avail ourselves to get rid of vermin. A decoction of the roots destroyed bugs; the same part being powdered, and administered in bread or some other palatable vehicle vehicle to rats and mice, corrodes and inflames their intestines, and soon proves mortal. The juice of the plant is used to poison flesh with, for the destruction of wolves, foxes, and other ravenous beasts.
Anon (1797). ‘Aconitum’. In Encyclopedia Britannica, 3rd edition, volume I, p. 77.
It's possible that Keats is using the adjective "tight-rooted" here to keep the tone of the first half of the poem consistent, in that tight-rooted has a connotation of being trapped and imprisoned, not free to roam the world. This is in keeping with the previous mention of the river Lethe, which was associated with lethargy in Greek mythology.
The root-structure of Wolfbane has a networks of thin roots that make their way through the soil, but also a fat, tuberous part, which is the bit that is still sold for 'herbal medicines' today.
enlarged structures used as storage organs for nutrients in some plants. They are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growing season,...
That is, they are the part of the root structure where all the water and nutrients are collected, the bit you want to be dealing with if you want to squeeze out 'poisonous wine'.
As to 'tight', this may be describing their condition in terms of how plump and juicy they are. The relevant definition of 'tight' may then be, per OED:
Drawn or stretched so as to be tense; not loose or slack: said of a rope, etc., or of a surface
and used in a transferred sense to apply to the whole. or in the sense of
Of such close texture or construction as to be impervious to a fluid
where the skin of the tuber does not allow for the escape of liquid from within. in the same way that a tight barrel contains liquid, a tight root would then contain sap.
Dried aconite root would need to be processed with liquid to turn it into a 'wine', but fresh, plump, 'tight' roots could just be squeezed.
So, my interpretation is that Keats is describing to the plump, hydrated character of the roots that one might squeeze 'poisonous wine' from.
Underlining that the roots are characterised by their juiciness, in The Year-book of Facts in Science and Art 1857 we can read the investigations of Mr Bentley, M.R.C.C, professor of Botany and Materia Medica to the Pharmaceutical Society about the character of the root, as he explores the likelihood of the alleged common confusion between it and horse-radish accounting for domestic poisonings. he writes:
The roots of monkshood and horse-radish may be also distinguished by the different appearances they present when scraped with a knife. Thus the former will then be observed to ne of a succulent character, and the scraped portions soon to acquire a pinkish or reddish hue, while the later scrapes firm and dry, and does not alter in colour.
The readiness with which juice is obtained from the root would tend to support my view that a 'tight root' is one that contains liquid in appreciable amount.
I note also the reference to the colour change Bentley observes in the scraped flesh. if that colour change also applied to the squeezed juice, and I'm extrapolating as he doesn't claim this, then it would also support the description of the juices as being an otherworldly wine.