The Walnut tree has been cultivated in the UK for around 500 years not only for its nuts but for the high quality wood used in furniture making. It is indigenous to Southern Europe but has spread outside of cultivated areas in the UK due to the action of birds and mammals such as squirrels. It normally self sows in warm areas and hedgerows.

The very distinctive nut is best harvested in Late October, early November as by this time the fruit is fully ripe and dry inside as younger fruit can be wet and tasteless inside the shell. Very young ‘green’ fruit can be picked in July and are often used in pickling. This fruit is pickled in brine for several days, then dried and sealed in jars with pickling vinegar and left for another 3-4 weeks. During the initial pickling the green walnuts turn black.  Walnuts are a good source of calories, protein and fibre. As Culpepper mentions below Walnut has supposedly healing properties against poisons.

The tree is deciduous and can grow up to 30m or 100ft tall often seen in planted parks and has a grey fissured bark, male and female trees have catkins with these forming on new growth for the male tree and in clusters on old growth for the female tree.

What Culpepper says
Walnut (Juglans Regia)
Description. The Walnut-tree rises to a great height, and spreads irregularly into branches. The leaves are pinnated: the pinnæ vast, oblong, and of a fine green, and the fruit is covered with a green rind.
Place. It grows wild in many places in Scotland; and is planted every where for the fruit.
Time. It blossoms early before the leaves come forth, and the fruit is ripe in September.
Government and virtues. This is also a plant of the Sun. Let the fruit of it be gathered accordingly, which you shall find to be of most virtues while they are green, before they have shells. The bark of the Tree doth bind and dry very much, and the leaves are much of the same temperature: but the leaves when they are older, are heating and drying in the second degree, and harder of digestion than when they are fresh, which, by reason of their sweetness, are more pleasing, and better digesting in the stomach; and taken with sweet wine, they move the belly downwards, but being old, they grieve the stomach; and in hot bodies cause the choler to abound and the head-ache, and are an enemy to those that have the cough; but are less hurtful to those that have a colder stomach, and are said to kill the broad worms in the belly or stomach. If they be taken with onions, salt, and honey, they help the biting of a mad dog, or the venom or infectious poison of any beast, &c. Caias Pompeius found in the treasury of Mithridates, king of Pontus, when he was overthrown, a scroll of his own hand writing, containing a medicine against any poison or infection; which is this: Take two dry walnuts, and as many good figs, and twenty leaves of rue, bruised and beaten together with two or three corns of salt and twenty juniper berries, which take every morning fasting, preserves from danger of poison, and infection that day it is taken. The juice of the other green husks boiled with honey is an excellent gargle for sore mouths, or the heat and inflammations in the throat and stomach. The kernels, when they grow old, are more oily, and therefore not fit to be eaten, but are then used to heal the wounds of the sinews, gangrenes, and carbuncles. The said kernels being burned, are very astringent, and will stay lasks and women's courses, being taken in red wine, and stay the falling of the hair, and make it fair, being anointed with oil and wine. The green husks will do the like, being used in the same manner. The kernels beaten with rue and wine, being applied, help the quinsy; and bruised with some honey, and applied to the ears, ease the pains and inflammation of them. A piece of the green husks put into a hollow tooth, eases the pain. The catkins hereof, taken before they fall off, dried, and given a dram thereof in powder with white wine, wonderfully helps those that are troubled with the rising of the mother. The oil that is pressed out of the kernels, is very profitable, taken inwardly like oil of almonds, to help the cholic, and to expel wind very effectually; an ounce or two thereof may be taken at any time. The young green nuts taken before they be half ripe, and preserved with sugar, are of good use for those that have weak stomachs, or defluctions thereon. The distilled water of the green husks, before they be half ripe, is of excellent use to cool the heat of agues, being drank an ounce or two at a time: as also to resist the infection of the plague, if some of the same be also applied to the sores thereof. The same also cools the heat of green wounds and old ulcers, and heals them, being bathed therewith. The distilled water of the green husks being ripe, when they are shelled from the nuts, and drank with a little vinegar, is good for the place, so as before the taking thereof a vein be opened. The said water is very good against the quinsy, being gargled and bathed therewith, and wonderfully helps deafness, the noise, and other pains in the ears. The distilled water of the young green leaves in the end of May, performs a singular cure on foul running ulcers and sores, to be bathed, with wet cloths or spunges applied to them every morning.

Fresh Walnuts are somewhat lenitive; so are the green ones, preserved with sugar. but when dry, they soon contract an hot rank quality, and not only by stuffing and obstructing the air vessels cause difficulty of breathing, but they irritate the nervous fibres by their acrinomy, and occasion tickling coughs, wherefore they should be eaten with caution. The inner bark of this tree, has a most violent emetic quality, and is not safe to be taken; but, for want of other vomits, the powder of teh juli, which are milder, may be used. Mr. Ray relates upon the credit of D. John Aubrey, that the spongy substance which is within the shell, and separates the lobes of the kernel, had saved the army in Ireland in a bloody-flux, when all the endeavours of the physicians had proved fruitless. At Hamburgh and in other parts of the lower Saxony, where the inhabitants kill, every Michaelmas, beef for the whole year's provision, they use the green Walnut tree, and Vine-leaves, to sweeten their powdering casks. The chair-makers steep the green, soft, outward shell of the nut in urine, and with it colour their chair frames, to make them look like Walnut-tree. Every body knows to how many good purposes the wood and root of this tree is employed, not to need to be mentioned here.