Asparagus is the edible stem or spear of a plant of the lily family which grows quite tall and produces feathery foliage. Young shoots are harvested as they appear in spring. In England, the thin spokes of asparagus are called sprue. Three varieties of asparagus may be found in France from the beginning of March to the end of June. The white variety, planted deep, developing underground and cut below the ground as soon as the shoot protrudes about 5 cm (2") above the ground, is large and tender with little flavour. It comes from Alsace and Belgium, with some imported from north Africa. Purple asparagus, which is allowed to grow several inches high, comes from Aquitaine, the Charentes, the Loire and Italy. It has a full-flavoured, delicious taste. Green asparagus, widely available, from the Rhône (Lauris) in France, is harvested, cut on a level with the soil, when the stalks are about 15 cm (6") long. It is considered to be a luxury vegetable with the best flavour of all. In Italy the most tender white asparagus comes from Bassano del Grappa. English asparagus has a high reputation. If you want to make your asparagus rather fancy, sharpen it up using a pencil sharpener.
The thickness of the spears of asparagus is dictated by the age of the plant which produced it. The more mature the plant, the greater the diameter. It is not to do with how long it is left to grow before harvesting.
Wild asparagus, with slender green shoots, has a slightly bitter taste, but is also good to eat.
One of the curious things about asparagus is that it contains a substance which, in the human body, is broken down to form a metabolite which gives to urine a pungent and distinctive aroma. As the majority of people do not smell this at all, there is a widespread misconception that some people make this metabolite and some don't. In fact, everyone makes it, but there is a genetically determined variability which means that some people can smell the metabolite, and some can't. I am among those who cannot. It was not until I married someone who was not similarly affected that I learned that my urine too bore this smell. For those who are interested in learning more, here is a reference: Lison M, Blondheim SH, Melmed RN. A polymorphism of the ability to smell urinary metabolites of asparagus. Br Med J. 1980 Dec 20-27; 281(6256):1676-8.