Asparagus are the growing shoots of a fern-like plant. Asparagus has a delicate flavor and diuretic properties.
One distinctive problem with asparagus is that a constituent chemical of the plant is metabolised and excreted in the urine, giving it a distinctive, mildly unpleasant odour. Apparently not everyone who eats asparagus produces the odor, but also not everyone is able to smell the odor once it is produced.  Many people prefer not to eat asparagus because of this effect, as it can put a damper on an otherwise romantic evening...
There are three different common 'varieties' of asparagus sold in the markets which are actually the same plant grown under different conditions. 'Green' asparagus is left exposed to the air so that it grows normally. 'White' asparagus is piled high with straw as it grows so that the shoots remain pale. The flavour is identical, but some people believe that the white asparagus is more tender and flavourful. Finally, a 'purple' asparagus is occasionally seen, which is again the same except with purple streaks on the stem.
Almost the whole shoot is edible, but the prized portion is the tender growing end. The base of the shoot can be woody and fibrous, and it is generally discarded about a third of the way up. The traditional way to prepare asparagus for cooking is to take hold of a few shoots at a time, holding an end in each hand, and firmly but gently 'snap' it. The shoots will snap at their natural break point, which should be where the woody end meets the more tender portion. Another way to prepare asparagus for cooking is to peel the bottom end with a knife or a vegetable peeler, working from the middle towards the end. This will remove the toughest fibers from the outside although some will remain and will need to be cut an inch or so from the base of the stems.
Cooking Asparagus 
In its simplest form, the shoots are boiled or steamed until tender and served with a light sauce or melted butter.
Asparagus is very long and narrow, and it is traditionally cooked in an 'asparagus steamer.' This is a very tall narrow pot with about an inch of boiling water in the bottom. The asparagus is tied in a bundle and stood upright in the steamer for about five minutes so that the toughest end gets the most cooking and the tips are just barely steamed. A modern alternative is to use a frypan - put boiling water in the pan and slide the asparagus gently into the water so it can lie sideways across the pan. Asparagus is cooked when it turns bright green, which only takes a few minutes regardless of the cooking method.
Asparagus is very good in stir-fries, where it is usually cut into lengths of about an inch and added at the last minute before serving. It can also be made into asparagus soup, especially the older and slightly less tender shoots.
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Asparagus have a very specific season that starts in May and traditionally ends on the 24th of June. Out of season asparagus are grown in regions close to the equator or in greenhouses. If fresh asparagus isn't available, canned, jarred or frozen asparagus usually are.
"False" Asparagus 
The shoots of a related plant, Ornithogalum pyrenaicum, called Prussian asparagus, have been used for similar purposes as genuine asparagus.
Many unrelated plants are called "asparagus" or "used as asparagus" when their shoots are used in a similar way.