Why are your onions dying?
When the onions leaves put their enitre energy into the bulbs that turned into onions, it start dying. Pull your onions out once the leaves die.
Onions are classified into two categories. Green onions or scallions and dry onions, which are mature onions. There are many varieties of onions and range from red to white to yellow. Some are classified by the pungency, hot and mild. Because onions are being hybridized it would be hard to know how many kinds there are.
The collective noun is a rope of onions. There are actually many collective nouns for onions. For example: a bushel of onions, a bag of onions (the word bag is a concrete noun, naturally, but is also a collective noun when used in this context), a lot of onions, or a bunch of onions. All of these are legitimate collective nouns that can be used to describe onions (there are of course many more).