1. It comes up every year all by itself?
Yes, and no! It's a perenial plant, and it will come up every year; but it won't keep doing that by itself for very long. It needs your help.
2. I only have to plant it once. That's all I have to do?
Good luck! You have to weed it, even when you are not getting anything from it, because......
3. I heard it takes 3 years before you can get any spears to eat?
That's right! The first summer they will just be tiny little things, and will grow no bigger than a fern. A year later many will be as big around as a pencil; leave them alone!! The second year, you may pick a few of the larger ones and sample them. The third year, go ahead!
4. I didn't have much luck with it; mine got all overgrown with weeds.
Who's fault is that? Go back and reread questions #1 and #2.
5. The shoots were thin, so I waited a few days for them to get bigger around, and instead, they shot up to "seed".
The shoots never get any bigger around than they are when they first come up; they only get taller; they branch out. If the shoots are thin, then that plant is not developed enough.
6. I didn't go out to the garden for a few days and they all went to "seed".
No kidding! They grow at least six inches a day. When the shoots are a foot high, they MUST be picked. This means some are ready EVERY day. Go look!
7. Some of my plants didn't come up; will they spread? They'll seed themselves, won't they?
To some extent each plant will "spread"; that is, it will send up many shoots. The new plants that come up from seeds in the spring should be weeded out. That's right; they are WEEDS! It is the same thing as if you planted too many carrot seeds, and didn't thin them out. None of them would ever get very big, would they?
8. So, how long can I harvest them? And after that, that's it for the year?
You can keep picking the spears until the end of June, here in New England. After that, you need to let the plants grow for the rest of the summer so that their foliage can gather sunlight and replenish the roots. Is that "IT" for the year? Not on your life! See #4! Let's look at your tomato plants, as an example. You plant them late spring or early summer. You fertilize them. You water them, you weed them. Then, in late summer, you harvest your tomatoes. Then you can forget about them!
Asparagus is just the opposite. You harvest them first, THEN you take care of the plants for the rest of the summer. If you don't, DON'T count on having a good crop the next year!