Answer: You had so many good questions, I copied them and answered one by one below.
Does the soil need to be prepared the year before or can it be done before planting?
The soil can be prepared in the spring before planting.
If you can space 18" apart why can't you have 18" between rows as well?
The spacing allows for mature size of the plants, they have huge root systems and the plants themselves get very large over time. You also need room to get in between them to harvest.
Is it better to remove volunteers and can I use grass clippings as mulch?
Volunteers should be removed, the originals should be better quality plants. You also do not want to allow them to crowd each other.
Grass clippings are ok as mulch if applied a thin layer at a time and if not treated with chemicals such as herbicides. It is better to compost them and use the compost.
My space is small 6'x2', how many plants do I put in such a small space, any advise? Is it to small?
Three plants would be about it. Honestly, they grow big!
Would 2yr. old crowns be better? I know that the following year would only be a small harvest but worth the wait in the long run.
Two year old crowns will allow you to harvest one year sooner. It depends on how much of a hurry you are in. You could also start them from seed if time is not important.
I hope this answers your questions -- enjoy that asparagus!
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