You can buy crowns anytime now. You dig a ridge with furrow each side - so that the crowns (which have lots of thick roots - like dreadlocks) can sit with the roots down each furrow, then replace soil and water in well. Make sure you have removed ALL perennial weeds, asparagus does not like competition! I would recommend mulching with compost, minced up comfrey or any well-rotted material to retain moisture. Personally i use seaweed collected from the beach (i have just mulched my bed today!) Asparagus is one of the few plants that can tolerate salt. In France it is often grown with thick sand on top - easy to cut the spears. And yes, you have to let it grow for a whole season before cropping. The second year you can harvest some spears, but just for a couple of weeks, then let it grow on. The third year you can harvest as much as you like, april till june. It seems a lot of agro but a good bed will last 20 years or so - so it is worth the wait!