From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156702-828627
Question: I have a 10-year-old asparagus bed which is not producing as
it once did. It is becoming thin and in some places the patch is very
weedy. Also, the soil seems to be compacted from years of walking on
it to harvest the spears. Should cultivate the bed to loosen the soil?
What can I do to bring my patch back to full production?
Answer: Although asparagus patches are long-lived, after 10 years it
might time to start a new patch, especially since the soil is
compacted and the bed is becoming weedy. If you have the space, you
may want to start a new patch in a different area to help minimize
problems with overwintering pests. Site the bed carefully, keeping in
mind that it will be there for many years. Consider what the size of
nearby trees and shrubs will be in 10 years and make sure they won't
shade the sun-loving asparagus plants. Also, before planting, add lots
of organic matter such as compost to the soil. To manage weeds, try
placing a thick layer of newspapers in the paths between the rows,
then mulching over that with a layer of straw or other organic mulch.
Renew the mulch every year.