Thanks, everybody.
I found this in the linked site that Groundhog posted:
Quote:
Six to eight inches is the optimum depth for crown planting. Shallower planting depths cause production of spindly, thin spears, whereas deeply planted crowns produce fewer spears of larger diameter and emergence is delayed. In addition, as crowns grow in mass, they 'migrate' upward making the crown more susceptible to frost damage during first spear emergence. Planting crowns closer than 12 inches results in reduced spear size and quality.
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Although that is talking about planting crowns, it sounds like an old bed could migrate shallower and get more crowded---both of which could cause reduced spear size.