Typically, when growing asparagus, you won't harvest any spears the first year, only a few (early) spears the second year, and will be able to harvest plenty of spears the third year. You want those early years to produce as much vegetation as possible, since that is where those third-year spears will gain their energy from. Those spears will grow long stems with lots of lacey leaves those first and second years. If you harvested the spears for eating, you would take them when they are a foot long (or less), and they would not have a chance to produce leaves. By the third year there will be enough roots/vegetation growth to sustain those plants even with fairly heavy harvesting of spears. You should still leave those thin spears to grow long though and only harvest those spears that are approximately the diameter of your little finger.
