[wfraser]

The part of this metaphor that I really like is how a garden is always changing. Unlike a bridge or a building, software doesn't stand in isolation; it's always relying on other software for something. These other pieces of the system are in constant churn, and your software must change with it. This is even disregarding other factors like changing customer demands. You have to continuously prune and maintain your garden, or the weeds (continuously changing software/hardware/services/etc. environments) will choke out your plants. Software is never "done".