seedlings, seedlings everywhere...

Not all seedlings are the same.  Seedlings can be from perennials and ornamental grasses, they can be saplings starting from trees or they can be unwanted intruders that have blown in from the winds of change. Newly installed landscapes can have waves of seedlings just like established landscapes.  If the ground was tilled and amended for the preparation of the bed it would have stirred up soil and any seeds in it. Seeds need warmth, air and moisture to germinate. They can sit dormant in the ground until these three needs are met. Once the soil is tilled it will allow more warmth and moisture to get to the seed beginning germination. At that point weed seedlings can start to pop up all over the place. Seedlings can be left over from previous plants that were in a neglected or refurbished landscape.  Seedlings can simply wander over from wind, or be carried over by animals.  We recommend using a pre-emergent at the soil level to help eliminate most of those weeds from germinating.  Pre-emergents activate once wet and can remain active for several weeks depending on the brand and weather.  A growing season can be nearly 40 weeks long, so reapplication is needed.  The rest is up to gardening grit.