It smothers pests and their eggs while they're sleeping out the winter in the tree bark. Recycle plastic knives. I hope you've saved your old plastic picnic knives. They make sturdy, durable row markers for seeds or even as identification tags in the perennial bed. Otherwise, cut strips from old white food containers.
Marker pens write well on plastic and show up clealy on the white background.
Check your calendar. Do this and they should be just the right size when it comes to planting time. The clear tops retain moisture and warmth that speed up germination. But once the seedlings are up, partially open the top for a day or so and then remove it altogether. Otherwise you risk creating too moist a situation for the seedlings to thrive in.