P.S.
It goes without saying that the marker you used in a seedling tray can be re-used when you pot up, or transplant out.
But if you grow densely, lower branches may hide even a large marker in the ground. It will be more visible (some would say "ugly") if you punch a hole in that slat, thread some twine, and tie it around the stem or a branch of the mature plant.
If your soil is too stony or clay-ey to easily push mini-blind slats into it, hold the slat just 1/2" or 1" from the bottom and push slowly. Slide your grip up another tiny bit, and push that down.
When that isn't enough, grab 3-4 slats at once and push those in 1/2" to 1" at a time, maybe wiggling side to side, then pull out all but one. They keep each other from buckling.
When that isn't enough, get a long used knife from Goodwill, with a big fat handle. Shove that in the ground, wiggle slightly back and forth, and you've made a big opening for the plastic slat. Insert it, and press the soil back around it. The longer the knife, the less you have to bend over.
Corey