A bit of relevant info for folks using TerraMap to hunt for these little jerks! The entries "Strange Plant1" through "Strange Plant4" that come up when you search are not actually the Strange Plant tiles! I planted one I had to test:
Those seem to refer specifically to the items that you get when you pluck the Plants, so if you search for those things, TerraMap will only highlight chests that have Strange Plants in them! TerraMap unhelpfully refers to the actual tiles as "Dye Flowers" which does not seem to also include the normal dye flowers (Marigold, etc). The "Strange Plant#" search came up with four tiles (the chest I had them in), but when I switched to "Dye Flowers" I got six specks in random caves.