Originally posted by Gunther Ultrabolt Novacrunch:

For thistle farming I try to find a large stand of Black Forest biome, I mark each thistle with a dot (no label just a dot). Make sure you are zoomed in as far as you can go to accurately mark the spots.

Onice I have marked a whole forest I simply go back after the thistle has re-spawned (which takes 4 hours according to Valheim Wiki).

For rasberries and blueberries I just hunt for them as needed.

i pretty much do the exact same but in reverse. i mark blueberries with dot and a 'b' (also mark mushrooms w/ a dot and an 'm')... then hunt the thistle. harder to see the berries and mushrooms while the thistle stands out.
ultimately markers are your friend. you can chart a path connecting the dots on your next foraging trip and save a lot of time, however you choose to do it.