I usually mark all of the early pickables like red berries, blue berries, mushrooms and thistle. It is perfect if there are a lot of them on the starting island. Then when i need them i just go and gather several stacks in one run. It usually takes almost the whole day, but then im good for more than ingame week.

If you're willing to mod, almost anything is possible.

https://www.nexusmods.com/valheim/mods/1042

Wish one could plant berries without mods, so many trolls and dwarfs love to destroy the raspberries and blueberries I find.

Originally posted by The Big Brzezinski:

Can't plant them, but you can mark clumps of berry bushes on the map. You can also build portals next to particularly large clumps.

Also in addition to this, having multiple maps where such things are pinned and portaled can add to the volume harvested in a short amount of time(though not always necessary if someone has explored extensively on their main map).

I don't mark them, I don't care about planting them. I just pick berries wherever I find them and before I finish the black florist I have more berries than I use for the rest of the game.

You can with mods. As far as I know you can't in vanilla.

As for the planting, please see here:
https://steamcommunity.com/app/892970/discussions/2/3409803709040150041/

As for mods:
Please remember that as of now modding is not officially supported. It's done at your own risk, there are no guarantees for future compatibility. Please keep backups of your save files before you start modding. If you need technical support, please always include a full list of active AND prior mods, since that can influence the game's behaviour with regard to bugs.

Can't plant them, but you can mark clumps of berry bushes on the map. You can also build portals next to particularly large clumps.