Weed Stem is your second tier type material that you will need if you're going to fortify your base with stronger structures! This is a lot easier than first thought, because you can get it with your basic axe, However, if you've got an upgraded one, you have additional options available to you.
To get Weed Stem in Grounded, you can chop down Dandelions with any axe to get them. Weeds will also drop them, but you will need the level two axe to get it that way. Dandelions spawn pretty readily around the map, so you shouldn't have trouble finding them.
If you've run out of Dandelions then you can hack down a Weed. Weeds are red stocked plants that spawn around grass and pretty much anywhere on the map. They aren't as plentiful as grass, but you should find it pretty readily in and around your current spawn location. You can only chop one down if you have a level two axe! This is pretty hard to get because you'll need to slay yourself a Ladybug and a Bombardier Beetle. Not only that, you'll need to track down some Spider Silk, which believe it or not is probably easier to get than killing the previously named bugs! We have an entire guide dedicated to getting this tool right here.
Weed Stem is very much like Grass Planks in that you can only carry it by hand and it cannot be placed in your inventory! So, bring it back to the nearest Field Station to research it so you can start crafting a more durable base!