You need actual biomass to physically exist, only then is carbon actually bound. Trees have much denser cellulose and stay around for longer. Ultimately, though, the answer is both. And bushes and shrubs. Just build up a whole forest. The denser you can make it, the better.
Almost every plant is carbon neutral in its life cycle, it’s a great sentiment, but it doesn’t work in the end.
I wonder how carbon-negatove a bamboo forest would be if you harvest it, turn it into charcoal (or bury in a bog or something?), rinse and repeat. Afaik charcoal sinks carbon fairly effectively (???), unless you burn it obviously.
Literally from people selling you it though lol. It’s possible, but I think the greater insect life more diverse lawns facilitate is a better net overall for the world.