Plants in Guild Wars refers to a breed of plants that are not exactly rooted in their habitat. They can move and fight!
Like real plants they have reduced armor against slashing damage and fire damage. On top of that, weapon upgrades "of Pruning" (for example an Axe Grip of Pruning) increase the weapon's damage against all kinds of living plants. Nearly all Plants are fleshy creatures, which means that they are vulnerable to bleeding, disease and poison, and leave corpses. The only known non-fleshy plants are Reed Stalkers.
Hard mode levels are in brackets.
For a list of all plants in the game see: Category:Plants.