Area: plants in a 40-ft.-radius spread
This spell causes tall grass, weeds, and other plants to wrap around creatures in the area of effect or those that enter the area.
The spell does not create the vegetation, the vegetation described above needs to exist for the spell to function. If the area is partially covered, only squares that have the required vegetation in them actually count for the purpose of the spell.
It is worth stating that a radius means the spell works in a sphere, focused on the point where you cast it. (often at ground level, so a hemisphere). Any vines and the like that are on walls, or hanging from trees will be affected. The spell doesn't require the plants to be on a flat or horizontal surface.
This lack of creating vegetation is reinforced by the mythic version of entangle (same page) which states
The spell functions in areas without any vegetation, causing plants to spring up out of the ground.
Note that roots are valid for this spell (assume they go out as far as the branches of the tree from the trunk for easy of play), but the trunk and branches of a tree tend not to be (although a weeping willow might count). I personally would recommend allowing anything no thicker than a rope and nothing stiffer than roots to work with the spell (So no young trees) but as usual the final decision is up to your DM.
It is always worth asking your DM / telling your players how exactly your version works when someone takes the spell for the first time.
Try web if/when you get access to it. Web works really well indoors (and is nonfunctional in open spaces).