When a plant growth spell is cast, the wizard causes normal vegetation to grow, entwine, and entangle to form a thicket or jungle that creatures must hack or force a way through at a movement rate of 1 per round (or 2 if larger than man-sized).
The area must contain brush and trees for this spell to work. Briars, bushes, creepers, lianas, roots, saplings, thistles, thorn, trees, vines, and weeds become thick and overgrown so as to form a barrier.
The area of effect is 10 feet on a side per level of experience of the caster, in any square or rectangular shape that the caster desires. Thus an 8th-level wizard can affect an 80' X 80' square, a 160' x 40' rectangle, a 640' x 10' rectangle, etc.
Individual plant girth and height is generally affected less than thickness of brush, branch, and undergrowth. The spell's effects persist in the area until it is cleared by labor, fire, or such magical means as a dispel magic spell.