There are a couple of ways to create grass and weeds. Using AnythingGrows by DCG is the easiest. You can then convert it to a mesh and edit it further to create some "stock" grass with bent leaves. Also starting in with a mesh plane in the vertex room with a fine mesh, and then grabbing vertexs and pulling then up is another. Make sure to multiple-select vertexs or you will spend a little too much time clicking and moving! For curving grass, like what is showing in the lower left corner of the image, you can use the spline modeler as well.