Sorry not to answer sooner Rhiannon.
I have some raw sheep wool here, coloured sheep so all different shades of brown. Couching it down with various chain stitches in brown threads/wool will be the trunk, heaps of texture and interest there. Mental note after first experiments, lanolin leaches into the cloth, maybe should wash it first! Will certainly have to afterwards
Leaves I will probably do in some variety of straight stitches, probably 6 strands of cotton embroidery thread. A mass of French knots in stranded cotton of various colours could make a believable potting mix.
Been looking at pine pads on various forums to work out how the stitches should lie to look believable.
Wont be machine washable, the couched wool isn't really attached at the back, maybe will iron on interfacing to secure it all once finished.
I do have a cross stitch design program, mine was one of the first produced tho, so yours is probably much easier to use and better all round. But this lends itself to creative embroidery techniques more than cross stitch. To use the raw wool, I'm just rubbing the ends of each staple until they fit thru the needle eye, and tucking both ends down under. Stitching over the top with 'normal' thread should secure it pretty well, can also use the couching thread colour to lighten or darken the final effect as needed, and add piney looking scales.
If you are not killing plants, then you are not extending yourself as a gardener..