Canada thistle is actually edible, if one is seriously desperate. The roots, stems and leaves can be eaten. The roots are easiest, but are said to be very gassy. The stems and leaves are better, but you have to work around all those thorns. In my garden, I've never noticed that anything else eats Canada thistle. Of all the plants, it best survived a recent deer invasion. Its aggresiveness gives it some value as a plant that quickly grows and helps replenish distubed ground. The seeds blow around, and take hold in any dry soil.