Just got home from Vancouver, BC - spent most of the day on a plane. The rest - grocery shopping as the larders were bare here.
A couple of things should be mentioned - make sure the shirts are 100% cotton if you want them to decompose in the compost. I used to use old nylon stockings (remember them - haven't worn anything other than knee-highs for awhile), but they would go through the compost again and again. I suspect polyester would as well.
I've also sometime dyed the shirts with a packet of green dye or just strong tea and coffee.
You can also cut up the top of the shirt between the shoulders if you wish, but the strips will have to be tied together. I don't usually bother with the sleeves. If you decide to crochet with them, you won't want to tie knots, but crocheted mats might be a good way to use all the multi-coloured shirts you have.
As Lee Anne said, the strips stretch with the plants and won't hurt them. I've done this for awhile with great success.
Ann
