Like Karamarie says, hay is the leafy part of the plant that is used for animal feed and will include grass seeds, which is why I don't use it in my garden. Straw is the stalk part of wheat,rye, etc and is used for mulch, animal bedding, etc. It will totally decompose with time and not "take root". Last year I used straw bales and have them ready for this year too. I put them next to fencing (in my case next to the horse round pen) and water well. I'll put about an 2" of soil or compost on top where I'll plant seedlings or seeds. This year I do plan to put tomatoes in bales--I'd heard that tomatoes were not good cause they get too top heavy and tip the bale over. But I'll tie the tomato cages to the fence and it should not be a problem. The soaker hose idea is best, but I didn't notice needing more watering than normal. This year I'm planting my winter squash in an area that the bales did not completely decompost from last year--I'm just going to toss in my squash and zinnia seeds (was told zinnias will keep away the squash bugs), water and keep my fingers crossed. My new bales will get tomatoes, peppers, zuchinni, maybe eggplant--warm weather stuff. One nice thing about straw bales is that you don't have to bend so much and weeding was almost totally unneeded.