Theresa, spring would be the best time to plant your straw bale gadern because it is already to late to start a fall gadern, planting in a bale that has not already had a chance to get past the heating phase of the decomposition/composting process may kill plants if placed in the bale to soon . Purchasing your hay or straw bale around December (here in SO CAL), would allow your bales a few months to absorb the nitrogen from the winter rains as well as give the bale some time to go through some of the earlier heat bearing compost decomposition process that may burn the roots of plants if exposed at the wrong stage of decomposition. Over the course of the winter, periodically add the cotton seed and bone meal powder so that it absorbs slowly into the bale. Key Point Your bale will become to heavy to move after it absorbs water over time , have a good design planned out for the location and size of the Straw Bale Raised Bed Keyhole Garden. The best time to plant is determined by your location and what you are choosing to plant. The Farmers Almanac is a great resource. Usually the local nursery will have a selection of seeds or sprouts marketed towards the next or current season for home gaderning. Good luck and most importantly ..have fun!