Locals are your best advisors--call a local nursery, county extension agent or a neighbor who grows vegies. Ask locals for advice when to start. This is determined largely by your average date of last frost. In the Southwest this can vary widely, especially with altitude.  In some parts, it seldom freezes.  Where I live it freezes a lot and the average last date of frost is May 20 (northern NM, 6300 ft). I don't plant anything that can't tolerate frost until June 1. Also ask them how long the average growing season usually lasts.  I have a very short growing season, but in some places you can garden all year.  Much soil in the Southwest is alkaline. Usually you don't want to add wood ashes or lime to Southwestern soil. I add manure and compost to my soil--mostly manure.  I'll second the recommendation for Square Foot Gardening--it conserves water.

 Annie