Don't buy potting soil, buy cow manure. Use one bag per mound and make certain you also purchase bone meal and spread that in with the seeds. You've got a problem. Low moisture and no help from the soil. Start a compost pile. I grind up my kitchen scraps in a blender and pour it over the compost pile after turning. You might want to get some bags of pine bark mulch and cover the mounds after the seeds germinate and then in the fall turn the mulch under. Get the nuggets. Also because the mulch will rob what nitrogen you have make certain you fertilize, but at night. You are going to have too much evaporation. I lived in Denver and I know about the heat and you are worse off. Is there a way you can create some shade? Garden centers have shade houses to protect plants from being burned. Hey it is not going to be easy and actually could be a losing battle unless you put some sweat equity into it.

http://propagatingperennials.blogspot.com