Here's a basic soil temperature guide that I follow with much success. Successful seed germination is determined by soil temperature, growing time, moisture, soil type.
Semi Hardy Vegetables - Cool Soil Temperature
If you don't wait to plant these vegetables after the Cold-Hardy vegetables, you're taking a chance at your seeds or seedlings dying from frost bite.
*Remember you can always get a head start on germination by planting indoors in containers. That way your plants are 2 - 4 inches in heigth and have a good root system.
Tender Vegetables - Warm / Hot TemperatureThe air and ground temperatures are beginning to warm. Here's a list of early Spring vegetables that Burpee, Gurney's, Park Seed, Johnny Seed and Harris Seed Company have all agreed are ok to plant at a ground temperature of 60F and above.
*It's important to know that these vegetables require a warm daytime temperature, prefer warm summer days and are completely intolerant to frost. This means, if planted in too cold of soil they will die or be severely damaged.
Plant these tender vegetables after frost-free dates. These dates can be easily found Farmers Almanac, Victory Seed, (Alaska, Idaho, Oregon, Washington) Humes Seeds, (California) Victory Seeds, Burpee Seeds. EmmaDear.com
Extremely Tender Vegetables
Here's a list of common vegetables that comletely thrive during the hot summer days. Most varieties are intolerant of frost and cool spring winds. So, if you reside in zones that get late frost up until April - May, start your plants indoors and plant by Mid May.
An aqua dome, newspaper, wall of water (plant protector), tarp, cardboard box, light blanket will help protect these vegetable while they get their start in the spring.
Plant these very tender vegetables 1-2 weeks after the tender vegetables listed above.
Late Crops for Fall Harvest - Warm / CoolIn most areas of U.S., you can get a second crop of: Beans, Beets, Broccoli, Cabbage, Cauliflower, Lettuce, Onion, Peas, Spinach. Plant the last crops in Mid July through August to ensure a bumper crops.
I reside in southern California where we have long growing seasons and most vegetables grow all year round. The only vegetable that I have a hard time growing is lettuce varieties. They don't like heat and will either bolt quickly or turn bitter to taste.
If planting lettuce varieties in the southwest were it is often hot and arid during summer, I would recommend a canopy of shade and moniter water levels. They enjoy a consistant moisture level. If your plants are too moist, they'll surely rot. If you don't have consistant moisture, they'll bolt, flower and then go to seed. Shade is your friend when it comes to lettuce in the southwest.