When should you plant? Timing is very important when planting a garden. If you plant too early, your plants might get killed by the cold. If you plant too late, the hot weather might create a problem, or the growing season may end before your plants mature and bear fruit.
I live in Colorado. In this area, the Rocky Mountain region, the general rule is that you can start planting the cool weather hardy seeds such as lettuce, spinach, beans, squash, peas, carrots & green onions in early April. These plants thrive in cooler weather and tend to bolt or wilt as it gets hot.
Tomatoes, peppers, flowers, celery and pretty much any other plants that are less tolerant of cold weather & frost should not be brought outside until Mother’s Day or slightly afterward. (We have had snow on Mother’s Day in Colorado.) These plants thrive in the warmer weather.