Unfortunately, it’s not that simple. Plants can be divided into several categories based on light requirements, sensitivity to frost, and ideal temperature for growing.
Types of Plants
Hardy plants require less light (3-6 hours of direct sun), can tolerate hard frost, and grow best in cool weather. Many will die, bolt, or go to seed when temperatures rise. These should be planted early in the spring, as soon as the soil can be worked, or late in the summer to be harvested in the fall. In areas with short periods of frost, hardy vegetables can be grown all winter.
Semi-hardy plants require more sun (6 hours), tolerate light frost, and grow best in cool weather. These should be planted in the spring, about 3-5 weeks before the frost date. They can also be planted late in the summer for a fall harvest, or during the winter in temperate regions without any frost.
Tender plants need more sun (8 hours), do not tolerate frost, and grow best in warm (not hot) weather. Any frost will kill them, so they should be planted on or after the frost date, which usually occurs in late spring or early summer.
Very tender vegetables require a lot of sunlight (8-12 hours), will be stunted by cold temperatures, and grow best in very warm weather. They should be planted after all danger of frost has passed, about 3 weeks after the frost date.
These vegetables have long growing seasons (they take a long time to go from seed to harvest), so you want to wait until the weather is warm enough for them, but you can’t wait too long or you will not have a chance to harvest before hot or cold temperatures set in. For this reason you should use transplants, not seeds.
With this in mind, you can see why it’s not best to plant onions and tomatoes at the same time. Hardy and semi-hardy vegetables benefit from growing in the cool, early spring while tender and very tender plants can’t be planted until there is no chance of frost and the temperatures are warmer.
Here is a chart that lists many vegetables and their category.
You can use this as a guide, but if you want to know EXACTLY when to plant for your location, the easiest way is to sign up for my newsletter or purchase my Planting By Color e-book. You will receive an outline of which plants should be planted each week–if it’s on the list, go for it. If it’s not on the list, it’s not time to plant that yet (or it’s too late to do so). This makes it simple and easy, and you don’t have to keep track of anything.Happy gardening!