Soil Temperature 40 F: Arugula, Fava Beans, Garbanzo Beans, Kale, Lettuce, Pak Choi, Parsnips, Peas, Radicchio, Radishes. Spinach, & fragrant Sweet Peas
Soil Temperature 50 F: Beets, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Chinese Cabbage, Leeks, Onions, Swiss Chard, & Turnips
Soil Temperature 60 F: Beans, Corn, Cucumber, Melons & Summer/Winter Squash
Indoor Seed Starting Schedule
There are some crops that it may be a bit late to start indoors from seed. Onions are one. Perennial herbs are slow growers and should have been started one to two months ago. For every rule, there is the gardener that pushes the envelope. I encourage you to do that!
To start (indoors) as soon as possible:
Eggplant, Peppers, Tomatillos, Tomatoes and all the Brassicas (Broccoli, Brussels Sprouts, Cabbage, Cauliflower). Typically it is recommended to not set out the warm season crops until end of May/Early June. Brassicas can be either direct seeded at 50 F or started indoors now, about two weeks before setting out. Don't let your brassicas sit in containers longer than 6 weeks as this can lead to poor growth patterns and low yields.
There is so much more. Which seeds to soak, which to not. When to use inoculant. And flowers and herbs. But, you can send any gardening questions to firstname.lastname@example.org