These two sites are my go-to for freezing temperature tolerance —
Southern Exposure's Fall & Winter Gardening Guide
https://www.southernexposure.com/southe ... zp-38.html
Broccoli (6/1-7/1): 28 degrees - leaves can handle to 15 degrees, but heads are more tender
Fall & Winter Growing Guides - Winter Planting Chart
https://www.territorialseed.com/product/ ... ing_guides
...note though that there are different types of broccoli varieties, generally indicated as “spring” and “fall” broccoli that have different kinds of cold as well as heat tolerances depending on the variety. Which variety are you growing?
Also, and I looked but didn’t find similar guide as above for broccoli at Johnny’s Selected Seeds, but THEY have excellent germination temperature guides for most vegetable seeds, and if you are sowing these seeds outside in the ground, they may not sprout until temperatures are conducive, and depending on the temperature, it may take up to 3 weeks.
Broccoli can germinate in 3-5 days at optimum temperature indoors, and they grow well at around 50-60°F under lights. Slower at lower temps. They will need to be acclimated to the outside for at least 1 week, but by the time the outside seeds sprout in 3 weeks, you may have nice starts to plant out. Other options are to start them in coldframe, etc. and/or plant them out under low tunnels.
Cabbage can be started the same way. Spinach and Kale are generally more cold tolerant.