A lot of our customers ask me when to plant specific crops. Onions and garlic should arrive in the store around mid-September and can be planted as soon as they are available. Planting tomatoes and peppers can be a little risky in September, but you can do it. Choose tomato varieties with less than 65 days to maturity labels and keep a good eye on the weather.
When it comes to beans and peas, I try to be on the early side of the planting guides. For the other fall/winter crops I tend to be on the late side of the planting guides. I have found it easier to protect plants from the cold than the hot and dry. Towards the end of September is a good time to plant lettuce, spinach, radish, beets, mustard and turnips, kale, chard, cabbage, broccoli, cauliflower, kohlrabi, and brussels sprouts. Actually, I never plant brussels sprouts. I hope you get the idea if I forgot to mention a specific favorite vegetable.
If it is still hot when you are planting these cool season seeds, here are a couple of tricks. Furrow (trench) the row and soak the inside of the furrow with water. Sow the seeds and cover with dry dirt, to the correct depth, as you normally would. You can also cover the rows with water soaked burlap bags. The burlap bag works very well but you need to check the row daily. Once a seed breaks the top of the row remove the burlap.