Check with your state's Extension Service. Each county should have it's own Cooperative Extension Office which provides free publications and information for the asking. They will have valuable vegetable/gardening tables available specifically for your area determined by universities and horticultural research scientists who have collected data from growing those crops in your state. Your tax dollars are already paying for this service so you may as well get some use out of it.

Vegetable Planting and Planning Calendar for Missouri (download the pdf) complete with spring and fall planting dates (underneath the spring planting dates for appropriate crops), how much to plant per person, etc. They can also tell you the average last frost date for your area and ideal planting times for specific crops and varieties in your area.

See how detailed this example is:

{{gwi:4005}}

things to plant mid-summer for fall/winter harvest:

beets
broccoli
cabbage
carrot
collards
leek
onion
parsnip
peas (maybe wait?)
rutabaga

• Turnips.
• Radishes.
• Dill.
• Chamomile.
• Spinach.
• Mustard.
• Kohlrabi.
• Chives.
• Basil.
• Chinese Cabbage.
• Cauliflower.
• Brussels Sprouts (early variety).

I'd do a bit of research before trying leeks, parsnips, onions or rutabagas, as it's far too late here for these to produce a fall crop.

Thanks very much for the suggestions. I live in Canada, so I'm not sure if we have anything like an 'extension service' here, but I'll check.

I think I'll start by throwing in some basil and dill.

cheers.

The cool-weather plants (like carrots, beets, and arugula) I've sown recently seem to stall out in the heat before they even put out their first true leaves. Do I need to either start them indoors (and keep the air conditioning on) or wait until the weather cools and then keep my fingers crossed the cold weather doesn't arrive too soon?

I'm trying beets and turnips. Here's hoping they actually make it. Last year I didn't plant the fall crop until mid August and they were shaded out by corn, so they didn't make it. This year it's earlier and they won't be shaded, but seeing nygardener's post, now I'll be worrying they're too hot!