I'd appreciate information on what fall crops anyone has successfully grown in zone 4 (Minnesota). What does well, when should seeds be planted? Is it too late to start broccoli or cauliflower from seed?
Zones refer to the average cold temperature in the winter. A better method of deciding what to grow and when to plant is average last and first frost dates. What's your average first frost date?
I've had frost the first week of September (unusual) and not until the last week of October (also not usual). Average frost date is probably around October 1.
According to Eliot Coleman's "4 Season Harvest" you can plant broccoli and cauliflower through early August for harvest in late October. They can survive light frost but can easily make it through October with a little protection like a row cover. The tricky issue for growing cold-hardy plants in fall isn't frost as much as the decreasing day length that greatly reduces the rate of growth in plants.
Based on my Z4 experience, I will be planting broccoli, lettuce, radicchio, chinese cabbage, and other heading plants the last two weeks of July. (I gave up on growing cauliflower many years ago.) Faster growing plants like spinach, arugula, radish, cilantro and mesclun salad mixes can be planted later in mid August. They can be planted again a couple weeks later if you can provide cold frame or hoop house protection later in the fall.
I'm going to try some faster maturity beans, soybeans and blackeyed peas. I heard it's suppose to add more nutients to the ground for the next year.
Our first frost date in Minneapolis is September 15. I just did research and posted lots of great sources including a planting schedule for zones 3-4 on my blog A Minneapolis Homestead below.
Here is a link that might be useful: A Minneapolis Homestead
I harvested a bucket full of carrots yesterday that I planted after early potatoes here in Maine. They are sweet.