Last fall was the first time my husband and I had tried to grow a fall garden. Some things did extremely well, like the arugula and kale, and some things didn't come up at all, like my carrots, shallots and leeks. When things didn't work out, I did additional research.
Vegetables like leeks, shallots, and Brussels Sprouts are considered fall harvest veggies, but here's the thing; they need to be planted now! I didn't start my seeds for these until July last year (about the same time I started my seeds for kale and cauliflower). My leeks didn't even sprout last year, and I blame that on the fact that it was too hot on my deck. I have started all of my seeds indoors this year and so far so good.
Brussels sprouts mature 80-115 days after TRANSPLANT, not from planting the seeds. I planted the Brussels sprouts last week indoors. Seeds take 5-10 days to germinate, and then you don't transplant for another 6-8 weeks. This should give me a harvest sometime in September for this batch. I'm going to try to plant some more seeds in a week so that I have a staggered harvest. Plus, Brussels sprouts are said to be sweeter if they are out in a frost, so the second planting should go through one frost at least.
Leeks take even longer after transplant to mature, at 110-135 days. I planted my leeks a few weeks ago and then again last week, for another staggered harvest. They should be started indoors 4-6 weeks before the last frost date, which in my area of Indiana is May 5. You can find out what yours is by visiting your state's extension service website. I find a lot of helpful garden information and planting guides at Purdue's Extension website.
Shallots also need a very long growing season. Seedlings mature about 100 days after transplant, which should be done 1-2 weeks after the last frost date. I may still have started mine too late by starting them this month, but we will see. Shallots also need a lot of fertilizer and mulch to keep the ground moist.
Besides the fact that there are some fall harvest vegetables that need planted now, you want to start thinking about your fall garden as you plan the layout of your summer garden. Last year, we had many plants like our tomatoes and spaghetti squash still in the garden when the time came to plant the fall vegetables. This caused us to have to fill in with our fall veggies in odd places. If you know that you will have certain things in the garden for a long time, you can create a more intuitive layout for summer. Planting as early as possible will also help to get the summer harvest completed sooner, freeing up space for fall spinach, kale, Swiss chard, etc. Planting the fall seeds too late may increase the likelihood that you'll have duds....like my fall planting of carrots.
If you've ever been on the fence about trying out a fall garden, I HIGHLY recommend you try it! Last fall we supplied salads with greens from our garden at Thanksgiving and I was still picking spinach and kale in December! There is nothing like fresh vegetables straight out of garden, and having a fall garden extends your ability to enjoy them.