What and Where to Sow:
First, consider where you have space from harvested crops or will have space in the next few weeks. Is there room to sow a little spinach in between your tomatoes? Maybe you could pull up your bolted lettuce and sow endive, escarole, or collards.
Container plantings do well in fall, because with cooler temperatures, they don't dry out as fast. If cold weather comes unexpectedly, they can be moved to shelter. Consider some edible container plantings like mixing kale, mustard, or lettuce with pansies and violas. Green onions could add a spiky centerpiece. A container of mixed herbs like chives, oregano, sage and thyme could be started in the next few weeks to give you a punch of flavor for your fall cooking.
When choosing varieties, consider crops that are complementary to vegetables that you'll be harvesting in the next few weeks. Would some cilantro make a good salsa ingredient to go along with those tomatoes and tomatillos that you'll be harvesting in late summer and early fall? How about some fresh dill to add some zip to grilled salmon and roasted sweet corn, or a cucumber salad? With a browse through your favorite recipes, you might find a handful of varieties that you can sow now for inspiration in the kitchen.
Mild climates and southern areas of the country that do not get frost have a wider range of options besides cool season crops. In those areas, warm season crops with a short crop time like bush beans and cherry tomatoes can be planted in August or September for a late fall harvest.
When to Sow:
To decide when to sow, look at the crop time listed on each packet, then count backwards from your average first fall frost date. One thing to keep in mind is that the hours of sunlight are declining toward the end of summer. So, it may take a couple more weeks over the crop time listed on the packet for your crop to mature. Crop times aren't as important for greens (like lettuce, kale, mache, spinach, etc.) that you can pick at any stage of growth.
If it's very hot outside in your area, you can start seedlings indoors then transplant outside in a few weeks, or start them in flats in a shady area outside where the temperature is slightly cooler.
Need more help? Check out our Late Summer/Fall Sowing Guide