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Lettuce is bolting when itLettuce is bolting when it starts to form a long flower stalk, usually in the center of the plant. The leaves on the stalk will be smaller and more elongated, and eventually, the stalk will form a flower spike. Leaf flavor becomes bitter. Bolting is caused by increasing day length as well as increasing temperatures.   For Illinois, you might want to stop the succession planting until late summer (about mid August or so), when you can sow for fall crops. If you do want to give summer growing a try, provide filtered shade all day for your plants, or complete shade in the mid afternoon onward, to protect them from the heat. Be diligent about watering.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?