Monitor the jalapeno peppers as they reach their full size, typically between 1 and 4 inches long for the “Mammoth” variety. Pick the peppers once they develop a deep, dark green color but before they begin to turn red.
Support the pepper in one hand and cut its stem off the plant with a pair of shears or a clean knife. Leave 1/4-inch of stem attached to the jalapeno.
Pick jalapenos every two to three days as they ripen. Picking the peppers promptly ensures the best flavor and encourages the plant to flower and produce more peppers.
Pull up the entire plant when the first fall frost is expected. Remove any remaining flowers or small peppers that aren't near maturity. Hang the plant upside down in a warm, dark area so the remaining peppers can finish maturing. Pick the jalapenos from the plant when they reach their mature shade of green.