Rated at 2500 - 5000 Scoville heat units, Jalapeno peppers are in the medium hot range. That said, the amount of heat in a pepper can vary, even when picked from the same plant. What determines the heat of a pepper? Typically, the longer a pepper stays on the plant, the hotter it will be. Plants that are stressed after the fruits begin to form, either through high temps or reduced water, generally produce hotter peppers as well. Harvesting peppers as they mature will encourage more fruit formation, so keep picking those peppers!