Greenhouse users (both Polytunnel and Glasshouse) need to know what the temperature was when they weren’t around to check it. Did it get too hot yesterday afternoon, stressing the plants and stopping the lettuce seeds from germinating? Did it get too cold last night, and should I have closed the vents a bit more? Did I overheat the greenhouse last night? A maximum and minimum recording thermometer will answer those questions. Both digital and traditional analogue types are available. Analogue types are generally cheaper and simpler to use, and need no power source. Instead of a single tube and bulb with mercury or coloured alcohol, they have a U-shaped tube with a bulb at each end and a column of mercury in the middle. A temperature change pushes the mercury down one arm and up the other. The mercury pushes a floating pin along each tube as it moves and leaves it behind when it moves back, showing how far it went since you last reset it. The thermometer is reset by tipping it up on end or pulling the pins back with a small magnet. Digital models are reset by pushing buttons as per the user’s manual. For best results place the thermometer or its digital sensor at the level of the plants and shade it from direct sun, which overheats it and gives a false high reading.