Other factors that come into play with mist propagation.
When a misting timer is programmed, it will follow the program until the propagator makes a change or some other device tells the system to operate differently. These devices can include an electronic leaf (which I don’t recommend in most cases) or a rain and freeze sensor (also not needed in most cases).
Electronic leaves run well over $200 and are not really a requirement when rooting cuttings. They theoretically control the misting cycle by the rate of evaporation which can vary with any change in weather. The leaf gets wired into your misting system so when it is activated it cuts off the electrical signal to the misting solenoid valve.
Electronic leaves like the Mist-A-Matic have an arm that pivots. On one end is a screen that sticks up in the air that water collects on during the misting cycle. When enough water has collected on this screen, the arm pivots downward. The other end of the arm then comes in contact with a small switch that will interrupt the electrical signal to the solenoid. When water evaporates off the leaf, it again raises and allows the electrical signal to operate the solenoid. The theory behind the electronic leaf is for it to simulate an actual leaf and the rate of evaporation.
While many propagators have had great success with electronic leaves, others have run into disasters. Something as simple as a bug or bird dropping will keep the leaf in the downward position, stopping ALL moisture from getting to the cuttings. Unless this is immediately noticed, the cuttings will surely die. Read this PDF for pictures and more drawbacks to using an electronic leaf.
Rain and freeze sensors
Rain sensors and freeze sensors can also be wired into a misting system to interrupt the flow of moisture to the cuttings. The theory behind a rain sensor is to stop the solenoid valve from opening during wet weather. Unless your cuttings are being rooted in a greenhouse or hoophouse (more on why you don’t need one of those later…), your misting system does not need to run during a rain event. The rain is keeping the cuttings moist and there is no need to waste water. The rain sensor has a small cup with a plunger inside. When this plunger gets wet it gets pushed downward and contacts a switch that interrupts the electrical signal to the solenoid valve. When the rain stops and the plunger rises, electricity can now get to the solenoid. One drawback to the rain sensor is if a bird dropping gets into the cup and keeps the plunger down, but the cup is so small the likelihood of that is slim.
Freeze sensors work in a similar fashion except they disconnect the power to the misting solenoid when the temp falls to around 37° F. When the sensor detects the temperature at or below the target, it interrupts the signal to the solenoid.
If you do decide to use either a Mist-A-Matic type electronic leaf to control your mist or a rain/freeze sensor to keep it from misting during wet or cold weather, be sure you check your systems operation often. Something as simple as a bird dropping on the leaf or a stuck plunger in the sensor could wipe out the entire crop of cuttings very quickly under the right circumstances.