A propagation mat is a mat used under seedlings to encourage germination. Another name for propagation mats is heat mats. Propagation mats increase the rate of success with seedlings by providing a warm, even temperature where the seeds can germinate in warm soil. Different seeds have different levels of sensitivity to the environment, but in general, seeds germinate most efficiently at temperatures between 65 and 75 degrees Fahrenheit (18 to 23 degrees Celsius). The propagation mat goes underneath the seedlings and provides gentle, even warmth through the bottom of the seedling pots.
There are many tricks that people use to encourage seedlings to grow without using propagation mats. Techniques such as setting the seedlings in a warm area of the home, for example, on top of the refrigerator, or creating a propagation mat with a hot water bottle or other source of heat may provide success. Commercial propagation mats, however, provide the most consistent, reliable heat for starting seeds.
A propagation mat is made specifically for use in damp conditions, such as starting seeds, and is safe and waterproof for use in those situations. It is not necessary to adjust a thermostat either. The mat raises the ambient temperature by between 10 and 20 degrees, rather than keeping it at a specific set temperature. Simply choose a location for the seedlings, set the mat in place, and set the seedlings on top of the mat. Plug the mat in and wait for the seeds to germinate.
Thermostat free propagation mats are a nice choice for most at home settings, but in some areas, they will not warm the soil enough for germination to occur. In larger, commercial applications, or in the case of someone starting seeds in an unheated garage or outbuilding, a propagation mat that has an adjustable thermostat may be required. These mats are typically more expensive, and are adjustable in a range from around 40 to 100 degrees Fahrenheit (4.5 to 38 degrees Celsius).
People new to using propagation mats may be surprised at how rapidly the soil dries out when the propagation mats are used. The extra warmth will increase the speed that water evaporates from the soil, so plan on more frequent watering sessions. This accelerated evaporation time is beneficial to seedlings, as it prevents the soil from becoming waterlogged. Soil that is too moist can lead to the young seedlings developing a condition known as dampening off, which results in rapid death.