This add-on unit for the commercial heat mat. The mat will maximize your seed starting area by daisy-chaining up to 4 of these units to the MTMDU unit (MTMDU sold separately) and is required. The new, stronger heating wire and thicker multi-layer construction offer more uniform heating and the durability to withstand rugged greenhouse environments. The waterproof construction means you don't have to worry about splashing when watering your plants with rugged and durable waterproof connectors. Bottom heat is one of the most important and effective ways to improve rooting and germination. Our new heavy-duty modular commercial heat mats will provide uniform heat for hundreds of your seedlings or cuttings. Warms root area 10-20°F over ambient temperature to improve germination.