Put a heater under your plants?
Poly tent covers help to hold in the heat early in the season when the bales are “conditioning” they give off tons of heat like a natural furnace underneath. It is the bacteria that are doing the decomposition that create the heat as a naturally occurring phenomenon of the process. People often see a freshly stirred up compost pile will steam for a couple of days when new organic matter is added, this is another example of the heat generated by the actively consuming bacteria munching away on the freshly introduced organic matter. The little seedlings and transplants love the warm root zone created by this process inside the bales, and the new growth loves the cooler temperatures of the early season. The combination causes very rapid root development and plant growth. The poly tent covers allow much earlier planting (2-4 weeks depending on weather). This natural heat from the bales is one reason folks in the far north, Canada, Alaska, and in the Arctic circle love the Straw Bale Gardening method. They don’t have to wait for June when the soil normally thaws to plant.