Which of the following is NOT one of the main forms of protection against spring frosts?

The main forms of protection against spring frosts typically include methods that either add heat to the environment or create air movement to prevent cold pockets from forming around plants. Heaters raise the temperature in a specific area, providing warmth directly to vulnerable plants. Wind machines circulate air, helping to mix warmer air from above with cooler air at ground level, reducing the likelihood of frost settling in low areas where plants could be adversely affected.

Sprinklers are also used as a frost protection method; they create a layer of ice around the plant. This ice actually insulates the plant tissues, helping to maintain a temperature above freezing as long as there is sufficient water and the system is functioning effectively.

Chemical sprays do not offer physical protection against frost in the same way. While they might influence the plant's physiological responses or create some form of barrier, they do not actively prevent the temperature from dropping or keep frost at bay during critical frost events. Therefore, they are not classified as a primary method for frost protection, making this option the correct choice for the question.