Worm eggs are everywhere, they dont discriminate whether the soil is frozen or hot. Some eggs survive for years in soil no matter the type of soil. That said, it's your soil that dictates when and how often you should worm. Warm moist wet soil requires frequent wormings. Cool or cold mountainous soil or desert like soil may require less frequent wormings. Chickens that are penned on the same soil require frequent wormings. The bottom line is if their feet touch the ground, they'll get worms.