There are a number of car insurance companies which offer commercial coverage to seasonal business. If you are in landscaping, if you drive a snow plow, even if you drive an ice cream truck in the warmer months, there are insurance companies for you.
Even if your vehicle is parked in the off season, you should not cancel your insurance. You can protect your vehicle and still save money by switching to a comprehensive only policy. That way you will get basic protection against things like theft, vandalism, and damage caused by nature.
As well, your insurance coverage will be continuous. That way you will not have to pay out the nose to get an insurance policy when your in season comes to pass.