Ray, a heater failure would normally be an insurable risk, if you have a policy that covers equipment, boilers and machinery. You also need consequential loss coverage for the plants themselves. This is no different from a freezer compressor breakdown and resulting food spoilage.
Your insurance broker did a lousy job with your old policy, and in some cases you can actually take the broker to court over an uncovered loss. Of course, if the coverage was offered to you and you turned it down, you're on your own.
I used to adjust claims for a boiler and machinery insce co. in what seems like a past life.
Adam, you should mention the plants to your insurer, especially if their loss would exceed your deductible. Make sure all likely risks are covered, and the plants are covered under a consequential damage rider.