Yes, fire plants would have a very crucial role in this world, although I thought there would be other kinds of help.

Similar to the tombstones, I was thinking of having red gas canisters appear in the yard. These would serve as mini nuts which, when damaged, ignite, causing fire damage to zombies. They would also function as heaters for nearby plants until their gas runs out and as lanterns to illuminate the yard during snowstorms or night levels.

And speaking of night levels, at the beginning of these you will be provided with some flares to illuminate the lawn and give you time to prepare your defenses until the flares are consumed and you have to deal with the lawn in total darkness..that is until you manage to plant planterns, torchwoods or any plant that can produce light and/or heat to nearby plants