There are many ways to heat a greenhouse meant for commercial use, ranging from electric heaters to passive, geothermal heating. Wood-fire stoves or propane heaters will require vents to pump out toxins from the greenhouse. Heat pumps and heat sinks are great examples of geothermal heating that can be used to passively warm a greenhouse.
Make sure that the greenhouse's frames and windows are insulated and well-sealed. Use a dual-cover to add extra insulation and maintain warmth inside the building.
