Most is still done with liquid CO2 due to a fairly cheap local source. There are generally two modes of enrichment. When vents are open, target levels are 400 ppm and liquid CO2 is often used because it is cooler and has less tendency to rise out through the vents. In Holland where cool nights extend through more of the year, water tanks are used to store heat for use at night while producing CO2 for day time needs. Combustion source CO2, usually from high efficiency low NOx condensing natural gas boilers is diluted with 4 parts of fresh air to reduce CO to less than 10 pmm and to cool it for distribution through pvc pipping.
Air quality is a primary concern as greenhouses contain people as well as plants.