If you’re already finding that your garden room smells not of flowers but of mildew, it’s time to do a thorough inventory of what has gone damp. Usually, furniture like armchairs and couches or rugs and pillows will be the culprits. It may be wise to replace any furniture you have with outdoor equivalents. Outdoor furniture and outdoor rugs are made from moisture-resistant materials and are less likely to go moldy.
Regular cleaning can also prevent some of the odors, but it probably won’t eliminate them altogether, especially if your garden room isn’t insulated. If it is insulated, then make sure you open all your garden room windows regularly. This will prevent condensation and the resulting excess moisture that can cause damp smells.