If your cushions are specifically designed for outdoor furniture, they are generally built to withstand some exposure to the elements. However, this does not mean they are invincible or should be left outside permanently. Most outdoor cushions are made from weather-resistant fabrics like solution-dyed acrylic and have quick-drying foam fills. They can handle a brief rain shower or daily sun exposure better than indoor cushions.
Nevertheless, consistently leaving them outside, especially during harsh weather, will shorten their lifespan. Prolonged rain can lead to mold and mildew growth inside the cushion, even if the cover is water-resistant. Constant UV rays from the sun will inevitably cause the fabric colors to fade over time. Dirt, pollen, and bird droppings can also stain the material.
For the longest life, it is best practice to store your outdoor cushions in a dry, covered place when not in use for extended periods, or during heavy rain, snow, or winter seasons. Using a waterproof storage bench or deck box is an excellent solution for easy storage. If you must leave them out, consider investing in a protective, breathable cover designed for outdoor furniture to shield them from direct rain and sun, and ensure they are placed on furniture that allows for air circulation to prevent moisture buildup. In short, while designed for the outdoors, a little care in storing them will keep your cushions looking fresh and comfortable for many more seasons.