The answer to whether you can leave your outdoor furniture outside all winter is: it depends. The decision hinges primarily on the material of your furniture and the severity of your local winter climate.
For truly all-weather materials like powder-coated aluminum, teak, or high-quality synthetic resin wicker, leaving them outside is often acceptable if properly prepared. These materials resist rust, rot, and moisture damage. However, even these benefit from protective measures. Using high-quality, breathable waterproof covers is highly recommended to shield them from snow, ice, and UV degradation. For cushions, storing them indoors in a dry place is non-negotiable to prevent mold and mildew.
On the other hand, furniture made from steel (unless stainless), iron, standard wood (like pine), or wicker not designed for all-weather use should be stored indoors for the winter. Moisture will cause steel and iron to rust, while wood can warp, crack, and rot.
Regardless of material, proper preparation is key. Thoroughly clean your furniture before winter to remove dirt and organic matter. For storage, choose a cool, dry place like a garage, shed, or basement. If leaving furniture outside, ensure it is on a level surface, consider elevating it slightly with feet to prevent water pooling, and secure covers against strong winds.
In summary, while high-end all-weather furniture can often brave the elements with protection, storing less durable pieces or taking comprehensive protective steps will significantly extend the life and appearance of your outdoor investment. When in doubt, storing or thoroughly covering is the safer choice.