Harsh climates can be brutal on rattan furniture. There's no doubt about that. Where you live will have an impact on how to care for your wicker, plus the type of wicker will also impact what you need to do. So here's some information on both rattan and resin furniture care.
Never leave rattan or "real" wicker outdoors during the winter (see above!). Although rattan is a natural material that grows in the rain forest areas, it is dead after being harvested, so it can be easily damaged by rain, snow, sunshine, and wind. The less you expose your wicker furniture to the elements, the better (even in the summer rattan furniture will fare better if it is on a covered porch and protected from the elements).
Rattan is not affected by variations in temperatures; however, it is highly sensitive to humidity. Air that is too dry can cause the wicker to become brittle and crack, while too much humidity can cause mold, mildew, and warping. The fibers in rattan will deteriorate with exposure from the sun, becoming dry and brittle and the glued joints in wicker furniture can loosen as the glue dries out. The fibers can also deteriorate from excess moisture from dew, rain, and snow, causing the furnitures hardwood frames to warp.
Left outdoors it only takes about 5-7 years for wicker furniture to deteriorate and quite literally fall apart. Storing your wicker inside in the winter or taking it indoors during inclement weather will help to keep it looking beautiful for years to come.
If your wicker has a natural finish, a light coat of lemon oil or boiled linseed oil applied with a rag or paper towel once or twice a year will protect your finish and add to its shine. Wipe off any excess. Vacuum thoroughly with a soft brush attachment before you do this.
Some people recommend using urethane, varethane, marine varnish, or paint on your rattan furniture to prolong its durability. Some of these finishes provide protection from ultraviolet rays from the sun.
Others don't recommend painting or varnishing wicker as it seals the reed and does not allow it to "breathe" or absorb moisture. An oil stain will allow it to breathe and is an alternative.
Ideally, store your wicker for the winter in a garage, basement, or shed. Clean it first with a vaccum or a soft cloth, then cover with furniture covers (not plastic, as the plastic can trap moisture inside). Store the cushions separately to prevent mildew.
Natural wicker is not intended for outdoor use. If you want wicker furniture for your deck or patio you're better off to buy synthetic or resin wicker.
This relatively new type of wicker can be beautiful but is not nearly as high maintenance as real wicker (something I enjoy).The advantage of resin wicker is its durability and easy care. It will wipe clean with mild soap and water and will withstand humidity and temperature changes. Some brands have been temperature tested to withstand prolonged exposure to below -40 degrees Fahrenheit (- 40 Celsius) and above 176 degrees Fahrenheit (80 Celsius), but as with other resin patio furniture, resin wicker will eventually show deterioration from the elements, so it's probably not a good idea to leave it out over winter.
If you can, store it inside. You could consider leaving it outside in a sheltered location and covering it to protect it from the sun and snow. Store the cushions separately.
One supplier suggested applying car wax to resin wicker every two years. Some car waxes do contain UV-protection agents, but the amount of protection a thin layer of wax can provide is limited. It may may add some sheen to your furniture.
Here's another thought - how about bringing your wicker indoors for the season - a hint of summer for your winter enjoyment.