Whenever you build outdoors, any wood that will be in contact with the soil must be decay-resistant or it will rot because of the moisture and decay-producing organisms. Fence, porch and patio posts, cold frames, borders and ties must all be decay-resistant to last for more than few years.
There are three ways to get decay-resistant wood: You can buy wood that's naturally resistant because of its resin content, wood that's been treated with chemical preservatives, or a preservative to use on the wood of your choice. Cedar, redwood and to a lesser extent cypress are the most common decay-resistant woods. Redwood is the most resistant of the three, and most people find it the most attractive, but heavy cutting and laws turning redwood forests into parks are making it scarce and very expensive, and the quality is beginning to drop. Today you see less deep-red heartwood and a lot more of the lighter sapwood, some of it dyed to make it look the way we all expect redwood to look.
Treated wood - ordinary lumber such as fir, pressure impregnated with preservatives - is starting to take the place of redwood. It's cheaper, stronger and so rot-proof that it can be used for foundations. But the treatment leaves it a sickly green that can't compare with the look of redwood or cedar. Still, you can color it with a redwood stain, making it look almost like the real thing.
Another common treated wood is the railroad tie, a heavy timber soaked with creosote that sees a lot of use in landscaping. If you use ties, remember that the creosote does not penetrate throughout the wood; if you cut or drill a tie you should soak the newly exposed wood with creosote.
You can extend the life of any wood by treating it. Creosote is probably the best-known preservative, but it's smelly and messy and can't be painted. It also turns the wood a dark brown that you may not want.
Clear preservatives, such as pentachlorophenol (PCP) or tributyltin oxide (TBTO), don't alter the color of the wood, and they take paint very well. They're also incorporated into some exterior stains so you can fight rot and improve the appearance of common woods with one coat.
If you use one of these preservatives, aim for maximum penetration. Soak the wood rather than brushing it whenever possible. This will provide the maximum protection. Still, don't expect wood you treat yourself to last as long as pressure-treated lumber.
One warning: Always treat wood preservatives with respect. They are essentially pesticides. Avoid inhaling them, and keep them off your skin and away from children. Be especially careful with PCP; it contains minute traces of dioxin, probably the most poisonous chemical made by man.