The best way to protect plants from frost comes down to being aware of any pending freezes, knowing which plants are particularly vulnerable to damage, and using a suitable covering material to protect them. Generally speaking, you should start to cover plants in the late fall, and use breathable fabric to wrap them. More delicate species may need loosely packed soil instead. Most plants respond well to covering, but particularly delicate ones may not survive the winter outside, even with the added insulation.
When to Cover
Frost generally happens when temperatures drop below 34°F (1.1°C). The best time to cover plants is after a few early freezes in late fall. This allows leaves to fall and the plant to naturally move into winter mode. Covering them up too early could increase the number of times they have to go through dangerous cycles of freezing and thawing. If a plant starts to get black, or if its leaves and buds start to get mushy, it may be too late to save it.
Covering Basics
When there is a threat of frost, cover your plants before sunset. Wrap as breathable a material as possible around the plants — like burlap, linen, newspaper, or old bed sheets — then secure it against the wind with heavy rocks or clothespins. It's best to avoid using plastic or vinyl sheets alone to protect plants from frost, since these materials can trap condensation inside, which can freeze and damage the plant. Plastic can also get very cold, and transfer that cold to the plant. If you do want to use plastic, put a fabric sheet underneath it.
For smaller seedlings or flowering plants, you can cut the bottoms off of milk or soda jugs, remove the caps, and then place them over the plants to make individualized greenhouses. This keeps the plants warm, and still allows any condensation inside to escape out the top. Alternatively, you can dig them up and move them to a container inside or take cuttings of them and keep them to re-plant in the spring.
Commercial coverings designed specifically to protect plants from frost also are available, but they generally work about the same as burlap or bedsheets. If you have plants that you particularly value, you may want to make a temporary greenhouse for them. You can do this by placing a large opened ladder over the plants, and then wrapping the ladder in breathable fabric.
If the temperature is expected to rise the next day, you should uncover the plants in the morning so that they can warm up and be exposed to the sun, and also so that any moisture that did build up overnight can escape. Keeping the plant covered during the day can also allow the temperature under the fabric to rise too high — especially if you're using plastic — causing additional damage. When temperatures stay low, you can leave the plant covered.
Container Plants
Whenever possible, it's best to move container plants inside a home or into a greenhouse or other protected area to protect them from frost. If this isn't possible, moving the pot up against the side of the house or a shed can offer some protection. Container plants can also be covered or wrapped, and an extra layer of insulation around the pot can be especially helpful to shield the roots. If possible, you should lift the containers off the ground to help the soil drain well and surround them with jugs of hot water to keep them warm.
Other Strategies
There are also things you can do before the threat of frost to help your plants survive, such as fertilizing them in the late spring. This provides the plants with the nutrients they'll need through the winter. Ensuring that they are disease and insect-free also will help them survive the winter. Don't over-fertilize them, though, or prune away anything other than dead branches and leaves. This can create new growth, which makes a plant particularly susceptible to frost.
Plants like roses and strawberries need special care. To give them the best chance of surviving the cold, wait for a few early freezes to ensure that the plants have dropped their leaves. Then, tightly pack mulch or dead leaves around the rose's upper trunk, called a root graft or bud union, which, if damaged by frost, can kill the whole plant. For strawberries, you should put mulch over the plants to protect early growing buds that will someday be fruit.
Most Vulnerable Species
No matter how much advance preparation is taken, a cold-intolerant species is unlikely to survive in too cold an environment. The agriculture departments of many countries publish plant tolerance guides that can help you choose plants that will survive the colder months in your area. The temperature zone might have nothing to do with whether a plant survives a frost, though. Flowers that bloom in the early spring are more likely to be damaged by a late frost, but if the roots are still alive, they could come back later in the year or in the next. Soft woods, actively growing bloomers, and potted plants are naturally more susceptible to harm. New plants or sapling trees also are at a greater danger than their more established neighbors.