If you might experience frost, but not extended stretches of freezing weather, your main concern is protecting valuable plants from predicted frost events. In those cases you gather up all the sheets and blankets or cardboard you can find and cover the most important plants first and keep going until you run out of covers. Besides southern frost events, these tips also are for early spring or fall frosts in the rest of the country. Place the coverings before the temperatures get below 40 degrees to trap heat under the cover. Remove the coverings in the morning after the temperatures are safely above 40 degrees. Don't leave them on too long. If you can use stakes to keep the covers from resting on the plants, the coverings will be most effective. Thin, clear plastic is usually not a sufficient cover.
If water use permits and the temperature is not going below the upper 20s a light overnight sprinkling also can be a good frost deterrent. And, if you get a surprise light frost, a sprinkling in the morning before the sun hits the plants may alleviate frost damage. The warmer water keeps the plant tissues from freezing or helps them thaw gradually, rather than too rapidly.
For those of you in the colder climates where freezing temperatures can last for days, weeks or months the other factors to consider are the types of plants, their cold hardiness and your desire to protect them. Plants that are naturally cold hardy don't need protection from the cold, but they may need protection from freeze/thaw cycles. It is the back and forth that causes the damage. A good mulch to help keep the ground frozen is the best for these kinds of plants. Most perennials, bulbs, roses, shrubs and trees will benefit from a good mulch layer.
In some areas, evergreens, particularly broad-leaved ones may need protection from the sun that warms the leaves too much and is followed by a sudden drop in temperature when the sun goes behind a cloud, building or down for the cold night. A burlap shield usually provides great protection in these cases. Make a frame of stakes as tall as the plant. Place it on the sunny side about a foot away from the plant and cover the frame with burlap
Spraying plants with an antitranspirant/antidessicant can provide added protection. These products place a coating over the needles or leaves that hold the moisture by cutting down the transpiration or drying out.
Another tip to help plants through the winter is to make sure they go into winter with plenty of moisture. They can obtain moisture from frozen ground, but frozen ground repels new moisture. If you have a very dry fall and winter, make sure you water plants before the ground freezes. It might even pay to dig the hose out and water if there is an excess of dry air during the winter.Roses
Roses are probably the most common plant we think about for winter protection. Many varieties are quite hardy and need no protection other than those mentioned above. Personally, I treat all of our roses as hardy. If a variety doesn't survive, I'll try a new one in its place. The over 90 varieties of Buck roses, developed by Iowa State Horticulturist, Dr. Griffith Buck, were bred for cold tolerance. Roses that are grown on their own root stock are usually hardy. Some other varieties, especially grafted roses, are much more tender and need protection to survive. If you're not sure, ask others in your area and learn from their experiences.
If you have roses with special significance that you want to make sure you don't lose, t you'll want to cover them for the winter. The number one protection for grafted roses is to place the plant deep enough so that the graft, or bud union, is a couple of inches below the surface of the soil. The next step is to place a mound of soil six to ten inches high over the base of the bush. Finally, place a one to two foot layer of leaves or other mulch over the rest of the bush. The mulch can be over individual roses or over the entire bed.
If you mulch the roses, you can cut the canes back to about the mulch layer. Don't do much pruning in the fall on roses that are not covered. Leave at least a couple of feet of canes. Then trim to remove all dead wood, and to shape and size the bushes in the early spring.