Plants outdoors in pots, tubs, barrels, window boxes and other containers are likely to be killed by winter weather unless given protection.
The reason is that the container and consequently the root system are above ground, surrounded by the cold, circulating air, rather than in the relatively warm, insulating environment of the soil.
The temperature inside the container will be the same as the air temperature. If the air temperature is 5 degrees F., that's the temperature the roots are subjected to; if the roots were in soil, the temperature three inches below the surface would be about 21 degrees F., according to research at Cornell University.
There is another factor, according to Cornell specialists: The roots are significantly less hardy than the aerial portions of the plant. For example, the above-ground portion of Laland firethorn (Pyracantha coccinea 'Lalandii' ) can survive -15 degrees F. when fully acclimated, but mature (woody) roots are killed at 2 degrees F. and young (feeder) roots cannot survive 22 degrees F.
When field-grown, the low hardiness of the root system isn't a problem because the soil temperatures do not get low enough to cause injury. But when plants are in containers, the root systems are exposed to damaging temperatures.
In addition to freezing injury, low container temperatures can cause desiccation damage. Once the water in the container soil is frozen, conditions are the same as having a dry soil. Obviously, ice cannot be taken up by the roots. Watering won't help because the extra water will freeze also.
The aerial portions of plants -- especially evergreens, but deciduous species also, to a lesser extent -- are continuously transpiring (giving off moisture), and this condition can lead to dessication.
One of the things that can be done with plants in containers is to store them in a garage or basement where temperatures range from 25 degrees to 45 degrees F.
Mulching of containers that have been moved close together is the only protection required for many hardy species. This provides a buffer against cold soil temperatures but does not prevent them from going below freezing. The soil temperature in heavily mulched containers has been found to be 20 degrees higher than in unmulched ones.
Probably the best protection of all is to lay upright plants on their sides as close together as possible, so the foliage will be on top of the adjoining container, cover the plants with duPont microfoam, cover the microfoam with clear plyethylene film and seal the edges with soil. It does the job. Q: My pointsettia is getting 14 hours of continuous darkness at night and is looking good. Should I fertilize it? A: If the plant was fertilized generously during June, July and August, and the leaves have a nice green color, it probably does not need fertilizer. Otherwise, fertilize it lightly every week until it blooms, following directions on the label for application. Q: how long will my apples keep? A: Varieties that mature in September, such as Grimes Golden and Jonathan, will not keep long. Late-maturing varieties such as Golden Delicious, Delicious and Staymay will become overripe in three to four weeks if temperatures are not below 50 degrees F. Q: My black walnut trees have nuts this year for the first time. How can I tell when the nuts are ripe and how should I handle them? A: Walnuts are allowed to mature fully on the tree. They will fall naturally or with mild shaking when they are ripe. As the harvesting season approaches, the ground under the tree should be cleared of weeds, trash and prematurely dropped nuts. Pick up the walnuts soon after they fall, while the hulls are still fresh and green. Hull and wash the walnuts immediately. If the hulls are left on too long, the kernels will be stained and have a strong flavor. After hulling and washing, spread the walnuts in thin layers in a cool, airy place to dry and cure for about four to five weeks. They store much better in the shells than as kernels.Dry walnuts will crack more easily if the shells are first dampened. Soak the walnuts in water overnight and crack them the next morning. Q: When is the best time to take cuttings for rooting apple trees? A: According to specialists, it's not a good idea to root cuttings of apple trees. It's best to graft the apple tree, using a water sprout. Do this just before the tree breaks dormancy in the spring. Q: Our dwarf Japanese red maple is red and beautiful in the spring but turns green in early summer and stays that way the rest of the season. Is there a way to make it stay red all summer? A: Most red maples, including the Japanese, are that way. It's their genetic makeup and there's nothing that can be done about it. Q: I have two miniature roses that were given to me; they are growing outdoors planted in the ground. What should I do with them this winter? A: They should survive the winter outdoors, in fact are more hardy than standard hybrid teas. You can also grow them indoors under fluorescent lights. Q: I dug a small persimmon tree from a field 15 years ago and took it home and planted it. Now it is a large tree, nice-looking, but it doesn't bear persimmons. Is there anything I can do to help it? A: Like the American holly, the native persimmon has female flowers on one tree and male on another. If you have a female tree it needs a male nearby to provide pollen, and if it is a male it does not bear fruit. Q: My spruce trees lost a lot of their foliage last month. What can I do to help them? A: During late September and October evergreens lose some of their inner and older foliage. This occurs in pines, spruces, arborvitae and junipers, and is a natural phenonenon. Each year they get a new set of needles. It can be assumed that as long as the new terminal or current year's growth is fresh and vigorous, the life of the tree is not in danger. Q: Last spring we bought sunflower seeds to plant to feed the birds. The plants came up and grew, but the seeds were not any good -- they were hollow. What went wrong? A: Moth larvae sometimes damage the seed heads, spittle bugs sometimes cause some damage. Next spring plant a variety intended mainly to produce feed for birds, such as Mammoth. Q: Our pear trees had a lot of fruit this year. What is the best way to store it for later use? A: Pick pears when fully mature but still hard and green. Let them ripen by storing at 60 degrees to 65 degrees F. immediately after picking them. Keiffer pears will ripen in two or three weeks, Barletts somewhat faster. After ripening, the pears should be placed in a refrigerator and kept as close to 31 degrees F. as possible. You can prevent shriveling by putting the pears in perforated plastic bags.