Wintry weather is upon us and we watched our gardens succumb to the cold early this year. No gardener likes to see his plants freeze with the onset of cold weather especially the premature onset of cold weather. Is there anything that can be done to at least delay the loss of plants to an early frost or freeze? The plants can of course be covered but this probably makes more sense in the spring than in the fall. While it is possible to cover an emerging plant with a pot or bucket in the spring it is far more difficult to cover a large plant in the fall. There are other techniques involving air movement and watering that can help. The work involved may be justified in the spring when the plant will be enjoyed all summer but employing these methods in the fall may give you only few more weeks and this year with the sustained cold it probably would have bought no time at all. A better option is to plant smart to prolong your garden.
First it helps to understand the difference between a frost and a freeze. A freeze is easy. It occurs when the temperature drops below 32°F. A hard freeze or killing freeze (often called a killing frost) is when the temperature drops below 28°F. This low a temperature will cause the water in the cells of a plant to freeze and the cells will rupture. Place a jar full of water in a freezer with a lid and you get the same effect. The water expands and either the lid will be forced off or the jar wilt break. There is no coming back from this and the plant will die or go dormant for the winter.
A frost is different. It is a light coating of ice on the surface of the plant. This occurs when the moisture content of the air is high and the air cools to the point where it cannot contain the moisture. In warmer temperatures the moisture coming out of the cooling air would form dew. When the temperature is hovering around the freezing point ice is formed. Various conditions that affect localized temperature results in patchy frosts. A plant may well survive a light frost as the water in all of the cells does not necessarily freeze and the cells remain functional. A few leaves may be lost but usually only the top most leaves will be affected. The wise gardener will leave these in place and they will continue to protect the lower leaves.
There are several theories that have been put forth to explain why plants like succulents, broccoli, parsley rand evergreens are able to survive heavy frosts and even freezes. One is that they produce antifreeze like substances that prevent the water in the cell from freezing. These substance lower the temperature at which the liquid in the cell freezes. If the water in the cell does not freeze the cell walls do not burst and the cell remains viable. Some plants seem to be able to bind the water in the cells to other constituents to prevent freezing. Yet other plants manage to contain the ice crystals in the cell walls. As long as they do not enter the central portion of the cell they do little damage.
The best solution then to keeping a garden at least growing if not flourishing as long as possible is to seek out the plants that are able to deal with lower temperatures. The frost resource is to check the tag on the plant or research the plant’s hardiness. This has nothing to do with the zone classifications which depict the lowest and highest possible temperatures that the roots can survive. For those who wish to keep their yards green as long as possible the criteria must be for the ability of the plant to keep its leaves green. Obviously, the first category are the evergreens. Pines, spruces, hemlocks, arborvitae and their ilk stay green all winter. Sedums and other succulent ground covers retain their color and form. Bugleweed is another ground cover that survives the snow. Hollies and many viburnums will add color to the landscape all winter. Those of you who grow hellebore have probably noticed that they also retain their leaves until the new one pop out in the spring.
Fully hardy plants are plants that will retain foliage when temperatures go well below freezing for short periods of time. Flowering cabbages and flowering kale can tolerate temperatures as low as
5 °F. They do not even begin to develop color until temperatures dip below 50°F.. Catmint is another plant that will retain its silvery green foliage well into the winter. Many of the artemesias are also extremely cold hardy.
The next category are frost hardy plants or cold hardy annuals. These are plants that can take a light to heavy frost. As far as perennials go these are usually plants that bloom in the fall such as aconite and asters. There are also annuals or plants that are treated as annuals that will survive a light to moderate frost and temperature as low as 23°F. Geraniums and petunias will continue to bloom after a light frost. Pansies are not much bothered by frost or low temperatures. Annual dianthus is quite hardy in the fall and may even bloom better at lower temperatures. Sweet allysum is another hardy annual.
Half hardy plants will survive temperatures that hover around 32°F.. The term is usually used for annuals as in half hardy annuals. This is a fairly large category of annuals and these are the are desirable for planting in areas need to retain color as long as possible. They are great for permanent beds and decorative plantings. Half hardy annuals include calendula, larkspur, bells of Ireland, stock, verbena and nasturtium. The lists of half hardy annuals and hardy annuals may overlap. The hardiness of a plant is often determined by where it is planted. Trees for instance will often prevent the forming of frost on plants that are under them as they radiate heat as the temperature cools.
Tender plants will succumb to temperatures around 40°F. They are frequently tropical perennials that the northern regions grow as annuals. These usually perish at the first frost They include begonias, impatiens, cockscomb, marigolds, zinnia and other common bedding plants.
While the best way to prevent the premature demise of a fall garden is to plant cold hardy plants IE plants that will survive low temperatures and frosts at the foliage level there are a few other tricks that might be helpful. Preventive measures work best in a fall garden as the things used in spring are usually more labor intensive and not practical in the fall for the short term results that they bring.
The location of a garden is important. It is colder at the top of a hill and as cold air sinks low areas are also more likely to be coder. A south slope is the best location for a frost free garden. If there is a frost pocket in a yard that is not the place to put in tender annuals. Planting near he house or other structure will often protect plants from frosts. Even the heat radiated by a stone wall or fence can be helpful. Group plants around trees or shrubs for the heat that they radiate at night. Sometimes just a few degrees will make a difference.
Do not plant variegated or gold leafed varieties of annuals as they tend to be more susceptible to the cold. Bronze leafed or maroon colored leaves are less likely to freeze as they absorb more heat. Fuzzy leafed plants are more cold tolerant than smooth leafed plants and narrow leafed plants work better than large leafed ones. Don’t encourage new growth by pruning or using high nitrogen fertilizer in the fall. New growth will freeze faster.
A little thought and research goes a long way when landscaping a yard or planting a garden. It is important to know what will grow where and how it will survive. Using plants that will stand up to less than ideal weather conditions will not only prolong the life of a planted area but will cause less work for the gardener. Read those tags and google that plant!