The concept of the hardiness of a plant is frequently considered purely in terms of the minimum temperature that a plant can survive on a cold night, but in reality it is much more complex than that.
A major factor for many plants is moisture. Some plants that can survive 0F if kept dry will succumb to 15F if their roots are in wet soil. Some cannot stand having their foliage wet outside the main growing season, almost regardless of temperature (anyone who has grown saintpaulias indoors will be familiar with that problem). The age of the plant is another important factor, older plants generally being more hardy than younger one's. Even given adequate age and dryness, many palms can survive happily a very low temperature for one or two nights but would suffer severe damage or death if the cold was maintained for a long time or occurred too frequently, and some can stand it longer or more frequently provided the daytime is relatively warm.
It is also important to realise that wrong feeding can reduce a plant's hardiness. Soft, lush growth produced rapidly, especially if grown late in the summer, is more tender than shoots which grew more slowly and earlier in the year, which have been well ripened/hardened by summer sunshine. Lush tender growth is encouraged by feeding with a nitrogen-rich fertiliser, while hardier growth results from plenty of potash, from tomato fertiliser for example. This is the general rule, but some plants depend for their effect on fast, lush growth, and simply must be given plenty of nitrogen to avoid being stunted (e.g. Musa basjoo). Even so, slowly switching to a fertiliser containing a higher proportion of potash late in the year can be expected to help winter survival.
The general climate of the area is not the only factor affecting the temperature of a plant's environment. The microclimate (i.e. the climate in the precise locality of the plant) is what matters, and although this will in broad terms be determined by the general climate, it can be influenced quite significantly by other matters. Most important is the presence or absence of a frost pocket. If the plant is located at a low point relative to the immediately surrounding area, then in cold, still conditions cold air will flow to it from the surroundings, making it several degrees colder than neighbouring spots. This can happen on a large scale (such as if your garden is located in a hollow surrounded by hills) or on a much smaller scale (when one spot in your garden is lower than others). In the latter case sometimes a solution can be found by making a small hole in a fence or by replacing a section of wall or fence by a hedge, so as to let the cold air flow away to somewhere lower.
The microclimate affecting a whole garden, or a major part of one, can sometimes be ameliorated by screening it from wind. Hedges (or, on a larger scale, lines of suitable trees) are much more effective as windbreaks than walls or fences. This is because the network of branches filters the wind and reduces its force, whereas a fence or wall deflects it upwards for a short distance, after which it descends again in a way that can cause more damage than in the unprotected garden. In a similar way, an individual plant can be screened from the wind, either temporarily or permanently, by surrounding plants or a simple mesh screen. A net curtain thrown over a palnt and tied down can make a big difference to the survival of a newly planted evergreen in cold windy conditions. Wind protection can be worth several degrees change in temperature, as well as helping to avoid dessication when roots have been damaged by transplanting.
The microclimate of an individual plant can also be improved by shelter. In this case layers of horticultural fleece can be surprisingly effective, each layer raising the temperature by a degree or so. Even more protection can be achieved by covering the plant with loose dry material such as straw or bracken, with a waterproof outer layer to keep it dry and prevent it being blown away. The most extreme case I have heard of is a London gardener who protects banana tree (Musa basjoo) stems by surrounding them with whole bales of straw (after removing the leaves from the tree). As a result not only the root but most of the stem survives, growing bigger each year and eventually producing flowers and (inedible) fruit. More common, and less unsightly and less effective, treatment for this plant is to cover it with an old chimney pot stuffed with straw, bracken or dry leaves, with a waterproof cover on top.
Another important way of helping particular plants is to grow them at the base of a south or southwest facing wall. The brick or stone absorbs heat during the day and radiates it again at night, warming adjacent plants. If the wall is that of a house this is even better, of course, because of the extra heat generated inside the house for the benefit of the occupants. Even growing a plant on a south facing slope helps because the soil then absorbs more heat from the sun during the day. East facing walls should be avoided, because much damage to plants can be caused by too rapid thawing in the morning sun. Some early spring and winter flowers are particularly prone to this form of damage.
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