Garden doctor: tackling frost
Nick Bailey reveals how to guard against the damaging effects of frost
A dusting of frost sprinkled across the garden might look beautiful, but it’s potentially devastating to plants. As temperatures drop, frost can lead to leaf and stem damage, heaving of the plant crowns (when cracks in the ground expose roots to air) or even plant death. But with the right plants and the correct protection it’s possible to deal with another beastly blow from the east relatively unscathed. There are three main forms of frost that can damage plants – air, ground and hoar. Air frost is when the air temperature falls below the freezing point of water. This type of frost is exacerbated by the wind and can burn foliage and kill plants. Ground frost is the formation of ice on the ground or objects that have a temperature lower than the freezing point of water. It has the potential to kill roots, tubers and bulbs as well as cause frost heaving. Hoar frost is when the surface temperature falls below freezing point before dew begins to form.
How to protect from frost
The places that generally get the most severe frosts are upland locations, high elevations and frost pocket valleys. In terms of individual gardens, the following settings are often the most vulnerable: exposed plots, valley gardens, walled gardens (that cold air can’t escape from) and plots on a slope with no exit for dropping cold air. In extreme situations, UK frosts can strike from September to June. Here is a rough guide to the last annual frosts: coastal areas 1 March-30 April; inland areas 10-20 May; the Highlands 1-30 May. Protecting plants from damage is relatively straightforward. Below ground plants such as dahlias and bulbs can be lifted out of the ground over winter or protected with a deep, dry mulch. Vulnerable evergreens and shrubs can be wrapped in fleece (with good ventilation) or boxed into homemade polycarbonate frames. Tender plants should be lifted before the first frost and only planted out after the last predicted frost of spring. Container plants are best placed in a sheltered spot, while tender perennials such as penstemon are protected from frost by their own dead stems if you avoid pruning them back until spring. It’s worth considering the situation of plants. Early flowers such as camellia and magnolia are often damaged as a result of thawing too quickly after an air frost. The simple solution is to grow them against a west wall. The RHS has developed a hardiness rating chart to help gardeners make sensible plant choices. The rating scale starts at H1a, which indicates plants that require a +15ºC glasshouse situation year round, through to H7, which specifies plants that will tolerate -20ºC. By using this online guide − go to: bit.ly/rhs-hardinessrating − and a thermometer to check microclimates in your garden, it’s possible to make safe plant choices. Fail-safe plants include cornus, geranium, symphyotrichum, Anemone x hybrida, bergenia, digitalis, Rosa rugosa and aquilegia. These plants can cope with -20ºC! But you can grow a much wider range using a few tricks to keep your garden a little warmer. Shelter belts of trees or shrubs radically reduce wind chill, and if your plot is walled or fenced make sure there is a low-level escape point for cold air. So, armed with a mix of protective techniques and the right plants, it’s possible to shield the garden from Nature’s wicked winters.
NEXT MONTH: Nick Bailey explains how to improve seed germination rates