Peter Gibbs recently visited RHS Wisley to explore frost protection measures with Guy Barter, the RHS Chief Horticultural Adviser.
Watch the video here:
Outside in the garden, plants have to cope with whatever the elements throw at them. We are fortunate in the UK that our relatively benign climate means we can use a really wide selection of plants from around the world. But as gardeners we always like to push the boundaries of what is possible, so some of our plants do need a helping hand to get through the coldest winter weather. The term ‘hardiness’ refers to how tolerant a plant is to frost – plants from subtropical regions for example, need a lot of help to get them through the winter. The RHS has developed the ‘RHS Hardiness Scale’, which runs from 1 (very tender) to 7 (fully hardy). Ongoing research at RHS Wisley explores the ability of plants to respond to frost – for example, large succulents, such as cacti and agave, are borderline hardy and it can be hit-and-miss as to whether they survive a winter.
So what can gardeners do to help out less hardy plants?
Some tender plants are taken into a greenhouse for winter at RHS Wisley. However, greenhouses do not provide complete protection from frost as they consist of a single, thin pane of glass. They are excellent for keeping rainfall off the plants, which keeps soils dry, however internal temperatures can still fall below zero degrees, especially as heat escapes during the long winter nights (check your min/max greenhouse thermometer to see the difference in temperature). Greenhouses cannot be completely ‘sealed’ because this can lead to the humidity becoming too high on warm, sunny days. Nevertheless, they can be very effective, and additional frost protection measures can also be implemented. For example, straw and fleece can be added directly to the soil for extra protection. Bubble wrap can also be added to the underside of glass panes to provide an extra layer of insulation, though it will also reduce the amount sunlight entering the greenhouse. Some gardeners even introduce means to heat the greenhouse over winter, such as thermostatically-controlled electric heaters, or gas or paraffin heaters – but care must be taken to control the internal temperature and not raise it too much, which can also have a damaging effect on the plants. A temperature of around seven degrees is adequate, and keeps the cost of heating down. Plants will not, however, grow during winter in a greenhouse, until sunlight levels increase in February/March at which time the thermal gain allows plants to start growing again.
For those plants remaining outside at RHS Wisley, structures are built around certain plants to help to protect them (e.g. palm trees, which are particularly vulnerable since the upper tip protrudes and is more exposed to the elements). Insulation is also wrapped around some plants, such as the core of banana plants. The wrapping is made of either hessian or ground cover fabric, packed with insulating straw, and covered with a waterproof material to prevent rain getting through. Finally, the whole cover is wrapped in chicken wire to prevent animals burrowing in, and the insulation is finished off with a layer of trimmed plant material in order to camouflage the cover. This wrapping works effectively, even in hard frosts. For the home-gardener, something as simple as a double layer of thin horticultural fleece can add 4-5°C of frost protection.
The microclimates in your garden can also help with frost protection. South-facing walls will gather some heat during the day and release it slowly overnight. But any wall gives some protection if plants are placed close to it and this can easily be an extra 4-5°C, or more if there is an overhanging roof or branches. Walls also protect against winds as well as rainfall. The lowest temperatures are usually at ground-level. Plants in pots can be stacked against walls and raised off the floor (using pallets or a bench, for example) so that the coldest air lies beneath them – indeed, it is the ground cooling down that is the real driver for frost on a calm, clear night, as heat is rapidly lost into space, cooling the air above the ground and allowing the development of frost. Furthermore, cold air can pool at the bottom of hills – if the air becomes trapped by a hedge or fence it can accumulate and ‘frost hollows’ can form. This is another aspect of garden microclimates that gardeners need to be aware of, and should ensure there is some way of the cold air to drain away from plants.
Such measures will increase the chance of tender, less-hardy plants surviving over winter. Plants also become acclimatised as they mature, so if a young plant survives the first few years, there is a good chance they will survive unwrapped in following winters.
Podcast 'Gardening in a changing climate':