It is the lack of through-breeze that is probably the most significant factor in whether a frost will settle in your garden. My former garden in the Clare Valley of South Australia is situated in a low lying pocket in a region that is relatively high in altitude. Because of the topography of the region, this garden is situated in a dumping ground for gale force winds from the west. In order to reduce the impact of these winds, windbreaks had to be planted along the western boundary.
Unfortunately this also reduced the through-breeze, with the frost settling more readily in such a garden, than maybe the garden next door which was left more exposed. Because this former garden experienced frosts as low as -10ºC, the objective was to manage the frosts through wise plant selection while limiting the gale force winds, desiccating to an exposed garden.
It is remarkable how the canopy of trees can have a profound effect on air movement – even the framework of deciduous trees is better than nothing. Not only is this framework essential in a summer garden, especially for those further inland, but this framework also proved its worth in the garden over the years during winter.
Under-planting beneath these trees in garden beds exposed to the east is more likely to be frost affected than those facing west. This is because the easterly facing areas thaw more quickly than on the opposing side, and it is the quick thaw from the sun that often causes damage to plants.
Frost damage is more likely to occur when the plants have been well irrigated in autumn, and even during dry winters. When nature is left to manage on its own, it is remarkable how effective this can be we just need to be more observant of how this is being done.
The leaves of plants are full of cells. When these cells are full of water (following supplementary irrigation), it is this water that swells when frozen. If the cells are kept dry, as nature intended (when it doesn’t rain for instance), there is no water to freeze and hence swell and so the damage is significantly reduced.
‘Black frosts’ are far more damaging than the rime frosts that are more evident. These ‘black frosts’ are prevalent when drought is impending and are usually associated with a dry winter. Nature intended the affected areas to remain dry for a reason, so that the damage to the plants’ cells is reduced.
‘Black frosts’ are often not noticed until a few days later when a particular plant can appear blackened. Vehicles that are left out overnight will have ice on their windscreens whereas none will be evident on the ground in such a frost. They are always higher off the ground with the damage occurring above ground level. Don’t irrigate your garden in frost prone regions during winter – it is dry for a reason.
Another very important strategy to consider is how you irrigate and manage your garden during the growing season. In frost prone areas, it is essential to “harden” the plants leading up to winter. The best way to do this is to irrigate your garden very infrequently, say every 4 weeks, but for at least 3 hours at a time. This is the best way to harden the wood of susceptible plants, and it is remarkable how well this strategy works.
Once the night temperatures drop in autumn allow nature to take its course. If your garden needs extra water at this time, it is suggestive that you have chosen the wrong plants for your site which are not compatible with the climate, the soil or both. New plantings can be hand watered to get them through til winter sets in.
Gardening is not hard if you allow nature to have more control. The role of the gardener is really only to observe, and give a guiding hand where necessary. The sustainable garden isn’t heavily pruned or controlled in any way. It can still have its formal elements, but even these are more natural and uncontrived. Leave nature to take control and enjoy the pleasure of observing.