Frost can be a tricky thing for the permaculture garden. For many plants, an unexpected frost can cause damage to the plants and even cause a crop to fail. Other plants, in contrast, actually benefit from a frost. The flavor of broccoli, for instance, actually improves if the plant has experienced a frost, while carrots get sweeter as the temperature drops.
It can seem daunting to try to plant all your vegetables and fruits at just the right time so they can be harvested before, during or after the first frosts of winter depending on their individual needs, and many gardeners have a lot of anxiety about protecting those plants that need it from the negative effects of a frost. There are these days, with constant developments in technology, much more accurate ways of estimating when the first frosts in a particular area will arrive. This draws on knowledge we have about the way global weather systems move and interact. However, as with many things on the permaculture plot, such estimations can never be an exact science. Local factors such as aspect, topography, temperature and human activity can impact upon the timing of a frost’s arrival.
But there are certain methods that the permaculture gardener can use to try and predict whether a frost is likely to settle on their plot. These are techniques that use one of the primary tools in the permaculturist’s arsenal – observation.
It may sound simplistic, but one of the best ways of determining of a frost is due overnight is to gauge the temperature. The temperature information published by meteorological organizations is typically measured more than a meter from the ground (to try to give an accurate average over a locale which has topographical variation). Many of your plants will be lower to the ground than that so use a thermometer to check the temperature around at-risk plants. This is one reason why raised garden beds can be a good design feature if you are in a location that is prone to frosts. The other factor that will influence the temperatures across your site is microclimates. Permaculture gardeners make use of a variety of techniques to influence microclimates in different locations across the plot, and these can influence whether a frost forms or not. For instance, areas that are adjacent to a concrete wall that has been exposed to the sun during the day are less likely to experience a frost, as the wall will radiate latent heat during the night. Likewise, plants that are close to a body of water such as a stream or pond are less likely to get frost covered, as the water releases heat absorbed during the day.
Observe the sky. If it is cloudy, the chances of a frost settling are reduced. A thick covering of low clouds will reduce the risk of frost as it prevents the radiation of the heat the Earth has absorbed during the day back into the atmosphere. It acts rather like a blanket, keeping the heat nearer to the ground and so raising temperatures.
A windy night is also likely to reduce the likelihood of a frost. A brisk breeze will cause different parts of the air column – the colder parts lower down and the warmer areas higher up – to mix. However, be advised that a strong wind may in fact increase the chances of a frost forming as it sweeps warm air that is closer to the ground away and replaces it with cold.
Cold air is denser than warm air and so it sinks (while warm air rises). That means that if your site is at the bottom of a slope or in a valley, and there is no wind, it is more at risk of a frost, as the cold air will move down the slope and pool at the bottom. This effect can also happen on a smaller scale across your site if you have a variety of elevations.
Length of Night
Put simply, the longer the night, the more likely a frost. Energy losses from the Earth always exceed gains during the night, and so when the nights are at their longest during fall and winter, there is more time for the energy losses to accumulate, lowering the temperature and cooling the ground.
The Dew Point refers to the temperature at which the air is no longer able to ‘hold’ all the moisture within it. The moisture then condenses and a dew forms on the garden. When dew forms, the heat within it is released, keeping the air temperature at or slightly above the dew point, and thus preventing a frost from forming. So the more moisture in the air, the less likely a frost. This is why a light watering of the garden a day or two before a frost is predicted can help stop it settling. It is in this regard that trees can assist in preventing frost. Trees transpire a lot of moisture through their leaves, so having a lot of trees helps keep the air temperature around the dew point.
Having conducted observations such as those outlined above, and determined that a frost is more likely than not to form overnight on your site, the best way to protect your plants is to cover them. You can use old blankets, sheets or burlap sacks. Choose a material whose weight is appropriate to the fragility of the plants being covered, so you don’t damage them, and drape the material loosely over the foliage, securing it to the ground by weighing it down with bricks or stones. If you have a day or two before the frost is expected, watering the soil around the plants can help, as water holds heat better than soil. However, avoid soaking the ground as this can lead to the water freezing within the soil and damaging the roots. One common permaculture technique that can be a longer-term strategy for preventing frost is to mulch your garden beds. Mulching with materials like straw, pine needles and wood chips helps preserve heat and moisture and so prevents frosts forming.