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What is a night-frost

To prevent damage as a result of a night-frost, one has to understand which factors are of influence.

As a rule it is accepted that no night-frosts occur after 15 May, it has happened. At the end of May 1990, a severe night-frost created substantial damage, this after a very mild winter period. At the testing ground all young shoots suffered.

Weather-experts prefer to use the term 'Frost at ground level', as this is the best discription one can give for this phenomenon. The air temperature, just above the ground, is reduced. Higher in the air, this does happen to a much lesser extent, and at 150 cm above ground-level is seldom a frost to be noticed. Only a severe night-frost would see temperatures at below freezing, at this level.

The sun heats up the soil during clear and sunny days. At night, when there is no cloud over, the heat escapes as 'radiation'. The greater the warmth is during the day, the greater the radiation, should the skies be devoid of clouds.

Nachtvorst 33 KB.jpg (33072 bytes)

From within the ground, warmer air tries to compensate for the radiation (refer diagram above). Hence the soil itself will seldom if ever freeze.

The temperature just above the ground, however can be lower than freezing-point. The top layer of the soil extracts heat from the bottom layer of the air. When there is little wind, there is then little compensation from elsewhere. A light night-frost has therefor only below freezing point conditions at ground level. Usually it occurs in the early hours, as the supply of warmth from within the soil is exhausted.

The factors that contribute to a night-frost

- clear skies and lack of wind; ;

- worked soil, and 'open' soils as peat, will suffer from a night-frost, and a clay soil much less;

- when the soil is dry, a night-frost will happen rather than a moist soil;

- as the cold air drops to the lowest point, gardens at a lower level are more susceptible to a night-frost, than those gardens at a higher level.

Preventive actions to avoid damage from a night-frost

* When the forecast is for a night-frost, apply a cover of light cover, to create an artificial cloud-cover. Think here off bubble-plastic or other opaque (thus never clear) layer of film, secured to the ground with some bricks. In this way radiation is reduced.

* Wetting the soil is the other possibility as a moist soil radiates more heat, from within the ground. The cold air is thus better warmed. This method was once used by the working group, before an expected severe night-frost of 28 April 1991. Plants to be used at the Floriade '92, were left outside under a small 'tunnel cover'. Damage was limited as the soil underneath the tunnel cover had been well wetted.

Ground without any cover warms up easier, during the night it reduces less. A cover of leaves should thus be removed only at the end of April. When weeding, the soil is loosened up, and thus loses some of the insulating properties. It is better to delay weeding till the end of April.

* A rough surface has a larger surface area and thus is prone ti grater heat loss. Working the soil should thus only be undertaken, once you are sure night-frosts will not happen again.

* Another method is to maintain a fire that is smouldering and smoky. The neighbours will not be too happy, as you create this artificial cloud over, thus reducing radiation.

'Gelderse Fuchsia Info-site' - November 2008