A touch of frost is good as gold
If you can’t fight it, use it. So set up your garden and plants to benefit from the winter frost.
Just as we get our winter woollies out of storage, nature has its own way of coping with winter. As temperatures drop and days shorten, a plant is primed to respond with changes to its metabolism, anatomy and physiology to prevent frost or chill damage.
Like tiny windsocks, the individual cell receptors within a plant transfer external signals that activate its internal defence system.
One response to dropping temperatures is the thickening of liquid within cell membranes, from a liquid to a gel-like state. This slows everything down, conserves energy and lowers the freezing point, a bit like antifreeze.
At the same time, the cell membranes become more permeable, allowing water to leak between the cells as a kind of buffer. This is the process of cold acclimation.
Can I tell when a frost is coming?
The two main types of frost are advective or radiative, depending on the atmospheric conditions.
An advective frost occurs when cold air from another region sweeps in, usually with strong winds. A radiative frost occurs on clear nights where there is little or no wind.
In the latter, the sun warms the soil during the day, but there is no cloud cover to stop the heat escaping into the upper atmosphere and no wind to mix it up with cooler air, creating an inversion layer. This traps the cold air close to the ground, and if the temperature at soil level is the same as the dew point and both are below freezing, a frost will occur.
Errol Hewett, emeritus professor of horticultural science at Massey University, based some of his studies in the orchards of Central Otago and has published internationally on the effects of frost.
‘‘The heaviest frosts are those that follow low pressure systems. You get a sudden influx of winds from the south, which cools down the soil and air, then it clears, and the sun comes out,’’ he says. ‘‘That night you know you’re going to get a real frost, because there’s not enough heat during the day to compensate for heat lost from the soil at night.
‘‘In autumn this is not such a problem, as the soil is still quite warm, but kiwifruit growers can get hit badly if they haven’t harvested their fruit by June.’’
How can frost be good for my plants?
Deciduous fruit trees benefit from winter chilling, and cold snaps turn starches to sugar in crops such as parsnips, improving their flavour.
Frosts can also disrupt pest and disease cycles, and improve soil structure – when moisture within soil freezes, it expands, and splits open soil particles.
‘‘Deciduous fruit trees require a certain amount of winter chilling during dormancy, which is usually calculated as the number of hours under 7C,’’ says Kate Marshall of Waimea Nurseries. ‘‘Some varieties require a high level of winter chill to enable good bud break in the spring – some Central Otago apricots need up to 1000 hours.
‘‘On the other hand, low-chill varieties for warmer climates like Auckland and Northland only need around 150 hours.
Special low-chill varieties have even been developed for growing in sunny Florida and Israel.