The surprising ways frost can help a gardener – صحيفة الصوت

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.

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At the same time, the cell membranes themselves 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 explains. “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 7°C,” 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. (Read Marshall’s eight tips for getting fruit trees through winter.)

“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. Cold temperatures also assist with the ripening of some fruit varieties, like astringent persimmons.”

Should I stick to frost hardy plants?

“The main damage is caused by the formation of ice crystals in the plant tissue and sometimes even within the cells of the plants,” says Dr Tim Jenkins of the Centre for Sustainable Agricultural Technologies at Lincoln University. “Damage from the crystals can be sufficient to kill plant tissue directly or lead to moisture loss from cells and tissue.”

Leaves may go limp, or turn black, as if they have been burnt, and buds and stems may show damage too. How much damage occurs depends on the plant. While some plants will die, others may only die back and recover when warmer weather sets in.

Plants that are originally from cold areas generally have some frost hardiness, while some tropical plants receive chilling injury at 12°C and succumb to even the slightest of frosts.

Hardiness can also be improved by high concentrations of soluble nutrients including potassium and sugars. Some plants also have physical protection such as thick bark, furry leaves or a canopy that protects the inner shoots, according to Jenkins.

Annual edible crops could be protected under cover.

How can I prevent frost damage on fruit trees and treasured plants?

So what’s the best way to prevent frost damage? According to Jenkins, row covers work well in frosts after a cold, clear night, trapping heat from the soil to keep plants warm.

But covers and frost cloths must be made of a lightweight fabric that allows sunshine to recharge the heat source for the next night. In emergencies, blankets and sheets can be used, but make sure you remove them during the day.

Avoid mulching around sensitive plants during frost risk to allow the bare soil to absorb the heat during the day and radiate it out at night, affording some frost protection. “Good levels of compost can help soil heat absorption during the day,” says Jenkins.

“Watering the evening before a frost also increases the heat given off by soil during the night. And watering foliage in tiny amounts (no more than a few millimetres per hour) in nights when temperature falls to zero or less can help protect against frost damage.

“Water must be applied continuously; as it freezes, energy is given off to maintain plant tissue temperature at 0°C, which is above the plant freezing temperature. Watering a frost cloth on a really cold night can provide an ice blanket that further insulates the plants.”

You should also ensure good plant nutrition, and allow plants such as vegetable seedlings to harden off in a shade house or under a cover outside before planting out in cold times of year.

But also think when to plant:

• It’s the late frosts that home growers should fear most, says Hewett. Plants emerge gradually from dormancy in spring, but they can still respond quickly to sudden spikes in temperature, say in early August.
• This triggers de-hardening, and that’s when they’re most susceptible. Buds form, blossom and bees come out, and fruit are produced eventually – a process to help them regenerate.

“Fruit trees can’t cope with the unexpected drop once de-acclimation occurs. With apricots we used to lose 60 to 70 per cent of fruit buds in early spring, but this was still enough to give a good yield. Trees can acclimate over a week or two, but not within hours.”

“It’s best to plant frost tender plants in late spring so that the tree can be well settled and hardened off before the cooler temperatures of autumn and winter,” says Kate Marshall. “And don’t fertilise after late summer so that there isn’t a lot of fresh tender new growth going into the autumn.

Can I save plants and trees after they suffer frost damage?

Maybe. If a shrub or tree’s stems or branches show frost damage, don’t remove the damaged wood too quickly. Allow time for possible recovery when the weather warms up. Any frost-killed branches will be apparent later on, and can then be removed.

The most effective weapon is preventative action, so watch the weather forecasts and be handy with the frost cloth. Of course, frost can occur in one garden, and not in a neighbouring one:

• A protected courtyard next to a house will be warmer than a large open garden.
• A row of trees can act like a dam, preventing cold air draining away.
• Large bodies of water, such as ponds, lakes and sea, can moderate the air temperature of adjacent land.
• Water can store a great deal of heat and air close to it does not cool as quickly.
• Urban areas tend to have fewer extreme temperatures than open countryside, because buildings, pavements and roads all absorb heat from the sun during the day and radiate it back into the air at night.

Bear in mind, however, that barriers such as walls, fences and hedges, which can protect plants from wind, can also create frost pockets if they block cold air drainage from your garden. Make sure your fence or hedge has an opening through which the cold air can escape.

Read more about how to winter-proof your garden and keep plants alive.