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Up to recently, when severe frosts were a
common winter occurrence, it was a widespread practice to
protect plants from frost. Stems of perennial plants were
frequently wrapped with straw, and a plastic covering placed
over this, to insulate against the worst of the cold. These days
however, people have less time to spend looking after their
plants, and with milder winters, frost damage is much less
common than years ago. However, milder winters also have another
effect, and this is one that concerns fruit growers. When
weather is mild in January and February, trees begin to grow
earlier in the spring. This is turn means that they blossom
earlier, and this is what can trouble growers. If an apple tree
blossoms in early May, the likelihood of a severe frost damaging
the blossoms is small. If however, the tree blossoms in mid
April, and this is becoming more common with earlier springs,
then the risk of a cold night and frost-damage is much greater,
because blossoms can only withstand one or two degrees of frost
before they are killed.
Because of this increasing risk of frost damage, growers are
looking for ways to ameliorate the damage. Many possibilities
have been tried over the years. In the vineyards of France,
smudge pots were placed between rows of grapes, and waste oil
burnt in these. The thick smog created in this way was somewhat
effective, raising the temperature by one or two degrees, but
this method is now considered environmentally unsatisfactory. A
more modern way to achieve the same effect is to use a
tractor-mounted gas burner, and drive up and down the rows of
trees at night. Some people do use this method, though proof of
its effectiveness is limited. In the United States, wind
machines are used. These work on the principal that hot air
rises, as is evident when we look at a hot air balloon. Because
the hot air rises, cooler air falls in to take its place, and in
certain environments, the area of air twenty metres above the
apple trees can be substantially warmer than that at tree level
or ground level. This temperature differential is used by the
growers, who, when the temperature in the tree zone falls, turn
on their wind machines. These mix the warmer air from the higher
areas with the cold air at ground level, and can give a rise in
temperature of a few degrees; usually just enough to protect the
fragile blossoms. Unfortunately, in this part of the world, we
seldom see temperature inversions such as those found in the
apple growing areas of the US and so this method has not been
adopted here. The last, and possibly most interesting way of
protecting against frost is used in the Netherlands. This relies
on irrigating the trees.
On a cold night when the temperatures drop to freezing point,
the grower turns on the overhead sprinklers. The water from
these may warm-up the trees, but on very cold nights an
interesting thing can happen. If the temperature drops well
below zero, the water applied from the sprinklers freezes on the
trees. However, the act of freezing actually releases heat (in
the same way as for ice to become water heat must he added, when
water turns to ice heat is released), and this heat prevents the
ice (and the plant) from dropping below 0°C, even if the
surrounding temperatures are significantly lower. This system
will work perfectly, as long as the grower can keep adding water
(which will continue to freeze), or until the air temperatures
rise above freezing again. However, this system requires vast
quantities of water, with reservoirs (or canals) containing
millions of litres of water for just a few acres. This does not
pose a problem in many parts of the Netherlands, but the storage
of huge quantities of water on the off chance of a frost is an
expensive option. This is probably why there is only one grower
in Ireland (located in Co. Armagh) with such a system. For
everyone else, there is the usual nervous wait between blossom
time and early May, when the risk of frosts has passed.
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