Apples in The Bahamas?

Mud-free dog

Is there a better sound in winter than crunchy footsteps on early morning dogwalks over heavily frosted grass? It's a welcome change from squelching in mud, with the added benefit that the dog might be a mud-free zone for a day, but it also means that we're racking up chill units.

We can all appreciate the significance of a mud-free dog; not having to bathe the reluctant creature in luke warm water and no damage limitation manoeuvres as she shakes water off herself and onto everyone and everything else. But what on earth is a chill unit?

Like teenagers and grizzly bears (what encouraged us to pair those two?) fruit trees need sleep. A lot of it. In effect, they need to hibernate and that only happens if it gets cold enough for long enough.

It starts when temperatures consistently fall below 7 degrees Celcius, but to begin with the process is reversible - a bout of warm weather in the autumn and the tree will wake up again and the whole thing has to begin again. In simple terms, apple trees need between 750 and 1000 “chill units” each winter, one chill unit (CU) being one hour where the temperature is between 0 - 7 Celcius, with some varieties needing more, others less.

The sharp eyed will have noticed that temperatures at or below freezing point don’t count towards chill units, so frost-forming hours may not themselves be that helpful. Correct - except that cold air falls and frost on the ground doesn’t necessarily mean freezing air 1 to 2 to 3 metres above ground, where the tree’s leaves and branches are. And, more importantly, in order to get to freezing the temperature will fall through the critical 7 to 0 Celcius range, and do so again in reverse as the air warms up again the following day, or whenever. A heavy frost has a lot of chill units hanging on its shoulders.

The chemistry behind this is that the cold breaks down the hormone responsible for dormancy; not enough CUs and there is still some sleepy hormone around come springtime, so the tree becomes a lazy teenager and doesn't wake up properly. The buds formed the previous year may not flower, leaf development may be inhibited, blossom will bloom irregularly but for longer which, in turn, exposes the tree to disease. Fruit production, inevitably, is affected. So, that's why we like cold, frosty weather; we're acquiring chill units (as well as chillblains??) and it's reassurance that vernalisation - the emergence of flowers in the Spring after prolonged cold weather - will occur is it should.

What we’re hoping for in the Spring.

Most apple varieties need 1,000 CUs, but some need less. Gala, that supermarket favourite, needs around 700-800 CUs. There's a new Golden Delicious-like apple called Anna that needs just 300-400 CUs. Most extreme is Golden Dorsett, which needs less than 100 hours and has been found growing well in The Bahamas.

And whilst it's reassuring to know that some varieties of apple may still grow well in Gloucestershire and elsewhere as the climate gets warmer, we'd prefer to keep that change in check and continue to enjoy Ashmead's Kernel, Blenheim Orange, Cox's Orange Pippin, Discovery, Ellison's Orange … and so on through the alphabet to McIntosh (after which the Apple Mac is named), Norfolk Beefing, Orleans Reinette … and onwards to William Crump, Yarlington Mill and Zuccalmaglio’s Reinette (there is no apple variety beginning with the letter "X” that we know of).

Thank you for reading … and please don’t complain about the cold!