Frost pockets and orchard siting

Radiation frost — the kind that matters in orchards — needs a clear sky and still air. The clear sky lets the ground lose long-wave radiation to space; the still air means the cooled layer near the surface is not mixed away. Once that layer exists it behaves like a fluid, because it is denser than the air above it, and it runs downhill along the surface of the land in the same way water would.

This produces a characteristic vertical pattern on frost nights: the coldest air at the bottom, a warmer band on the mid-slope, and cooler air again well above. The warm mid-slope band is sometimes called the thermal belt, and it is where fruit growing in hilly country has always concentrated. The difference between the belt and the valley floor on a bad night is not marginal. Several degrees is normal, and several degrees is the difference between a crop and no crop.

The corollary is that flat land is not automatically safe. A large flat field with no fall at all will simply pond its own cold air, and a flat site at the base of a long slope will receive everyone else’s as well.

Anything solid across the downhill side of an orchard will hold cold air behind it: a thick hedge, a stone wall, an embankment, a shelterbelt of conifers, a new building, a road on a bund. This is the most common way a previously reliable site becomes a frost pocket, and it is usually done for a good reason — shelter from wind, screening, a new access track — by someone not thinking about air drainage.

The remedy is to open a gap at the lowest point so the cold air can continue on its way, or to thin the barrier so it is permeable near the ground. A hedge that is stock-proof at the base and open above is the wrong shape for frost drainage; a hedge with gateways or gaps at the bottom of the slope is much better. Where a shelterbelt is needed against prevailing wind, it belongs on the upwind side, not across the fall of the land.

It is worth walking a candidate site on a frosty morning and looking at where the frost lies longest, and at what grows where. Cold-sensitive vegetation, late-leafing hedgerow species and the pattern of surviving old trees all record decades of frost behaviour that no single season of observation can give.

Cider apple cultivars flower late. Most of the classic West Country bittersweets are in the later flowering groups, opening well after the earliest dessert and culinary varieties, and that lateness is a large part of why the cider counties are where they are. A variety that blooms in the second half of May in a mild oceanic climate has missed most of the frost risk that a variety blooming in April has to survive.

Perry pears do not have that protection. Pears flower earlier than apples, often noticeably so, and perry orchards in Gloucestershire and Worcestershire sit on the frost-exposed side of the calendar every year. This is a real and under-discussed reason for the irregularity of perry supply, and it is why perry pear siting has historically been fussier than apple siting even in the same parish.

None of this makes a bad site acceptable. A late-flowering orchard in a frost hollow will still be caught in a late spring frost, and increasingly is, as bud burst advances faster than the date of the last frost.