Dormancy, winter chill and bud burst

What looks like a single dormant winter is two distinct states. In endodormancy the bud is held closed by internal signals and will not grow even if brought into a warm glasshouse; this is the state that winter chill releases. In ecodormancy the internal brake has been lifted and the bud is held closed only by the cold outside; warmth will now start it moving.

The transition between the two is the point of interest. Before it, the tree is safe from a warm spell. After it, every warm day advances development towards bud burst, and the tree is committed. In a maritime climate the transition typically happens in midwinter, well before the risk of frost has passed, so the second half of winter is a period in which the tree is accumulating heat towards a spring that has not arrived.

Chilling is conventionally counted as hours below about seven degrees, though this simple model is known to be crude: very low temperatures contribute little, and warm spells can partly negate chill already accumulated. More sophisticated models exist and disagree with each other. The concept is sound; the arithmetic is approximate.

Insufficient chilling does not simply delay spring. It disorders it. Buds break unevenly over an extended period, some not at all; flowering straggles over weeks rather than arriving as a flush; leaf emergence lags behind blossom so the flowers open on bare wood without the photosynthetic support they need; and fruit set is poor and irregular. The crop that results ripens unevenly, which is a serious problem for a single-pass mechanical harvest.

This is not currently a constraint in the traditional north-west European cider regions, where winters comfortably exceed the requirements of the cultivars grown. It is a live constraint in warmer cider-growing regions, and it is why cultivar chilling requirement is a routine selection criterion in parts of Australia, southern Europe, South Africa and the warmer parts of the United States, and a curiosity in Herefordshire.

It is also the mechanism by which a warming climate could eventually become a limiting factor in places where it currently is not. The effect would appear first as increased variability between seasons rather than as a step change, which makes it hard to detect early and easy to attribute to something else.

Once chilling is satisfied and temperatures rise, development runs through a recognisable sequence: dormant bud, swelling bud, bud burst as green tissue first shows, green cluster as the flower buds separate, pink bud as the petals colour, first flower, full bloom, and petal fall. The stages exist as named reference points because both frost tolerance and disease susceptibility change sharply between them.

Frost tolerance falls steadily through the sequence. A dormant bud tolerates severe cold; a bud at green cluster tolerates a few degrees of frost; an open flower is damaged at little more than a degree or two below zero. This is why the same night’s temperature can be irrelevant in March and catastrophic in late April.

Scab susceptibility begins at bud burst, not later. The overwintered fungus releases ascospores from fallen leaves as soon as there is green tissue to infect, so the first spray decision of the season and the first frost risk of the season arrive together. In cider, where the spray programme is lighter or absent, the equivalent decisions are cultural: how much leaf litter was dealt with, and how open the canopy was left.