Pollination groups and flowering overlap

The apple carries a gametophytic self-incompatibility system: pollen carrying the same S-allele as the style it lands on is arrested before it can reach the ovule. The effect is to prevent self-fertilisation and to enforce outcrossing, which is good for the species and inconvenient for the grower.

A few cultivars are described as partially self-fertile and will set some fruit alone. Dabinett is the most often cited among English cider varieties. Even where this is true, the crop from self-pollination is usually lighter and the fruit smaller and less well seeded than from a proper cross, so partial self-fertility is a safety margin rather than a planting strategy.

Seed number matters beyond the seed itself. Developing seeds produce hormones that hold the fruit on the tree and drive its growth, so poorly pollinated fruit is smaller, more likely to drop at June drop, and more likely to be lopsided where seeds set on one side only. Pollination failure therefore shows as a light crop of small misshapen fruit rather than as no fruit at all.

A flowering group is simply a position in the blossom sequence, assigned by observation over many seasons at a reference site. Group 1 flowers earliest and the highest-numbered group latest, and the interval between adjacent groups is on the order of a few days. Because flowering lasts a week or more, a cultivar overlaps usefully with its own group and with those immediately either side.

The absolute dates move every year with the weather; the sequence is much more stable than the calendar. A cold spring delays everything and a warm one advances everything, but a group 3 variety still flowers before a group 5 variety. This is why the groups are useful at all, and it is also what a changing climate puts under mild pressure, since cultivars do not all respond to warmth at exactly the same rate.

Apple pollen is heavy and sticky and does not travel on the wind in any useful quantity. It is moved by insects: honeybees, bumblebees, solitary bees including Osmia species, and a long tail of flies, beetles and other visitors that between them do a substantial share of the work. Bumblebees and solitary bees fly at lower temperatures and in duller weather than honeybees, which makes them disproportionately valuable in a cool maritime spring.

Distance matters. Pollinating insects work systematically along a row rather than crossing an orchard at random, so a pollinator variety two hundred metres away is much less effective than one in the next row. The conventional layouts — every third tree in every third row, or full alternating rows — exist to keep every tree within a short flight of a compatible partner.

Weather during the flowering week can override all of it. A cold, wet, windy blossom period keeps insects from flying, and a crop can fail in an orchard with perfect variety composition simply because nothing was foraging during the days the flowers were receptive. The receptive period of an individual flower is short — a few days at most, shorter in warm weather — and it cannot be extended.

Wild pollinator populations are part of orchard management rather than a happy accident. Flower-rich margins, undisturbed banks and dead wood for nesting all support the solitary bees that do so much of the work in a traditional orchard, which is one of the places where habitat management and crop production point in the same direction rather than opposite ones.