Flowering overlap and planting a mixed orchard

Apples and pears carry a genetic self-incompatibility system that rejects pollen carrying the same incompatibility alleles as the flower. It is a mechanism for avoiding inbreeding, and from the tree’s point of view it works.

From an orchard’s point of view it means pollination is a planting decision taken years before the crop. Two cultivars flowering three weeks apart in the same field are, for this purpose, in different orchards.

Bees make the arrangement work, and their behaviour matters as much as the flowering dates. A bee working one block will keep working that block, so alternating rows or interplanting pollinators does more than putting a pollinator block at one end of a field.

The distribution is the practical finding. Roughly half of the recorded varieties are in one band, and the two ends carry a handful each — so the difficulty of finding a partner is very unevenly spread. Barnet, Bunch, Gin, Green Longdon, Red Pear and Rock are all very late, and an orchard containing one of them and no other very late variety has a problem that will not be visible until the trees are old enough to crop.

The bands are also not evenly related to ripening. Hendre Huffcap flowers early and ripens in the middle of the season; Tumper flowers mid and is one of the very earliest to ripen. Flowering and ripening are two independent properties of a variety, and neither predicts the other.

A triploid carries three sets of chromosomes rather than two, and its pollen is effectively sterile. Two consequences follow: it cannot pollinate anything, and it needs two compatible diploid pollinators of its own — because a single partner would have to pollinate both the triploid and itself, and it cannot do the second.

Among perry pears, Barland is recorded as a triploid, which sits alongside its recorded poor cropping and helps explain why a pear of that reputation is not more widely planted. Among cider apples the triploids are a scattered group across the classes.

Planting a triploid without accounting for it is one of the more expensive mistakes available in an orchard, because it is invisible for the first several years and then permanent.

The other half of the flowering decision is frost. Blossom is killed at temperatures a little below freezing, and the damage is total for the flowers affected: a frost at full bloom removes that year’s crop from those trees.

That argues for late flowering, and it is one of the reasons late-flowering cultivars are valued in frost-prone sites. But a block that all flowers late is a block that will all be caught by the same late frost, and a block that all flowers together is a block with a single point of failure.

The traditional mixed orchard spread its flowering across several weeks without anybody planning it that way, which is a resilience the modern single-cultivar block does not have.

A tool that answered “will these two varieties pollinate each other?” is an obvious thing to want and the data does not yet support one. The perry pear bands are recorded for about half the varieties the site holds, and the collection does not place the rest. Apple pollination groups are recorded for a minority of the cider cultivars here.

More seriously, flowering overlap is necessary and not sufficient: genuine compatibility depends on incompatibility alleles, which are published for very few cider cultivars, and on ploidy, which is recorded for fewer still. A tool built on flowering dates alone would answer a narrower question than the one it appeared to answer.

The bands and groups are therefore published on the cultivar pages, where a reader can see what is recorded and what is not, and no calculator is offered on top of them.