Seed Counts and Biennial Bearing
Con Traas, The Apple Farm, Moorstown, Cahir, Co. Tipperary, Ireland
Slightly edited from a letter to the Apple Crop Internet mail group. By permission of the author.
.....I originally spotted this information in the Netherlands fruit growers journal Fruitteelt 'round about 1998. Some articles in this publication are peer reviewed and abstracts carried in the abstracting journals. I'm not sure about this one. The article seemed very logical to me, and the study well conducted, so I began applying its recommendations.
The main biennial variety which I grow
here in Ireland is Elstar (and clones). A count of 2 million seeds per hectare is
considered the threshold before biennial bearing sets in. We have applied this threshold
in our orchard. We simply allow pollination to take its natural course.
In 1999, fertilization was very good, with many apples having 10 seeds (we examine them
when they are about 12mm in diameter) With trees planted at 2500 per hectare this meant
that a safe level of cropping was about 125 apples per tree. Apart from chemical thinners
(which we routinely apply to Elstar), we supplementary hand-thinned to lower crop load to
about this level, although in reality about 150 apples or more per tree were recorded at
harvest.
In the 2000 season, fertilization was poor, many apples having only 2-4
seeds, and some none at all. However, fruit set was good, and again supplementary
hand-thinning was performed, but this time leaving 200+ apples per tree. As flowering for
2001 has not yet been determined, I cannot comment on whether bienniality has set in,
although I have faith that it has not. A feature of this system is that in a many-seeded
season like 1999, apples are strong sinks, and tend naturally to grow large. In such a
season I would normally have allowed a larger fruit set, and would probably have paid the
consequences. On the other hand, I would have thinned hard in 2000 so that fruit size
would be good, but because of the lesser sink size, trees could have lost
their balance and become over vegetative. Instead, in both years we harvested about 60
tons per hectare. In 1999 the fruits were larger and in 2000 smaller, but the trees stayed
nicely in
balance in both years If you don't hear from me in May you can assume that flowering is
good and I'm sold on this system.