Biennial bearing

By alternance is defined as the variation in the fruit yield in two annual rhythm of fruit trees, especially in culture and culture - apple - pear, as well as olive trees and plums, often triggered by influences of the weather.

This causes the apple to the so-called " apple years," which alternate with years directly very low yield. So would render the fruit-growing unprofitable Without horticultural interventions, as bring in the apple many years, but relatively little fruit at elevated offer little profit while at lower range though high prices can be achieved in the following year, but little fruit can be sold.

Especially susceptible to alternation are the apple varieties Boskoop, Delbarestivale and Elstar.

Expiration

The seeds as well as the still growing shoot tips of many fruiting trees form plant hormones ( phytohormones), which reduce or inhibit the differentiation of flower buds for the following year, when the tree has very many or too many flowers. Then little or no flowers for the following year are created. In contrast, a relatively large amount of flower buds for the following year will be created in a year in which the tree has little or no flowers.

Moreover, since the biomass is limited, which can generate a tree from the available assimilates through its metabolism ( energy sources ) are less in a year with very many fruits this average. Contrast, with only a few fruits invest more energy in order to create new branches and thus flower buds and let the individual fruits ripen relatively bigger a tree in a year.

Occurrence

The occurrence of alternation is an individual expression of the metabolic equilibrium of each individual plant. Nevertheless, apple years usually occur in an entire region by a late frost, the alternation triggers the same for almost all trees by a strong reduction of the flowers, for example, in a year. The second year becomes the " apple - Year " for the entire region.

This in turn drops the amount of new shoots and buds below the average, so that the third year again a low-yield year. This change weakens only with time, unless it is broken in the apple year by engagement of the gardener or tree warden.

Intervention options

In some varieties the flower buds differentiation for the following year has already ended a few weeks after flowering. In this case, a - for the desired fruit cultivation uniform income - too large a quantity of the developing flowers can only be reduced by poor fertilization, an early chemical or mechanical thinning or a suitably adapted interface.

However, If the flower bud differentiation at a late date, and is correspondingly long under the influence of hormones, which inhibit the formation of flower buds can be obtained with an early hand thinning of fruit a sufficient approach of flower buds for next year.

Alternation in Olive

The alternation concerns in apple and pear the whole tree in the olive, however, only the supporting branches, while the non-bearing buds begin next year. For this reason, experienced growers cut their trees right at the harvest by as far as to cut away the branches that have borne as useful and leave the others are, so that they can wear next year.