Today autumn sown onions constitute aprox. 14% of the total surface used for onion cultivation. Generally, all instructions for spring sown onions as regards planting and harvesting method, plant protection etc. are also valid for autumn sown onions. Special problems in the cultivation of autumn sown onions will be treated in the following.
Today almost exclusively hybrids and open pollinated varieties of Japanese origin are on the market, whose resistance to sprouting and winter hardiness in appropriate cultivation regions has been proven in intensive examinations (s. variety description).
Autumn sown onions must be sufficiently winter hardy and after overwintering sprout-resistant. The choice of the sowing time has a great influence on both goals. For all dates given in the following text we asume that the plants are immedialy irrigated and the beginning of the germination process is directly after sowing.
For autumn sown onions early sowing doesn´t mean early harvest. A late sowing date doesn´t lead automatically to delayed maturing. It is important that until mid to end of November, when the growth is interupted, the plants are large enough to ensure sufficient winter hardiness (rule: thick as a pencil), on the other hand, when the plants develop too quickly, rudimentary blossoms may be produced too early. In the later case, a cold stimulus may lead to sprouting (Durchschossen) in the following year and, thus, to losses in yield and quality of the crop.
In the fertilization of autumn sown onions it should be taken into consideration that excessive growth and overproduction of leafage caused by an increased dose of N applied in autumn may lead to a reduction of the winter hardiness. Apart from that, fields, where too much N had been applied, showed an unproportional high number of bolters in years with an increased development of bolters.
A general recommendation for the N fertilization can only be given on the base of soil examinations. In any case, since after sowing in autumn the temperatures are generally still high, the exploitation of the N supplies of the soil is very good. This process can be promoted by irrigation.
A slight insufficiency can be compensated by fertilization in September, whereas the consequences of an excessive N supply can´t be headed off anymore.