INFLUENCE OF SOWING-DATE ON THE TIMING OF MATURITY FOR HARVEST WITH LETTUCE, RADISH AND DILL
In Finland, timing of crops is realized to a fairly limited extent. Methods of timing, like raising young plants and polyethylene mulching, are used mainly for getting early crops. The timing of crops which ripen later in the growing-season, however, is not actively influenced with the exception of variety, and the cultivation measures are mainly directed to increasing the crops and improving the quality.
When starting, for instance, in our country, the planning of cultivation timing covering the whole growing-season, we find out soon how limited our knowledge about the influence of the different periods of the growing-season upon the growing-rate is, so far. Research as well as practice have started the cultivation of open fields straight from the beginning of the growing-season thus, unconsciously timing the cultivation obviously to the most favourable period of the season.
When this fact became evident at the Institute of Horticulture in Helsinki University in connection with an unexpectably slow maturity of a late cauliflower crop, it was considered necessary to start clarifying the influence of the different periods of the growing-season upon the rate of development. As test species for the primary investigations quick growing dill, radish and lettuce were selected, because it was possible to get crop yields with these even of seeds sown in August, towards the end of the season.
In Finland, the most characteristic feature of the growing-season in the open is the long day which is still lenghtened by the twilights before the sun rises and after the sun sets. For instance in Helsinki, where the following sowing-time experiments were made, in June the sun rises about 3 o'clock and sets after 9 o'clock in the evening. Including the twilights, the length of the period affecting the plants' light reactions should be more than 20 hours. Towards the end of the growing-season, the light period in Helsinki shortens to some 14 hours.
Another feature, characteristic of our growing-season, is a low mean temperature being in Helsinki on an average 13°C. On the other hand, daily temperature tends to vary greatly. For instance, in the years of the experiments mentioned, the monthly means of daily maximum and minimum temperatures differed with some 10° centigrade from each