Verfrühung von Süßkirschen im Folienhaus

Early ripening sweet cherry cvs ‘Burlat’, ‘Earlise’, ‘Samba’, ‘Souvenir des Charmes’ and clone M, which are all characterised by large fruit size and dark red fruit, on GiSelA 5 rootstock were forced – by closing a polytunnel from 24 March to 20 April 2006 – at Klein-Altendorf research station of Bonn University. A portable gas heater was used for automated frost protection in April. An adjacent uncovered planting at the same spacing of 3.8?m?×?1.75?m served as control. Forcing resulted in a 16–18 days earlier flowering in April relative to the 12–16 days earlier harvest at the beginning of June, resulting in a 2–4 days longer fruit development and retarded ripening. PAR was reduced in the polytunnel by 40% on sunny and 30% on overcast days with a concurrent drop in the humidity to 30% on sunny and 40% on overcast days and nearly 100% relative humidity at night. Polytunnel air temperatures on sunny days were increased – during ventilation – by up to 6?°C relative to the outside, e.g. 33?°C versus 27?°C outside. Forced cherries saved one spray application against aphids compared to those outside. Fully-grown cherry leaves contained 40–60?mg chlorophyll/g FM sufficient for photosynthesis with commensurate chlorophyll contents when grown in the polytunnel compared with those outside, and were deficient in calcium (0.5% DM) and magnesium (0.2% DM), but with a surplus in both nitrogen (3.5% DM) and phosphorus (0.42% DM). The surplus leaf nitrogen was due to excessive nitrogen mineralization in the rich soil under the higher temperatures in the polytunnel which caused the vigorous vegetative growth despite the dwarfing rootstock. Great fluctuations in fertilisation – despite strong flowering and sufficient pollination with honey bees – of between 4–71% may be due to the combination of S-alleles of the cherry varieties, infrequent water supply, nutrient imbalance and microclimate. Trees of cv. ‘Burlat’, ‘Earlise’ and clone M in their fourth leaf yielded an average of 2.5–7.5?kg fruit with 15?kg/tree in one case. Fruit of forced cvs. ‘Burlat’, ‘Souvenir’ and ‘Samba’ were larger compared with those of cv. ‘Earlise’ and clone M, which were smaller as a result of increased fruit load on the tree than those from uncovered control trees. For all five cvs tested, except forced cv. ‘Earlise’, a fruit mass of between 10?g (28?mm ) and 12?g (30?mm) and a sugar content of 12–17% with sugar:acid ratios of 20–37:1 made their fruit suitable for marketing as superior or premium fruit at higher farm-gate prices of 4.5–8?€/kg. Fruits were sufficiently dark red or black and firm for short distance transport and rapid sale. An economic evaluation showed financial gains, when yields exceed 8?kg fruit per tree per year at a farm-gate price of 4.50?€/kg.