Some Combined Effects of Size Grading and ‘Hardening’ Seed on the Establishment, Growth, and Yield of Four Varieties of Carrots

Carrot seed of four commercial varieties was separated into fractions by means of round-hole sieves. A fraction containing seed slightly larger than the average of the original sample was selected and called graded seed. Graded and ungraded seed batches were separately mixed with water in the ratio 10: 7 by weight, kept covered in petri-dishes for 24 h at 20 °C and then dried by exposure to the air at the same temperature. Three cycles of this ‘hardening’ treatment were carried out. The varieties differed in their response, but in general the combined hardening and grading treatment was often more effective than the separate treatments and resulted in a 25 per cent increase in seedling weight, a rise in the field emergence percentage from 64 to 75 per cent, and increases in yield of 10 per cent above a control of 46 t ha−1. The rapidity of seedling emergence was increased by the hardening treatment alone and the time over which the whole population of seedlings emerged was reduced by the grading treatment only. Weather conditions caused a double ‘flush’ of germination in part of the field experiment and the hardening treatment reduced the proportion of undesirable second flush from 50 to 30 per cent.