AbstractA field experiment with leek was conducted in 1990 and 1991. Plant weight and the contents of nitrate and dry matter of the leek were measured on ten successive harvest dates each year. The yield of dry matter per square metre was calculated and related to harvest time. A maximum and virtually constant average plant weight was obtained from the beginning to the end of October (weeks 41–44). After this time the plant weight decreased. The yield of dry matter per square metre was virtually constant during the last two weeks of October and the first week of November.The content of nitrate decreased steadily during the harvest period, while the content of dry matter was at a minimum in the middle of October (week 43).The changes in nitrate and dry matter during growth could be explained by results from analyses of the different composition of the fractions of the leek. The content of nitrate and dry matter decreased from the outer to the inner part of the leek, in which the major part of the growth takes place during the weeks up to harvest.A maximum yield of fresh weight and dry matter can be obtained after harvest in October or November (weeks 43–45) when the content of nitrate has decreased to a low and almost stable level.