A simple and potentially inexpensive implementation of a high-solids reactor is a single-stage, stratified bed reactor, in which the bed is made up of the plant biomass fed into the system. In the present study, the stratified bed was started up for a period of four weeks by either direct feeding of sugar beet leaves at four different feeding rates, or by introducing a straw bed primer which was batch digested without feeding. During weeks five to six both systems were fed with sugar beet leaves at such a rate that the total amount of beet leaves added at the end of week six was the same in each of the four corresponding pairs of straw and 'no-straw' reactors. Straw bed priming enhanced the methane yield of the sugar beet leaves, with 0.33-0.37 in 3 kg(-1) VSadded (volatile solids) accumulated at average solid retention times as short as 11-25 days, while the 'no-straw' reactors had lower yields at longer average solid retention times. The levels and speciation of the organic acids suggested that both the rate and extent of the anaerobic digestion of the sugar beet leaves added in the straw reactors were improved. At the highest loading rate, the straw reactor failed, while the 'no-straw' reactor did not. It is hypothesised that the microbial biomass was better established in the straw reactors than in the 'no-straw' reactors. (c) 2006 Society of Chemical Industry.