Changes in sward structure, plant morphology and growth of perennial ryegrass–white clover swards over winter

White clover (Trifolium repens L.) is at a disadvantage to perennial ryegrass (Lolium perenne L.; PRG) due to its limited cold tolerance and low growth rates at colder temperatures, which can affect subsequent spring herbage dry matter (DM) availability. The effect of PRG ploidy on white clover morphology and growth over winter, and its subsequent recovery in spring and the following growing season, is poorly understood. The objective of this study was to compare the effect of white clover inclusion and PRG ploidy on sward structure, plant morphology and growth of PRG–white clover swards over winter. Four swards (diploid PRG only, tetraploid PRG only, diploid PRG–white clover and tetraploid PRG–white clover) were evaluated over a full winter period (November–February) at a farmlet scale. The PRG ploidy had no effect on herbage DM production, white clover content or tissue turnover (P > 0.05) over winter. However, white clover inclusion caused a significant decrease in herbage DM production (P < 0.001; −254 kg DM/ha) and tiller density (P < 0.001; −1,953 tillers/m²) over winter. Stolon mass was not affected by PRG ploidy (P > 0.05); however, stolon length and number of leaves per stolon were affected by PRG ploidy (P < 0.05). Including white clover in PRG swards can alter winter sward dynamics, potentially causing difficulties in subsequent spring management and performance due to the reduced over-winter growth rate when compared with PRG.