Two glasshouse experiments were conducted to; (i) investigate the effects of soil mechanical impedance (MI) on individually grown plants of Lolium perenne, Trifolium repens (sown pasture species) and Agrostis capillaris (indigenous species); and (ii) investigate the effects of MI on the growth of L. perenne and A. capillaris grown in swards comprising pure stands and mixtures. The individually grown plants showed a significant (P0.05) reduction in dry matter accumulation in an impeding treatment (2.3 MPa PR) compared to a control (0.25 MPa) for all species. Each species showed significant (P0.01) correlations between root and shoot dry weight indicating that the root to shoot ratio was maintained across the different MI treatments. This suggested that MI caused some form of root to shoot communication which kept root and shoot growth in synchrony. L. perenne also shows significant reductions and delays in shoot development. This effect was less pronounced in the other two species showing that L. perenne was more sensitive to MI. MI also significantly (P0.05) reduced root length and axis numbers for both L. perenne and A. capillaris in both pure and mixed stands. Only in pure swards of L. perenne was there significantly (P0.05) less root dry weight in the impeding treatment (1.8 MPa) compared to the control (0.2 MPa). MI had no significant effect on any aspect of shoot growth of either species grown in pure swards. However, L. perenne grown in mixture with A. capillaris had significantly less shoot dry weight in the 1.8 MPa treatment compared to the control. These results reinforced the findings from the individually grown plants which also showed that L. perenne was more sensitive to MI than A. capillaris. From the evidence of the glasshouse experiments, it was concluded that the PR measured at the field sites, would have affected the growth of the sward species. It is also suggested that the greater MI at one of the sites would have played a role in the more rapid replacement of L. perenne by A. capillaris observed at that site. This is due to the greater sensitivity of L. perenne to MI compared to A. capillaris, reducing its competitive advantage.