As part of the North Street improvement scheme it was the original intention to plant trees directly into the ground. This would have saved pavement space and given the trees themselves a better chance in a difficult environment. Unfortunately, after several trial holes were dug along North Street, it became clear that the prevalence of existing underground utilities precluded this option. So, rather than have no trees at all, planting tubs were chosen. These come with 5 years worth of pre-paid maintenance, which should give the trees a reasonable chance of becoming established.
See also: North Street progress