Is Rotala wallichii hardy in the UK?

Rotala wallichii is rated RHS undefined. It should be matched to your own coldest winter minimum.

In most of the UK, yes, if your garden's coldest night stays within its RHS undefined band. In colder inland or northern gardens, give it a sheltered spot, sharp drainage, and fleece on hard-frost nights.

RHS hardiness ratings run H1a (needs a heated glasshouse) to H7 (survives below -20°C). Match the rating to the lowest temperature your own garden records in winter. The rating is an absolute minimum, not an average.

For a borderline-hardy plant, the UK winter killers are not just cold air but cold wet roots and wind. Improve drainage before winter, mulch the root zone, move containers to a sheltered spot or against a warm wall, and use horticultural fleece on hard-frost nights. Established plants in free-draining soil take far more cold than a recently planted one in heavy, wet ground.

UK winters vary far more by locality than by latitude. Mild western and coastal areas (the south-west, Pembrokeshire, western Scotland's coast) and urban heat islands run several degrees warmer than inland frost pockets, valleys, and exposed eastern sites at the same latitude. A plant marginal on its RHS rating often survives in a sheltered town garden but fails 20 miles inland — so judge by your own coldest recorded night and microclimate (a south-facing wall, a courtyard) rather than the regional average.