Is Yellow Water Trumpet hardy in the UK?

Yellow Water Trumpet is rated RHS N/A. 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 N/A 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.