Is Indian Mustard 'Tendergreen' hardy in the UK?

Indian Mustard 'Tendergreen' is rated RHS H5 (tolerates light frost, more heat-tolerant than most mustards) (hardy in a cold winter, hardy to about -15 to -10°C). It hardy through most of the UK even in severe winters.

In most of the UK, yes, if your garden's coldest night stays within its RHS H5 (tolerates light frost, more heat-tolerant than most mustards) band (around -15 to -10°C). 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). Indian Mustard 'Tendergreen''s rating, H5 (tolerates light frost, more heat-tolerant than most mustards), means hardy in a cold winter — it tolerates winter minimums of about -15 to -10°C. 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.