Here’s a link to the RHS Hardiness Ratings used more and more with plants
I’ve copied and pasted the information for H3/4/5 but the link gives you all the information in a table.
H3 1 – -5 Half hardy – unheated Hardy in coastal and relatively mild parts of the UK except 9b/10a greenhouse/mild in severe winters and at risk from sudden (early) frosts. winter. Maybe hardy elsewhere with wall shelter or good microclimate. Likely to be damaged or killed in cold winters, particularly with no snow cover or if pot grown. Can often survive with some artificial protection in winter. (Many Mediterranean-climate plants, spring sown vegetables for later harvesting). H4 -10 – -5 Hardy – average Hardy though most of the UK apart from inland valleys, at 8b/9a winter altitude and central/northerly locations. May suffer foliage damage and stem dieback in harsh winters in cold gardens. Some normally hardy plants may not survive long wet winters in
H4 -10 – -5 Hardy – average Hardy though most of the UK apart from inland valleys, at 8b/9a winter altitude and central/northerly locations. May suffer foliage damage and stem dieback in harsh winters in cold gardens. Some normally hardy plants may not survive long wet winters in heavy or poorly drained soil. Plants in pots are more vulnerable to harsh winters, particularly evergreens and many bulbs. (Many herbaceous and woody plants, winter brassicas, leeks). H5 -15 – -10 Hardy – cold winter Hardy in most places throughout the UK even in severe 7b/8a winters. May not withstand open/exposed sites or central/northern locations. Many evergreens will suffer foliage damage, and plants in pots will be at increased risk. (Many herbaceous and woody plants, some brassicas, leeks)
H5 -15 – -10 Hardy – cold winter Hardy in most places throughout the UK even in severe 7b/8a winters. May not withstand open/exposed sites or central/northern locations. Many evergreens will suffer foliage damage, and plants in pots will be at increased risk. (Many herbaceous and woody plants, some brassicas, leeks)