Hi GrowGuru, Well, yes we are hesitating !! I would imagine, like me, most of the growers of 'tropical' plants are at a loss as to what to suggest people like yourselves grow ?? For years, we kept pushing the boundaries and to be honest, we got away with it, we were getting mild winters with sometimes here in Cornwall hardly a frost at all !! How things have changed, given these last three winters !! After the first bad winter, we all said, 'well,its a fluke, a mere blip and next year everything will be back to its mild normal self ' so everyone replanted and then bang ! another, even colder winter, back to square one ! Last year, a few dedicated gardeners replanted but, I fear this winter will have knocked even thier confidence !! Down here, this last winter has killed even things that survived the previous two, Ive lost Agaves, Aloes and Ceanothus that have previously survived !!
Now to your question, All I can do is to tell you what customers of mine (who have for years taken back plants to London) have found do well.
Most grasses are ok but avoid Pennisetum, Yuccas are fine, Chamaerops humilis and Trachycarpus fortunei are the best palms. Cordyline australis should be ok (plain green the hardiest, avoid variegated) Furcraea and Beschorneria might be ok. Most Phormiums are fine, as are Kniphofia, Bamboo,
Fascicularia bicolor, Olives are ok
Now we come to the plants that we used to say were ok but now I feel might have to be pot grown, placed outside for summer and then given protection in winter, if these cold spells are to be the norm !! Agaves, Cordyline Australis (coloured varieties) Bananas (the best in my opinion, is Musa lasiocarpa) Hedychium, Canna, Agapathus, Coprosma, Puya, Sedum, Aeonium, Abutilon, Restio, Acacia, etc etc the list is endless !!
Alot will depend on the nature of your site, its size and what you yourself like ?? I would suggest you visit a few gardens locally and see for yourself what has survived?? Another good contact would be 'The Palm Centre' Ham Nursey, Richmond.
As a general rule, if I was to give you the single most important thing that will help you the most, especially with some of what must now be classed as 'borderline' hardy, it is DRAINAGE !! Dig in lots of grit and if possible mound the soil up, undulate it, use rocks and pebbles to cover it and this will help enormously.
Sorry to be a bit vague, but given these recent winters, im not feeling my normal enthusiastic self about 'exotics', if I'm struggling here right next to the sea in Cornwall, I wish you the best of luck !!
Hope some of this helps, Lannerman