Tropical garden Novice

You would be surprised by how cold dahlias are able to survive. I'm not saying risk your best ones but it's worth experimenting. Our neighbour had hers survive and they didn't have any mulch, were actually partly showing with no protection (it was a mlid winter).
With a really good mulch they are probably going to stay warmer than in the greenhouse because of the soils thermal mass. You can also help reduce the moisture getting to them by having an impermeable cover over the top. It's also possible to put them in the loft if you have one where they should be safe and sound.

I plant spring bulbs and wallflowers to fill in any gaps, although mine is more a cottage/wild garden, the tropicals are grown in pots.

Abutilons are also a nice species to look into, some are reasonably hardy but others should be ok with a little unheated greenhouse protection.

 

IMO the biggest problem with leaving Dahlias in the ground, is not the cold, or even the wet, but slugs eating the emerging growth, so I dig mine up and start them in pots, only planting them out when they are large plants.

 

Hello @flounder, what are eddoes?

 

I don't grow tropical plants per se, but I reckon Ligularias would be a good fit for tropical look.
I have Britt Marie Crawford which has maroon foliage & Desdemona for olive foliage, big acidic yellow flower spikes too.
They like constant moist soils and shade so your conditions should suit.
Slug magnets tho'...

 

Ooh, I like Ligularia. Another plant for my list. At this rate the whole garden may be tropical! Luckily we don’t have too big a problem with slugs (famous last words)

 

Colocasia esculenta, could be sold as taro or eddoe. Asda sell them, as do other supermarket chains. There's quite a lot of 'exotic' vegetables you can grow from store bought stuff. Even ginger will give a summer alternative to other greenery, or oca, cardoon, galangal, turmeric or even the humble runner or broad bean can give a bit of a jungly look if integrated.
Look out for asian or oriental greengrocers...they normally have some weird and wonderful things for summer bedding and you won't need a heated greenhouse to start them off.

 

Thanks @flounder I grew some ginger last year although I was quite late eating it iff so only just in growth come Autumn. We have lots of Asian shops in Brum so I think I’m going to have lots of fun with this tropical lark

 

If your ground gets a goodly amount of water then some nice structural plants can look good. What used to be called darmera peltata but has a funky new name I think, rheum palmatum and rodgersia can all look very tropical but are completely hardy. I also think the common persicaria like bistort and amplexicaulis look great in a tropical setting and are very hardy.

As @Punkdoc mentions, overwinter dahlias is the ground does make them very susceptible to slugs. We get a lot of slug problems but with the strong growth on dahlias here it just tends to knock them back, rather than kill them. The tubers are massive though and for small plants it's probably not worth the risk.

 

That is the exact opposite of what they are supposed to want.
Mine wilt at the merest hint of the soil drying out.
Perhaps you should have a word with them?

 

My old and trusted dried crushed egg sheĺls layers keeps snails and slugs àway from tender emerging dahlia leaf.

 

The snails round here eat that first, then demolish emerging shoots

 

Must be partial to egg shells I will get me thinking cap out get you a different recipe

 

Daylilys look quite tropical and so do Crocosmias and both are fully hardy.