I was too little when we lived in North Dakota to take notice of what was in people's gardens. In Minnesota, I watched as each autumn, our neighbour wrapped up her roses, then covered the wrapped plants in straw and finally put a big Styrofoam cone over the whole thing. Other people buried the roses in a shallow grave, unearthing them in the spring. I don't know if I would go to all that trouble, as much as I like roses. I watched my mom and dad plant lovely perennials in the back garden hoping that in the spring these plants will have made it through. Some did, some didn't. If the plant lived through a Minnesota winter then it had earned its place in the back yard. In Iowa, I just had a few pots of flowers and annuals. The deer ate the tulips, so I gave up on perennials.
Years later, after a move to the UK, I belonged to a UK gardening newsgroup. One of the things that really annoyed the UK gardeners were Americans coming in to the newsgroup and asking questions about hardiness zones. The US based gardening newsgroups were so busy and full that it was hard to get a question answered. Americans then turned to the smaller and knowlegable UK newsgroup. The UK gardeners were also annoyed when asked about how to deal with raccoons. Questions about racoons and zones usually resulted in the friendly gardening American being sent rude, flaming responses. It was a shame because the gardening newsgroup was a nice group. They just got tired of saying that racoons and hardiness zones weren't something they could talk about. Most UK gardeners have had no experience with a system that was set up by the US government.
Anyway, back to zones. The higher the number, the warmer the winter temperature. That's about it.
Please have a look at the official site.
In the US, the zones can be quite specific. Des Moines has a higher zone that the areas surrounding it. That makes sense as cities usually have a warmer micro-climate thing going on. Your local gardening centre in the US will always know exactly what the local zone will be.
You can see on this map of the UK that we only have a few zones to deal with. Here at Whitelees we are in Zone 8. If we were a few miles downhill towards the coast and we'd be in 7. However, as far north as we are (about 55 N) we don't enjoy the same light levels that somebody in Georgia or Texas has, even though we share the same zone of hardiness. We also get a whole heck of a lot more rain.
Winters are not that cold but the flip side is that summers aren't that hot. Not hot enough to grow things like corn and tomatoes without a little help. Folks in Texas don't have to grow tomatoes in a greenhouse to keep them warm. Imagine not being able to grow corn in Georgia! When thinking of growing something here in the UK, I don't really worry about hardiness. I worry more about the length of my growing season and the low light levels during the growing season. I think that this is one of the big reasons why the US system just isn't appropriate for us over here in the UK.
Having a short growing season is where greenhouses come into their own. A very large proportion of UK gardeners have one. I need mine to get things started in the spring. I germinate most of the vegetables in there before I transfer them to the outside on the first of June. I also use them to protect some things that have difficulty in the dark wet winters. My big pot of rosemary goes back into the greenhouse in the autumn. The fuchsias go in there too.
If the rain stays off today, I might be able to get out there and tidy up a few things. If the rain stays off until Saturday, I may even get the lawn mowed!