This video can help give you a little introduction to hardy citrus varieties.

These are not the usual citrus varieties (lemon, orange, etc) but other unusual varieties you have probably never heard of, many from Japan or China, and others hydrids with Poncirus trifoliata.

Now these hardy citrus varieties don't taste anywhere as good as the regular citrus varieties you will find in a supermarket, but if you are in a colder climate, say, USDA climate zone 8 or 7, and you want to a citrus tree to grow outside, these are pretty much your only options.

While most of these hardy citrus varieties do not taste that good, they do not taste as terrible as the hardy citrus plant people are most familiar with, Poncirus trifoliata. The fruits of Poncirus trifoliata are terrible, it's practically only an ornamental plant. (Although some people will still try to eat them, or find ways of processing the fruit to make it borderline edible)

Some of these other hardy citrus types, while they are much harder to find than Poncirus trifoliata and are not quite as cold hardy, at least produce fruits that can be eaten or used in cooking. Again, we are not talking anything as delicious as an orange, but they can be eaten, or used like a lemon, or for culinary purposes. (More like foraging for edibles in nature) Some people just like to know they can grow their own citrus. And it can be quite a conversation starter to see a citrus tree with fruit hanging on it in a place where citrus trees do not normally grow.

Some of these hardy citrus varieties have more cold hardiness than others. Their tolerance probably ranges from about zone 8b to zone 6b, with the more cold tolerant ones generally having worse flavor. Cold hardiness can also be a relative notion. Out of the "ordinary" citrus varieties, Satsuma mandarin, Meyer lemon, and kumquat are the most cold hardy, but they are not really what I would describe as "cold hardy" here. They might be able to do well down to zone 8b, and it can really depend on the situation. Probably do better in zone 8b in latitudes that are further south.

It can also be difficult because the word "edible" can be pretty relative too. There are different degrees of "edible".

This is a pretty complicated field, and I cannot begin to describe everything here. I know there are some cold hardy citrus collectors in France and Switzerland who are growing dozens of different rare cold hardy citrus varieties outside. Many of these varieties you can't find for sale, or are very difficult to find for sale. Some people might argue this is not really a practical thing, but these are gardening enthusiasts with a specialized hobby. I myself live in Washington state, and I've visited another man here who has a fairly big collection with fairly big trees outside his house. Cold hardy citrus probably has the most popularity in South Carolina, and there is some interest in Texas also.

Just something unusual you probably did not know about.