Hi, first post on Gardenweb...
TL;DR summary: You can absolutely grow outdoor in ground citrus in southern NJ, or almost any other part of the USA, even upstate MI. It just depends on how much effort you want to invest.
Here's my full 2 cents:
Citrus are quite content being dormant for several months of the year. They can tolerate prolonged periods (several days) at or slightly below freezing and short periods of a few hours (depending on the variety) down into the teens without much damage except for possible defoliation. The key is that the tree is healthy and fully dormant, active growing trees are much more sensitive to cold.
One alternate approach to outdoor growing (as detailed by Poncirusguy) is to bring containerized citrus indoors for winter. Doing this you can either keep them growing actively (not dormant) by supplying sufficient heat and warmth. Or if you have a cool spot inside (temps in the 50's, for example) that receives filtered (not direct) sun you can overwinter them indoors in their dormant state.
For outdoor growing, you also need to consider the fruit. In general the fruit will be damaged by temps below ~28F for more than a few hours. So you can either make sure to protect the trees so temps do not get that low or select varieties, such as early Satsumas, that are able to ripen their fruit before cold weather sets in.
Depending on the degree of protection, you could grow pretty much any variety of citrus in ground. Alternatively, in zone 7B you should be able to grow some of the more hardy Poncirus hybrids in ground with minimal or possibly no winter protection. But, depending on your personal taste preferences, you may (or may not) like the fruit you get from the Poncirus hybrids.
I know someone who has been growing Satsumas, Kumquats, Meyer Lemons, and some other Poncirus hybrids in ground in zone 5B for several years. Some of his trees are 6+ ft tall and many produce fruit. He uses extensive winter protection and supplemental winter heating. I have seen one of his Kumquats with well over 50 ripening fruit that he overwintered!
Personally, I have one Satsuma in ground in my yard (zone 6B Massachusetts) and have overwintered it without using any supplemental heat. Instead I used several water barrels (taking advantage of the large heat capacity of water) to protect my tree.
My tree saw a low temp of ~28F this past winter (the outside low temp was ~4F). During one stretch, the outside air temp did not get above freezing for 8-9 continuous days.
Here are some pictures that might be helpful.
Satsuma last Fall:
Bundled up for winter (Early November). The tree is surrounded by 6-7 filled 35 gal water barrels wrapped in plastic sheeting to keep the wind out and is topped with an old full glass patio door for a roof. There are many different setups for protection that would also work... you just need to experiment a little and find what works best for your specific area.
After 25" of snow in late Janurary:
Uncovered this spring:
...and putting on new growth:
Whether you choose outdoor (in ground), indoor active, or indoor dormant overwintering is really your personal preference. All methods will likely require some level of trial and error to determine what works best for you.
Personally, I have found that maintaining my outdoor tree during the winter was much less work than my indoor trees. Once I set-up the enclosure, I just left the outdoor tree on it's own all winter. (In full disclosure, I did keep an eye on the interior air and soil temps with some remote temp probes and had an emergency heat source that was set to turn on automatically if the interior temp went below 27F) In contrast, my indoor citrus require frequent checks to see if they need water and I typically need to spray them with Horticultural oil a couple times during the winter to keep the insects (scale, white flies, mealy bugs, red citrus mites, etc...) in check.
Cheers,
Scott
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