Criss
My weather search  Atlanta temp records shows that you are 4 weeks ahead of me.  I feed my in ground trees with hot water 110F to bring my trees out earlier to get them a head start.  I will be planting my Meiwa kumquat tree in ground with my first 2 week frost free forecast into heated soil.  I don't know if you ever  tried heating the roots early.  If you get a record setting cold snap you will need to be prepared to cover your trees.  This extra month head start has made a big difference in the amount of growth.  What variety of kumquats do you have.

Except for my two Satsuma mandarin trees in my semi-heated greenhouse, most of my citrus trees were quite damaged this winter, despite attempts to cover them.
I haven't heated soil, I think that would be too far beyond "cold hardy" for me to bother with. 
My desire is to find some varieties that will survive and fruit with as little assistance as possible.

My kumquats are home grown from seed, and it's been so long since I started them I don't even remember which variety they are.
I'm starting to think I may have to dig them back up, put them back in pots and let them get much more mature before I attempt to leave them outside during the winter.
(Mature trees are much more cold hardy)

A crazy idea that came to me: What about for small citrus trees, upend a plastic 55 gallon drum over the tree, then surround it with straw? that way you don't have wet/decaying stuff up against the tree (given that citrus don't like wet feet), but have an enclosed space to catch heat rising from the ground. White drums transmit a fair amount of light, and the tree is probably better off forced dormant before a hard freeze anyway. I suppose you could put a small light bulb inside to heat the space and give the tree light, too... might even be enough to survive winter outdoors in harsh climates.

Some LEDs get very hot (hotter than incandescent bulbs, as I was shocked to discover when I touched one of the samples at the hardware store) -- so would be both suitable and cheap. Also, even a 25 watt "lizard heater" (ceramic heater that plugs into a light socket) puts out a LOT of heat -- I use one to heat my garage in winter.

Some years back there was a guy on a citrus forum I frequented that basically did this exact thing.  The 55 gallon drums plus some lights or other low wattage heat source.
It seemed to work for him, and he was significantly further north than I am if I remember correctly.

Ah, then I'm not as crazy as I seem.   I'll have to try "barreled citrus".

C

Cris Bessette wrote:Some years back there was a guy on a citrus forum I frequented that basically did this exact thing.  The 55 gallon drums plus some lights or other low wattage heat source.
It seemed to work for him, and he was significantly further north than I am if I remember correctly.

On the now defunct CGF there was a guy doing this in MA with an Early St. Anne.  Came through his winters very nicely.  Was very sad to see that forum disappear as it was a great resource.

That's an interesting idea.  I can see how it would work on any small shrub that needs a few zones of protection.  I wonder if they get enough CO2 when they're contained in the "greenhouse"?  I don't have easy access to straw but I bet bags of leaves and piles of snow would do the trick.

Experiment:
I just tried to germinate some kumquat and mandarinquat seeds in a window here in POrtland.  Some I planted in a small, heated yogurt maker, which heats the seeds to 70-80 F I think.  Many of those sprouted.

The ones in an unheated tray didn't sprout at all. 

I removed some from the yogurt maker and planted them in a tray that is unheated nearby. They are doing fine.

Previously when I did this experiment, the kumquat seedlings grew so slowly that I lost them, even in the raised bed. 

I think I'll raise them for a full year first, then put them out next year in the spring.

I met someone who successfully harvested kumquats every year here, but he brought the plants in for a week or two each year, during very cold weather.  That's my goal.

Landrace kumquats!

John S
PDX OR

growing Meiwa kumquat seedling take a lot more light and warmth to get them through the first year.

Greg Martin wrote:

On the now defunct CGF there was a guy doing this in MA with an Early St. Anne.  Came through his winters very nicely.  Was very sad to see that forum disappear as it was a great resource.

That sounds familiar, maybe is the forum I'm thinking of.  They guy I'm thinking of went to an extreme amount of effort to have citrus in the ground. 
I don't really want to go to that much work, but have done so to get things going in their first few years.  I'm looking for that "sweet spot" of sufficiently
cold hardy plants with good tasting fruit.  So far Satsuma mandarins and Poncirus/Citrus trifoliata are the only things that have done well for me.
Both are very cold hardy, but only the first has good tasting fruit.

Seed grown Meiwa kumquat tree to be planted outside in ground this spring when tree is acclimated from greenhouse to direct sunlight.

I might have to dig my kumquats back up and grow them in pots for a few more years. Mine are all but dead after repeated temps below freezing.
More mature trees with larger trunks are generally more hardy.

One nice thing about growing questionably zone hardy citrus from seeds is that you often get a lot of seeds and then seedlings.  Hopefully, we can hedge our bets and protect some and let others try to survive cold winters.
John S
PDX OR

Has anyone considered burying a heat tape/cable  around the base , under the mulch layer?

I'm too far north to attempt outdoor citrus, but I use them to keep my masonry sand from freezing in winter , and for bottom warming seedlings early spring. Just a thought. Its the type of cable used to keep gutters ice free. My 400 w cable is about 80 ft long and costs only about 6 cents /hour to run.
Might be worth trying for those colder nights. My sand will stay thawed with a tarp over down to 0 F.   The cable is available in various lengths.

I know many people put Christmas lights on their tender trees. Seasonally appropriate and many times has made the difference between survival and death.
John S
PDX OR

I had never thought of using a heat tape to heat the ground under a citrus tree.  I thought of using a heat tape to wrap a fig tree trunk so I could have a 10 foot trunk that branch growth could grow back each spring after all the side branches died over winter and were removed.  I would only keep the trunk alive.

I'm in such a very hard winter area that we use them a fair bit. , Just thought it might work for you guys where it doesn't quite freeze rock hard like here, but the extra warmth might make the difference on those extreme nights. I do use them for bottom heating pots on a table , so the gentle warmth on the roots does help. Not too expensive either, and can be rigged with a timer to come on at night.
I still have one under my masonry sand at night. Makes quite a difference just to keep it above freezing.

Cheers ,

Mark

Mike Jay wrote:That's an interesting idea.  I can see how it would work on any small shrub that needs a few zones of protection.  I wonder if they get enough CO2 when they're contained in the "greenhouse"?  I don't have easy access to straw but I bet bags of leaves and piles of snow would do the trick.

I just saw somewhere that higher CO2 improves cold tolerance, much as it improves drought tolerance (either way, the plant doesn't need to work so hard to survive). Soil bacteria produce some, so if the dirt doesn't freeze there should be enough leaking upward. But maybe one could put a fermenter, say a jug of leaves-and-yeast, in the barrel with the tree and provide it with a little extra CO2.

My in ground seed grown Fukushu kumquat tree has completed its first growth spurt

Good luck with the kumquat.  My personal opinion is that kumquat is one of the best choices for cold hardy citrus.
Mine took a beating last winter, might have to dig them back up and repot, grow indoors for another year or two
so that they will have more adult resistance to cold.

In ground Fukushu kumquat has 2 flowers

Check also Yuzu tree.....as I read in several forum it should be very hardy..

Her is a Meiwa kumquat that is on flying dragon that should do well in Atlanta Ga with limited protection.  The Flying dragon adds 5F colder tolerance to its normal 15F.

http://ediblelandscaping.com/products/tropicals/Citrus/