My lemon that i have outdoors... seems fine so far.....

we have had far colder weather than that we have ever had here, some seriously cold weather for the time of year!
Air temperature of a high of 34F today here and last night 24F, we never get frost but there has been a bit of it for the last month or so, unlike any year in the last 25+ years these temperatures have been for for over a month on and off now, tree still seems fine, the only thing realy wrong is the bits that got snapped while moving it from the front to the back of the house in october

the top of the trees from the pot upward are covered in frost fleece with some christmas lights inside, air tempertaure of about 35F inside the fleece and roots at about 33F , no additional heating other than the lights i wrapped round the trunk and branches and a few circles of the pot

Its in a 280 Litre pot , that is waterproofed to stop rain getting in with a tarpaulin, ive left it quite dry, its fine, not dropping leaves, lemons are growing slowly, large lemons are going yellow slowly, the leaves look fine, even some new growth on the top looks straight and healthy (no its not frozen straight!)





My mandarin tree seems fine as well, it now in fleece with lights, but till last week it was like this with no lights, they do seem fine in cold weather

Most citrus trees bought in the UK will probably be grafted on citrange rootstocks.
Citranges themselves are perfectly hardy in the UK. I have been growing them unprotected in my Buckinghamshire garden for about 12 years, and it is in a colder than average locality. So with your trees in very large pots (which probably prevents frost from penetrating very far) the roots should be OK as long as they aren't too wet.
You have done all the right things with christmas lights, tarpaulin etc. However, beware that frost damage may not show for several weeks after exposure.
Nonetheless, I can assure you that all the commonly known edible citrus varieties will not survive and grow unprotected in the UK unless you have the very warmest microclimates. These would be either be in the south-western coastal fringes, or in large city centres, or close to sheltered, south-facing walls. I think it is partly the number of months that frost is possible here (September to May) and the fact that the remaining three months are often not hot enough to encourage strong growth. So even when the tree just about survives one year, it succumbs to cold and damp next time.

Im not sure what the rootstock is or the variety of lemon, i asked where i bought it, a few times and got no reply!
I wrapped the pot back in july as it wouldnt stop raining, only taking it off in dry weather, so i could control the watering, it is dry but not too dry so its not likely to get rootrot, the top of them is now wrapped in thick frost fleece not the tarpaulins

From pictures and info found on the net, i think its a lisbon lemon, it has large thorns, around 1 to 2 inch, all over it, the rootstock looks like a mandarin of some type, its leaves are very similar to the mandarin tree i bought from the same place

When it was delivered it had a mini rootstock sucker plant, i seperated it with roots, it looked like it had spider mites, but none were found, just the rootstock leaves, it died
the leaves are not trifolate , normal leaf , about 1/2 inch wide but close on 3 1/2 inch long