Surprisingly, there is a fully hardy citrus species: Poncirus trifoliata
Some different cultivars exist, like ‘Flying Dragon’ with twisted branches and huge crooked thorns. Poncirus is deciduous and the fruit is bitter and gummy. Poncirus is very important for us as a rootstock for oranges, lemons etc.; it tolerates a variety of soil types and grows well in our climate. Experts believe this is the best rootstock for North West Europe. (Most edible citrus varieties in the trade are grafted on other rootstocks to suit the mediterranean climate.) Most Citrus varieties in the list are not yet well known and difficult to obtain.
Citrus ichangensis is evergreen but less hardy than Poncirus. It can be cultivated in a sheltered garden against a South facing wall. The fruits are fairly good but full of seeds.
In the search for hardier citrus, Poncirus has been crossed with oranges, mandarins, lemons, grapefruits, and kumquats etc. . Hardiness has improved a lot, but at the same time the quality of the fruit has declined. The ornamental value however is enormous!
POSITION IN THE GARDEN, SOIL, FERTILIZER
100 % sunny and sheltered! Preferably against a south facing wall.
The best soil is a rich, light loam and well draining. Improve sticky soil with coarse sand and peat.
From march till september apply dried cow-manure monthly (no manure from birds, that is too chalky!). During periods of hot summer weather Citrus will need extra water.
During medium frosts (5 to 10 °C of frost) a thick layer of mulch (40 cm) and a protecting piece of fleece cloth around the top is enough. You may wrap the trunk with burlap. If the wind is strong and very dry, protect as indicated for heavy frost.
During heavy frosts (10 to 15 °C of frost): apply extra protection with more burlap around the trunk and cloth around the top.
During very severe frosts (15 to 20 °C of frost): wrap the tree in christmas-lights to give some extra warmth. In extreme situations (when the temperature drops even lower than minus 20), cover the whole tree with an old sleeping bag.
Citrus ichangensis (“Ichang Papeda”), the hardiest evergreen Citrus, is cultivated in SouthWest China for it’s aromatic lemons. New hybrids with this one seem to taste better than hybrids with Poncirus.
So far I only know little people in the Netherlands who have been succesful with citrus cultivation in their gardens. I do not know if ripe fruit has been picked already. I tried a fruit of the Ichang Papeda with a fish dish but the rather resinous flavour and texture was not very pleasant. Apparantly the fruit quality declines with increasing frost hardiness . Everybody who tries to grow Citrus in a temperate climate is still a lonely pioneer. Your only guarantee is my enthusiasm …