As a rule, lemon trees, like most citrus, prefer warm climates. If you live in an area with harsh winters, you will need to take steps to protect your lemon tree. This usually involves insulating your tree in some way in order to protect it from drastic changes in temperature and harsh elements. Soil, blankets, heating lamps and ice formation are some of the things you can try in order to keep your lemon tree safe. Multiple methods may be used in conjunction with one another for optimal results.
Insulate the base of your lemon tree. Contrary to what you may think, this will require you to remove mulch (if you have any) from the base of your tree. Use your shovel to move all of the mulch elsewhere. Next, pack topsoil around the trunk of your tree, at least a couple of feet up. Pack it nice and thick, sloping up like the outside of a volcano. Soil absorbs warmth and can insulate very well, providing a significant increase in temperature for your lemon tree's trunk and root system. Do this before the first frost of the winter.
Wrap a tree blanket around the top of your tree. You may want to try using this blanket like a big tent, in conjunction with a heat lamp. Make sure that the heat lamp does not touch either the tree or the blanket because tt may damage the tree or cause a fire. Use your ladder to get the blanket up over the top of the tree. You may want to ask a friend to help you, depending on the size of your tree.
Wrap bandages and felt in alternating layers around the trunk of the tree, all the way down to the base. While not completely ideal, this will act as a sort of winter coat for your tree. In climates with relatively mild winters, this may be all you need. Use a ladder to reach as high up the main trunk as you can. If you like, you can also wrap some of the bigger branches.
Insulate your tree using ice. This may seem counterintuitive, but one good layer of ice surrounding your lemon tree from the first frost through the worst of winter may help. When the first freeze occurs, spray the tree thoroughly with water and let it freeze. Spray it when the sun is down, if possible, so that the sun does not accidentally melt some of your ice. The weight of the ice may damage some tree branches, but that is surely preferable to your entire tree dying.