Potted lemon tree
Thank you for referring your question to Ask An Expert. It sounds like whatever you have been doing with your lemon tree is right as it appears to have remained healthy over the years.
Typically a potted citrus tree will outgrow its container in three to four years. A good way to tell when it is time to transplant is if the leaves start to turn brown or you start to experience twig die back (not from heat stress). You can also check your drain holes to see if roots are starting to escape through the holes. When any of these start to happen you have two choices. One is to transplant it to a larger pot or two, is to prune the roots back so that your tree will remain happy in the present container.
If you choose to transplant your tree, the bigger the container the better. Remember though that the size of the tree has been regulated by the present container. A larger container, at least 25 percent larger than your present container, will allow your root system to expand and allow the tree to grow bigger. Use a good grade potting soil when you do the transplant. A good potting soil will contain a lot of organic matter and can be found at most retail outlets that sell nursery products.
If you choose to prune the roots gently remove the tree from its present container. If the tree is large you may want to enlist another pair of hands to assist in the removal. Once the tree is out, trim off about a quarter of the roots or two to three inches from the sides and bottom. Shake off the loose soil and replant with potting soil as mentioned above. Since the root system is now smaller you should prune off at least a third of the foliage.
The best time to transplant or prune your lemon tree is in the spring when it is growing.
Good luck with your small orchard and enjoy your lemons.