Answer: Key Limes will usually set and ripen the most fruit in summer. So wait until most of the Limes have ripened before you prune your Key Lime Tree. Then when you are ready to prune your tree, pick all (or most) the remaining ripe fruit off. Then prune any dead, damaged or diseased stems right to the base. Then start the longer task of pruning back the long wispy stems. Weak stems do not hold fruit well. Cut any that are smaller than a pencil. One that is done, cut any remaining smaller/medium size stems that are intercrossing the plant. You want to open the plant up to improve airflow, reduce disease, and make it easier to pick future setting fruit.
Now when that is done, step back from the plant and look at the balance and shape. Is it the shape that you want it? If it is too high, cut all the branches and stems that are above the height that you want it. Remember that it will grow a lot on the top, so cut it shorter than you want it to be. If the plant is growing all to one side, remove stems and branches to balance it out. If you want a more open look, remove more branches and stems growing in the middle of the plant and clear the lower few feet of branches of side stems. If you want a more compact tree from a larger open one, cut the stems and branches back pretty hard. You can also prune to shape it round or square or oval or whatever. You can even use hedge trimmers to do that.
Traditionally a single citrus tree should be pruned so that it is smaller at the top, and bigger at the bottom. You could do this with a hedger so that you get an even shape. This allows for more surface area to receive sunlight. It may also be a good idea to skirt the tree (take off about the bottom foot of foliage) just to keep it tidy looking and make it easier to maintain.
Key Lime Trees are encouraged to bear lots of large fruits, pruning trees to develop a strong branch system capable of withstanding the annual load of ripe fruit is also critical. Prune Lime trees to allow ample sunlight to reach into the middle of the tree, otherwise fruits will not ripen properly and will lack good color. That can mean removing many more lateral branches and stems than might make the most pleasing-looking, bushy plant. Such pruning also allows for good air circulation through the crown, and that prevents disease. A citrus tree allowed to bear all the fruit it sets in the spring will produce scads of poor quality fruit, or it will produce well only every other year. For consistently good crops you must thin clusters of young fruit to a single fruit. Do this when the fruit is still small (marble to golf-ball size). Each fruit should be six inches or more from its neighbor. Such thorough fruit thinning is time consuming, but you will appreciate the effort when Harvest arrives.
Hope this information helps!
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