I have been watching this kumquat tree outside the hallway window here in my office building. It seems that it is really growing fast — faster than the kumquat right next to it. Is it a variety difference? Doubtful. Look at the photo below and see what you think.
Upon closer inspection, I see a couple of things that really stand out — the taller portion has no fruit on it and the leaves are different. Could it be a bud mutation? Citrus trees are notorious for mutations and reversions. But then I see another telling trait — can you see it in the next photo?
If you look closely, you can see thorns. Uh oh. When I saw the thorns, coupled with the larger leaves I knew right away what was going on. I followed the larger growth on down the trunk and found this.
Nearly all fruit trees are grafted onto a rootstock to ensure the clonal nature of the variety. Citrus trees are no different. In this case above, the rootstock has produced suckers that have grown large and threaten to overtake the kumquat tree. All of the benefit, and energy, of the rootstock should be going into the kumquat scion; however, the sucker shoots are robbing the kumquat of essential energy. If allowed to continue, because the suckers are so vigorous they will overtake the kumquat tree and end up reducing its production over time. Therefore it is crucial to monitor fruit trees for unwanted suckers that are detrimental to the scion plant. The best course of action here is to remove both of the suckers and then monitor the tree occasionally for new suckers, removing them when they are small. On this tree pictured, it is not too late to remove the suckers and for the tree to remain healthy, but the suckers should not be allowed to continue for much longer. Suckers can be removed at any time of the year, but it is certainly preferable to remove them when they are small, succulent, and able to be taken off by hand.