Root congestion is an insidious and stealthy thief that starts to rob plants of their potential in terms of growth rate, vitality, their ability to defend themselves against insect herbivory and disease pathogens, and often appearance. The loss of potential begins to become conspicuous about the point time that the soil/root mass remains intact, as a unit, when lifted from the pot. As the amount of time a plant spends in the same pot progresses, the amount of potential lost increases to the point where the plant becomes closer and closer to the point where it's no longer making as much food/energy as it uses during respiration. Plant's can't survive decline that steep (using more energy than they create) because their food/energy stores are like a battery that eventually goes dead. That type of decline, since it seriously limits a plant's ability to defend itself, usually ends in the plant's systems and process descending into life-ending disorder with the help of insects or diseases.
Unfortunately, unless you know what to look for, you can't readily SEE the slow increase in decline and loss of potential because a plant can still look reasonably healthy when it is only realizing a middling fraction of its potential; and, also unfortunately, lost potential (in terms of growth) can never be regained.
The reason it's essential to reduce root congestion regularly by dividing plants that are best divided and repotting other plants with branching lignified (woody) roots, is because the plant, no matter its size, is reduced to securing resources (water/ air/ nutrition) from the soil mass within the pot. Potting media shrink; so with time soil particles are consumed by soil life. The minerals/nutrients released are either used by the plant or are flushed from the soil, while the carbon in the hydrocarbon chains of the organic fraction of the medium gasses off as CO2. The plant gets bigger but the amount of soil becomes increasingly smaller. There are 2 ways to address this issue, one works completely and one is a half measure.
Potting a plant up to a larger pot ensures that the poor conditions in the center of the root mass (insufficient air/ water/ nutrients) remain as a permanent limiting factor that robs plants of huge amounts of potential. It also increases the size and weight of pots that must be used, yet still remains a half measure.
A full repot, which includes (in most cases) completely bare-rooting the plant, eliminating most of the large thick roots not attached to the base of the trunk, roots that serve as nothing more than plumbing (it's the finest roots that do all the plant's heavy lifting" maximizes the amount of soil available for colonization for fine roots and increases dramatically the ratio of fine roots to large heavy roots of no value to a potted plant.
A glance at the image ^^^ will tell you there is only a minuscule soil reservoir remains to hold resources, and there are precious few of the fine hair-like roots that do the plant's work. Without correction, even if this plant was planted out in the landscape, this plant would be destined for a very abbreviated life (in terms of tree time). Being knowledgeable about the benefits of repotting vs potting up allows us enjoy the added growth, vitality, and healthy glow of plants that comes with including regular repotting in our care regimen.
The tree below was purchased from a nursery which didn't care for the roots by potting up before the plant reached the stage where soil and roots formed a single mass. The plant was planted out in the landscape w/o correcting to problems with a result that we are gradually learning should be expected if we don't correct root issues at plant-out time. Keep in mind that these roots and the plant would have also died were the plant to have been left in the pot.
Most hobby growers have a deep-rooted but irrational fear of working with roots, and harbor the idea that any manipulation or mechanical maintenance of roots will surely cause the death of the organism. This is unfortunate because it's primarily that fear which keeps growers from the ability to maintain plants in a high state of vitality over a very long term. Where people have hearts, a plant has its roots to serve as its heart. It's as essential to the plant that it's roots enjoy the same good root health as it is people enjoy poor heart health. A healthy plant is unattainable unless we have the wherewithal to maintain our plants' roots in a high state of vitality.
If your inquiry was re what time of year you should repot, early spring is the best time, though as a subtropical, it can be repotted in summer as well.
Ask if you want to see a repotting sequence or two where the roots were reduced dramatically (by more than 90%), but the plants recovered quickly and were all the healthier for it. I typically remove an average of 1/2 to 2/3 of the plant's root system each time I repot, but that's because I practice bonsai and grow trees in very small pots. For those plants I'm growing on in pots (conventional container culture) I reduce the root mass by 1/3-1/2, focusing primarily on heavy (thick) and problem roots (crossing, encircling, girdling, roots growing straight up, down, or back toward the middle of the root mass. After the first or second time a tree is root-pruned, the roots should be mainly be radiating away from the trunk, which makes subsequent repots easier and faster than the initial repotting where the roots are little more than a rat's nest entanglement.
Al
