All house plants have visible growth above ground, but they also grow roots downwards into the soil to support that top growth. After growing around and around (what ever type of container they have been placed in) there will come a time when eventually that plant will need repotting because said container has been completely filled. This condition is known as being "pot bound", or "root bound".
The main reason for repotting is to upsize the current pot or container the plant is currently in and fill the remaining space with fresh nutrient containing growing material. The advantages of doing it mean you will likely get more visible growth, i.e. a bigger healthier plant. Generally it also means you have to water it less because if a plant has completely filled its pot with roots there is less space for water to be retained.
... You notice the soil is drying out very quickly and you are watering it a lot more than normal.
... It's starting to climb up out of its current pot.
... A crazy amount of root has started to creep out of the pots drainage holes.
... It hasn't grown for years, although you want it to.
... The pot has broken, either from age, accident or from the plants own root growth.
... The soil has become disgusting. It might be full of mold, starting to smell or has more salt deposits appearing on the soil surface than a Friday night fish and chip takeaway.
... You take the plant out of its container and it's all roots and no soil. Like this:
It's impossible to provide fixed rules which suit and apply to all plants, therefore we'd recommend viewing our plant profiles where we can be more specific about their needs. However there are some generally agreed rules about when not to repot which we can vouch for -
Never repot a plant which is in flower. House plants normally only flower when conditions are favorable to do so, and if you start turning the plant upside down, shaking it around and giving it a shockingly bad time it may abandon its flowering attempt. For example the flower buds may drop off, which can be observed in the Christmas Cactus.
Don't do it if the plant is a giant already. You may hurt yourself (and the plant) if it's too heavy to lift safely, in these circumstances it's best to simply top dress. This involves scrapping off an inch or so of the soil at the top of the pot and replacing with fresh compost. This will provide nutrients and increase the pots water absorbing capacities.
Some plants need to be pot bound in order for them to flower, for example the Bird of Paradise and the Peace Lily. Concentrating on growing flowers only happens for these plants if they can no longer spend their energy growing further roots.
It's almost a given that if you repot, the plant will grow larger. If there is no physical space for it to grow into and you can't move it to a bigger home, or you just don't want it to get any larger, don't repot it!
And last of all
If your plant has become sickly don't assume repotting will automatically cure all. Unless the soil is absolutely atrocious it's likely another problem and repotting may make matters worse. If you have tried everything else then you have nothing to lose, but of all possible ideas to try for treating a sick plant, repotting is one of the most radical.
Almost all books and website's will advise you to only repot during Spring which is typically when new green growth is starting up. The idea being that the new growth works in both directions and the roots will quickly grow into the new space. However when you've had some experience you find out that ultimately the majority of house plants don't mind so much when you do it, because its more about how you do it. So if you buy a plant in Winter and it's in desperate need of a new pot, don't be afraid to do it then if you feel it's needed.
First things first, gather everything you need. The new pot, fresh compost etc. It can be messy work so it's always best to do it outside, if you have to do it indoors then cover your work area. Pick a container that is only a little bigger than the existing one, if you jump up several sizes you will have to be extra, extra careful with watering because it will be exceptionally easy to accidentally over water. The Amazon links below might give you some pot inspiration:
Chances are the roots are filling the old pot and have ever so slightly distorted it, or the roots may be coming out of the bottom holding everything in place. Either of these things can create a challenge as the plant might therefore not want to come free without a fight. Gentle hands and patience are needed here, so no yanking the plant out by the base of the stem, because this is simply setting yourself up for disaster. The kind of disaster which involves ripping the plant in two! Try the following ideas to release a stubborn plant from its container.
Idea One - Release from a plastic pot: (if it's in a clay/hard material pot which cant therefore be squeezed go straight to Idea Two), squeeze the pot a little and rotate, repeating the squeeze as you go all the way around. Eventually you will loosen the root ball and everything should then hopefully slide out neatly.
Idea Two - Releasing any hooked on roots: If any roots have grown out of the drainage holes at the bottom be prepared to cut or brake them off. Even one of these roots can twist and hook on with amazing strength completely stopping the overall release.
Idea Three - Further release tips: If the squeeze trick didn't work, there are no hooked on roots to dislodge and the plant still isn't playing ball, you will need to apply a little more force. You can try pushing the blunt end of a pencil (or similar) through each drainage hole a little up into the root ball, the intention is for you to gently push the plant up and out.
On occasion some plants leave you no choice but to literally be smashed or cut out of their old home
Idea Four - When all else fails: It's time to get brutal. On the container. On occasion some plants leave you no choice but to literally be smashed or cut out of their old home, a very badly distorted pot is an indication you will need to consider doing this. If it's not in a plastic pot, you have one of two choices to make; smashing the container or abandoning the repotting attempt entirely.
Hopefully by this point the plant is free! This is a great time to check the roots for damage and the general health of the plant, any mushy roots which are black, dead or dying should be cut away. Bad roots should be quite rare anyway, but if the root health isn't good take it as a warning that you need to adjust how you are caring for the plant generally.
Some people at this point will also recommend cutting/slashing/burning off healthy roots to encourage new growth at this point. We really don't recommend this at all. You will just be forcing the plant to grow all these roots back which is a complete waste of energy that would be better spent fueling new leaf growth or flowers. Presumably this idea came from how gardeners "tease out" roots before planting outdoors. Teasing out just means you are gently pulling on the compacted roots to loosen and free them from the tight bundle. Tightly restricted roots can end up growing around in circles, not out into the new compost and space of the new container. So by all means give teasing out a go if this is the case for your plant, just don't go evil Gandhi on the roots.
There are two good ways to actually get the plant into its new home. Which ever you pick make sure when you have finished the plant is sitting at the same "soil level" or depth as it was in its old home.
The first method is very easy and ensures a very precise result, but it may not be practical if the new pot you've selected isn't of a similar shape to the first, or if the root ball has lost the shape created by the old container when you have been freeing the plant (especially if you had to go ahead with Idea Four above!).
All you need do is use the old pot as a "guide", placing it into the new container (with some fresh compost at the bottom already) visually measuring its distance from the top, bottom and sides. When you are happy you just fill all around the sides between the new and old pot with fresh compost, then pull the old pot up and out. This leaves an imprint which your plant can just be slotted into for a perfect fit, a little more soil is then needed to firm it into position before giving a good watering.
The second approach is just to use the plant itself as the measuring guide, its more fiddly as you need to get it just right, but the principle is the same. When you are happy with how it's settled, gently firm the the soil around the plant into place and water.
That's it, repot complete. you're done!