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I have read, seen and heard from folks here that the pot for planting a plant should be bigger by the previous pot by only either one or two inches. I don't understand why. No one has explained this anywhere on the web.

Is this has something to do with the owner of the plant (gardener) or specifically to the plant?

I mean, how would a plant know if it's being planted in a way bigger pot or not? In fact, isn't re-potting a plant in a bigger pot better for the plant, too? More space, infrequent re-potting needs to be done. Here I am stretching it:

Suppose a plant is potted in an 8 inch pot. It needs re-potting every year. Now if we plant the plant in an 12 inch pot. It will get more time to be in the pot. It will not be disturbed by moving it another pot every year or two.

Am I making a mountain out of a molehill or does pot size really affect plants? Like more water retention in bigger pots, etc.?

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    There's an explanation for that here gardening.stackexchange.com/questions/9043/… – Bamboo Oct 21 '16 at 16:15
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    @bamboo: that question is related, but not exactly the same. Possibly you need to answer here. – Giacomo Catenazzi Oct 21 '16 at 19:31
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    @Bamboo I agree with Giacomo Catenazzi. That question has some excellent information. If you'd provide an answer over here too that would be great. Thanks! – Sue Saddest Farewell TGO GL Oct 21 '16 at 19:38
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    Hi 4-K! This is a cool question! I notice you used the "houseplants" tag. Does that mean you're not interested in outdoor potted plants too? – Sue Saddest Farewell TGO GL Oct 21 '16 at 19:42
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    That other question - and answer - seems exactly the same to me ;) But given three others disagree I won't flag it as a dupe. – Tim Malone Oct 22 '16 at 21:48
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Roots head for moisture not walls. If you want a plant with deep roots, you water deeply and allow to dry before watering again. This trains the roots to grow towards the water so it will have the ability to access water way below the surface, thus drought tolerant/resistant.

The most important reason to maintain pot size to the plant size is water/moisture. A pot that is correct to the plant size will have room to grow roots that suck up the moisture...more importantly there is no excess water and subsequent root rot. You do not want water staying in the soil, in a saucer, unless you have a semi aquatic plant.

Another thing most people do not realize is that transplanting and 'fruffing' up the roots (slightly breaking and interrupting any circling of the roots) causes MORE and healthy root growth to fill the new pot. Babying roots is not a good thing.

And another thing about potted plants that most people do not understand is the roots maintain a certain amount of canopy or photosynthetic growth. If the photosynthetic growth is larger than the root mass, this would necessitate pruning the above surface growth to match. If the root mass is larger than the photosynthetic above surface growth, again, the roots would need pruning to match the 'food making factory'. Storage and uptake (roots) have to match the food factories (top growth, photosynthetic). Or the plant will be majorly stressed.

Planting in a pot that matches the plant keeps this relationship in order. Small plant, small roots, small pot. Large plant, large root mass, large pot. Too many people jump to the conclusion that a small plant should be planted in a large pot that it would/should inhabit once mature and seems so much EASIER. WRONGO!! Not only does a too small plant in a too large pot puts the plant at risk of root rot, fungal diseases, it also slows the growth WAY DOWN. The roots are furiously trying to make sense of the amount of soil and are putting more energy into root building than vegetative growth (not to mention reproductive growth).

Do not try to correlate any of this to plants planted in the garden soil...potted plants are 100 percent dependent on their stewards. Totally different situations; pots versus garden plants!

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The reasoning behind this advice to only use the right sized pot is that if you put a very small plant into a large pot filled with potting medium, most of the medium (potting compost) will not be occupied by anything except bacteria and other life forms, some of which may not be desirable. The potting medium also may become 'sour' because, if the rootball is muich smaller than the soil around it, it's necessary to keep the contents watered in an attempt to make sure the plant's roots are able to access it; that means a small amount of root sitting surrounded by a much larger amount of wet or damp soil for longer periods, and this may create an unsuitable environment for plants.

If you have a plant you know grows quickly, let's take Fatsia japonica as an example, and it's in a 6 inch pot - potting on into something that's 8 or 9 inches, maybe even 10 inches, if done in Spring, will be fine, because it grows quickly. By the time a plant gets to be in an 8 inch pot, growth might be slowing down because the plant is older and won't be growing so fast, but if its a large, woody plant which will just keep getting much bigger for a while yet, then you could use an 11 inch pot in an attempt to avoid potting it on the following year, but I wouldn't recommend anything bigger than that for the reasons stated in the first paragraph of this answer.

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    That's fascinating Bamboo! I've been taught that with flower seeds, going from a tiny germination cell, to a very small (2" or so) pot, to a bit bigger (4" or so) pot, was correct. After that I always thought bigger was better! Thanks for this! – Sue Saddest Farewell TGO GL Oct 21 '16 at 23:37
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    Yes. And a related aspect is that gradual promotion to larger sizes ensures a better distribution of roots throughout the pot. Typically, roots head for the walls of whatever pot they're in. If a small plant is potted in a large pot right away, there will be few roots in the pot but a dense wall of root matter around the edges. It's both a waste of potting medium and that configuration makes future transplanting more difficult. – Prester John Oct 21 '16 at 23:49
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    @PresterJohn well put, I should have mentioned that aspect too... just because you can see roots through the holes at the bottom, doesn't necessarily mean it needs potting on... – Bamboo Oct 22 '16 at 0:25

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