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I keep quite a lot of potted plants around my house, and usually I just move them to a bigger pot when I notice that they're looking like they need more space.

It would be kind of nice to be able to judge the size of pot I need as soon as I'm getting a plant, so I was wondering if there was a sort of rule of thumb for what size pot might be needed for a plant based on it's size it grows to?

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I think knowing a bit about the particular size a plant would ultimately grow to is the key. Obviously, one can grow a plant in a larger pot w/out any particular issues. I'm certainly interested in what some of our potted plant experts have to say on this question. –  itsmatt Aug 15 at 21:34

3 Answers 3

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I think there is no easy rules beyond knowing the size of the adult plant. However, there are rules that can help you know when you have to change the pot with a larger one.

  1. If you can see many roots coming out of the drainage holes of pots.
  2. If the soil dries out very soon.
  3. If the plant has stopped growing and you know for sure that can become larger.

Another thing: there are many studies showing that plants adapts to the size of the pots. The problem is that many of those studies... only have one common point "photosynthesis reduction". One of the last I have read suggest that photosynthesis is reduced by the lack of nutrients in small masses of substrates (thats mean less nitrogen and phosphorus). Obviously photosynthesis is one of the factors to consider in growth.

Perhaps you are wondering why I wrote the previous paragraph. It is to say that a plant do not need to be repoted indefinitely. Simply fertilize, prune, maintain adequate root density / substrate mass (to avoid root asphyxia) and renew the substrate from time to time (pruning roots if necessary).

Note: Normally when you repot, you will need a four centimeters of diameter more bigger pot.

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+1, good point on limited nutrient intake with smaller pots. –  J. Musser Aug 15 at 23:10

Firstly, this varies so much between different species that you could almost ask a separate question for each one (for instance, most cacti like abnormally tiny pots, while most vegetables like huge pots) but there are some good 'rules of thumb' you might like to know.

First, pots generally come in sizes, and you generally want to go one size up when repotting a pot-bound plant. Don't go by the size of the top, go by the roots. They shouldn't be circling the pot inside. If you see more roots than soil, definitely use a bigger pot next time. Otherwise, just trim the roots a little, and repot in the same container.

Don't put a houseplant into a pot which is far too big. This can cause rot, and/or kill the plant. Use a pot the next size up, or if you don't have it, something only a little (like 2" diameter) bigger. You generally only have to repot once a year. On round bushy plants, the pot is often half the diameter of the branches. It varies.

On most plants, I will slice a thin layer from the edge of the rootball before planting in the new pot. This takes off the unhealthy, circling roots.

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Isn't root rot due to overwatering? Is it just that people tend to water larger pots more so it ends up too soggy for the plant to handle? –  Matt S. Aug 15 at 22:18
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@MattS. yes, that's the biggest cause of root rot. It's harder to gauge moisture in a pot which is too large, and people tend to overwater. But there is another thing that causes poor growth. Many house plants have been selected from wild plants adapted to similar conditions, including very limited root growth. Too much new space can cause fast root growth at the expense of top growth, causing unbalance and stunted growth, sometimes even killing the plant. –  J. Musser Aug 15 at 23:46

The first two answers tell you most everything you need to know, but from reading your question, you seem to be asking, if you could work out what size pot the plant will eventually need, you could maybe use that initially.

The reason you don't is that you'll have a small rootball (presumably) surrounded by a large area of compost which will be more or less unoccupied for a significant period until the plant grows sufficient root material to take it all up. Unoccupied compost which is watered tends to 'sour' over time, and pathogens may build up which will cause your plant ill health or death. In a pot of the right size, the plant's roots will spread densely into the surrounding compost until they can't spread any more, so gas exchanges and nutrient/water take up are happening in all the compost.

What you are currently doing is absolutely the right thing to do, i.e., moving up a pot size when it's necessary.

And a word on watering - root rot usually occurs because the plant has been left standing in water, often contained inside an outer pot or tray. There should be drainage holes in the pot, and most plants should be watered when the surface of the compost is just dry to the touch, but not shrunken from the sides of the pot. Water thoroughly, let stand and then empty out any water left in the outer container after 30 minutes. If there's a lot of water, you'll need to check the container again 30 minutes later and empty any further excess.

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