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As with all things plant related, there’s no correct answer here. There are a tonne of things that can massively influence the size of the plant pot your indoor plant requires BUT the general rule of thumb is as follows:
The root ball of the plant needs to fit in the pot, with a small gap around the edge.
The reason I say ‘small’ gap, rather than giving an exact measurement is that as plant gets larger and more established, you can upsize more generously. When you buy teeny baby plants, you want to increase pot size more conservatively.
How to calculate pot size
One of the more frustrating things about buying new plant pots is that there are SO MANY different shapes of pots. If someone suggests you get a 4-inch pot, they mean a standard-sized pot (that tapers at the sides) that measures 4 inches in diameter at the widest part.
I personally think it would make more sense to measure pots by volume, but it seems I’m alone in this.
When I went to buy a 30cm pot the other day, a lot of the offerings were the same volume as my 22cm pots, because they were shorter.
I know I’m not being helpful here. It’s not my fault – blame Big Plant Pot.
What I suggest you do is carry around a (clean) pot that snugly fits your plant’s root ball. Test new pots by seeing if the old pot fits nicely in the new pot, leaving a gap of a finger or two all the way around.
I personally don’t do this, because that’s just not the sort of thing I tend to remember, BUT it’s gonna be the more reliable way of making sure your plant will fit (and not need repotting a month later).
Do not trust your eyes. They deceive us. I learnt this when buying fish – we’d buy fish that seemed huge in the tiny 50l store tanks, and bring them home to find we really should have bought 40 for our 240l, not six.
(Do we learn our lesson? We do not.)
Why is pot size important?
I’m gonna be honest here and say that for some people, pot size isn’t important at all. Sometimes a unique combination of the plants chosen, the conditions in the home, and the care you give will mean that a plant can live in a pot that’s way too small or way too big with no dire consequences.
However for the rest of us, get the wrong pot size and you’ll end with sad plants.
Too small of a pot and your plant won’t be able to get adequate water. The roots displace the soil so there’s nowhere for water to be stored. Unless you get into the habit of soaking your plant often, you’ll end up with crispy leaves (and eventually crispy roots).
Too big of a pot and you’ll end up accidentally overwatering, no matter how infrequently you physically add water to the soil. If you don’t fill up a lot of the pot with material that don’t retain water, you’ll end up with root rot because the soil holds more water than the plant can use.
Is it better to have a pot that’s too big or too small?
If a plant pot is too small, then there are steps you can take to combat this. It’s highly unlikely that the roots are going to strangle the plant and take it out that way. The number one concern is getting water to the roots.
If you take the time to soak the root ball weekly (or more often in warm weather) and add nutrient water to that water every month or so, you may find that the plant is totally fine. If you’re the kind of person that loves to spend time with their plants, and relishes watering them, then too small of a pot isn’t a big deal.
For people like me, that plant’s days are numbered.
Pots that are way too big tend to be more of an issue. However, if you have a pretty well-established plant and a very chunky soil mix, you can definitely get away with it.
Buuut if you’re using store-bought potting mix (or worse, gardening compost) your plant will probably end up with root rot in a few weeks.
Another issue with putting plants in too big of a pot is that it can lead to delayed growth.
The plant can sense (ok, I don’t know if that’s true, but it seems that way) that there’s too much space around it, and will work on creating a fabulous new root system that fills the big pot. Monstera in particular are notorious for this.
Whilst they’re growing new roots, all energy will go to that, so they won’t push out any more leaves until they’re done. When they are done (and assuming they didn’t get any root rot in that time) new growth should be pretty speedy, because they have a tonne of root to support a tonne of new growth.
How to make plant pots smaller
So, what if you have a pot that you want to use, but it’s too big for your preferred plant?
There are a couple of options:
- Put your plant in the correct size of plastic nursery pot, and set that inside the pretty pot. You can prop it up on crocks if needed
- Fill the pot with something that won’t retain any water, like gravel.
If you choose option 2, it’s of paramount importance that the pot has drainage holes. If it doesn’t, go with option one. Over time water will build up in the bottom of the pot and you’ve got no way of getting it out.
How much should I increase the pot size by when I’m repotting?
The standard answer here is usually an inch, but I think that could be too much for tiny baby plants (which are really popular now because they’re a great way of getting more expensive plants cheaper).
Instead, I go for a pot that the old pot can just about fit into. Probably more half an inch than an inch. If you want to go up the full inch (what a weird sentence), you totally can, just be extra vigilant about not overwatering OR use an especially chunky soil mix (more perlite, less coir).
Does the size of the plant pot matter more than the material?
In general, pot size will have a much bigger impact on the health of the plant than what the pot is made out of HOWEVER if you’re using terracotta it’s advisable to go slightly bigger than you would with plastic or ceramic. Terracotta draws moisture out of the soil so plants dry out a lot faster.
Does plant species affect the pot size?
Yes, but as long as you stick to matching the size of the pot to the size of the root ball, it won’t matter. Aroids tend to grow a lot of roots. There are big differences in the root, er, design. I mean, if you compare PPP with Anthurium roots, there’s a big difference:
However, they’re both pretty aggressive, root-wise. I’ve just repotted a lot of mine, so I couldn’t take a photo of a rootbound Philo, but in general, aroids grow a TONNE of roots and benefit from being repotted as they grow.
Other plants are less bothered. Hoya and succulents are more than happy to stay in the same size ot for YEARS. Sire, they will need repotting eventually, but I’ve had my Hoya bella for about four years and she’s in the same pot. I’ve changed the soil to add nutrients, but not the pot itself.
Also, some plants (MONSTERA) like to grow their roots down and circle the bottom. Rather than repot, I rearrange the roots in the same pot. I have a Monstera repotting article here that demonstrates what I mean.
Once you’ve got to grips with the basics of house plant care, you’ll have more of a sense of how big of a pot you need, however, newcomers tend to want to put the plant in a big-ass pot and hope to never have to repot it again. That doesn’t really work – too much soil = too much water = sad plant.