The Right Pot Size: As Big as You Can Keep Watering
Everywhere you read: big plant, big pot. My 1.50 m plant sits in 3 litres of coco. Here's why pot size depends on watering, not on plant height, how to spot a…
Ask ten growers about the right pot size, and nine look at the plant first: how big should it get? The pot gets picked from that, and when in doubt, one size up. My plant is 1.50 metres tall right now – and it sits in 3 litres of coco . That works because pot size depends on something completely different: how often you can water. Here's my rule for it, how to spot a pot that's too small or too big, how to find the right size for your setup in one week – and why I don't use fabric pots. Up front: this is about mineral nutrients, organic works differently in places – more on that at the end. 🌱
What the pot really does
A pot is above all one thing: a reserve of water and nutrients. Its size decides how long the plant can go without you. The bigger the pot, the longer the reserve lasts – and the less often you have to water.
Most pot-size charts online only ask how big the plant should get and work out the litres from that. Well meant, but the most important question is missing: who waters, and how often? If the roots get fresh nutrient solution often enough, the plant gets by with surprisingly little substrate – see my 3 litres. If they don't, even a giant pot that stays wet for days won't help.
Does a bigger pot mean more yield?
A large review of 65 studies came to this result: doubling the pot volume gave on average 43 % more biomass. Sounds clear-cut. It isn't, though, once you water automatically.
My GG4 (Gorilla Glue 4) yielded 400 g in a 3-litre pot. If you apply the 43 % blindly, it should have given around 570 g in 6 litres. It would never have been that much more. Why the figure doesn't fit a grow like this:
1. Biomass isn't yield. The review counts the biomass of the whole plant, not the flower yield.
2. Frequent watering makes up for much of a small pot. In a Japanese trial, chrysanthemums grew worse in small root zones – but far less so when they got nutrient solution eight times a day instead of once. With eight waterings they even grew as well as or better than plants with three times the root zone that were watered only once a day.
3. For cannabis, one review points the other way. A 2019 review of cannabis studies found no advantage for bigger pots in THC yield per square metre: going from 5 to 11 litres actually lowered it, most clearly in the variety White Widow.
With automatic irrigation and crop steering, volume no longer counts the same way. When fresh nutrient solution comes several times a day, the pot stops being a reserve and becomes a tool: you steer the plant through the irrigation events and the dry-back in between. Then it's not the litre figure that decides, but how well you steer.
To be honest: the pot effect doesn't disappear completely with automation either. In the chrysanthemum trial, plants in the smallest root zone still grew weaker than in the largest, even with eight waterings, and none of the trials was cannabis under crop steering. That's why I don't say "as small as possible", but "as big as you can keep watering" – and with automation you can water very often.
My rule: as big as you can keep watering
Without automatic irrigation, this is what I go by:
- 🟠 Needs water several times a day: too small for hand watering. Either go one size up – or get an irrigation system to do it for you.
- 🟢 Once a day: ideal. The plant gets fresh nutrient solution every day, and you look at it every day. You spot problems before they grow.
- 🟡 Every two to three days: fine. The compromise if you don't have time every day. Then the pot can be bigger.
- 🔴 Longer than three days: too big. The substrate stays wet for too long. And if you can't even manage every three days: don't go bigger, automate.
I can't give you a blanket litre figure for that – it depends on your setup, more on that in a moment. But you can find it out yourself in one week.
Why a bigger pot isn't the answer
If you're short on time for watering, it's tempting to grab the biggest pot in the shop. That goes wrong for several reasons.
1. The substrate stays wet for too long. A big pot with small roots in it doesn't dry down for days. The roots run short of air, and fungus gnats and root rot have an easy time.
2. Fresh nutrient solution less often. Between two waterings the plant draws water, and the salts stay in the substrate. The longer the gap, the further the EC in the pot can drift from what you mixed.
3. Less control. If you water every five days, you only measure every five days. A pH or EC problem then shows up days later than it needs to.
4. More weight, more space, more cost. A big, wet pot is heavy, hard to move and takes up room in the tent that you're missing elsewhere. And you buy the substrate for it again every grow.
How to tell your pot is too big:
- The substrate is still wet three days after watering.
- When you lift the pot, it's almost as heavy as right after watering.
- Fungus gnats show up.
- The plant droops even though the substrate is wet.
How to tel …
The Right Pot Size: As Big as You Can Keep Watering