Just guessing is totally fine – it's only about a first impression.
Dry air pulls water out of the plant – that's the vapour pressure deficit from the VPD course. Wind amplifies it further because it constantly carries away the humid boundary layer at the leaf. Both at once, plus a pot without shade: water demand can multiply on such a day.
In the VPD course humidity is something you set. Outdoors it is something that happens to you.
You cannot lower the vapour pressure deficit. No humidifier in the world helps against a dry easterly wind over an open garden. What you can do is give the answer – and the answer is water.
So the whole logic shifts outdoors: indoors you control the climate and water to a plan. Outdoors you read the climate and water to demand.
On a muggy, still day the plant almost stands still. On a hot, windy day it drains the pot while you're at work.
In a pot everything you know from the watering course applies, only sharper. The volume is limited, the sun heats the pot wall, the wind dries from above. And because every watering delivers salts the plant doesn't fully take up, salt build-up climbs precisely on hot days – exactly when you have to water most often.
In grown soil almost everything is different. The soil buffers: it holds water for days, and its cation exchange capacity catches nutrients instead of passing them through. In exchange you share the control with the weather – you can't take back a week of rain, and in a drought your watering can doesn't reach where the roots have got to by now.
The pot gives you control and takes your buffer. The soil gives you buffer and takes your control.
A week of steady rain in August. Your potted plants have had plenty of water without you doing anything. What is the more likely problem afterwards – too much fertiliser in the substrate or too little?
Too little.
Rain is irrigation water with an EC of practically zero. It brings no nutrients but it takes some away – every litre running out of the pot flushes dissolved salts with it. A rainy week acts like repeated flushing in a pot.
So after prolonged rain in pots, feeding is more the issue than flushing. In grown soil it's more relaxed: exchange capacity holds part of it back, which is why soil doesn't wash out as fast as a pot.
A note from the water course: rainwater has no roof above it – but the moment you channel it from a roof into a barrel, it has one. For the plant what counts is not the cloud but the path.
The watering course recommends letting the substrate dry back between waterings – roots need air, and a permanently wet substrate invites problems.
Indoors you plan that dry phase. Outdoors you get it for free, in portions you didn't choose. Three hot days are a dry phase that goes too far. Ten days of rain are a wet phase that allows no drying back at all.
That's no reason to panic – plants outdoors tolerate more than tent operation suggests. But it shifts what you watch: it isn't the calendar that tells you when to water, it's the weight of the pot and a finger in the substrate.
And it leads straight to the last lesson. Because if you can't get there daily, the substrate is no longer setting your rhythm – your visiting schedule is.
🎯 Hands-on mission
After the next longer rain spell: look at your pots and consider how much fertiliser has run through. The dosage calculator helps you plan the next feeding – but the decision whether to feed at all you make at the pot.
To the dosage calculator →✓
And if you don't even own the ground the plant stands on? Then of all the controls only one remains: the choice.
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