Why “every three days” is a useless answer
The most repeated piece of watering advice on the internet is also the least useful: water every three days. That number doesn’t depend on the plant, but on the pot you bought, the substrate you used, the temperature of the week and whether there’s wind today. A Mediterranean herb in a two-litre pot and the same herb in a thirty-litre pot have nothing in common, even though they’re the same plant.
What does work is answering three questions with data: how much water is available in the substrate, how much is lost through evaporation, and how often you can replace it.
The two numbers that control the whole system
The water reserve of the substrate (what the pot holds between waterings) and evapotranspiration (what the plant consumes and what evaporates) determine the interval between waterings. The relationship between the two is so direct that you can calculate it with a kitchen scale.
To estimate the water demand of the climate, the international reference method is FAO-56: a reference evapotranspiration phase, which describes the atmospheric demand, and a crop coefficient, which describes what that particular plant needs. If you look up the data for your local station on AEMET OpenData and apply a sensible crop coefficient, you get an estimate with a scientific basis instead of a guess.
The weight method: the one that doesn’t fail
It’s the most reliable method and the most neglected one.
- Weigh the pot with the plant and the substrate completely dry.
- Water until water comes out of the holes and let it drain.
- Weigh it again. The difference is the water the substrate has held.
- Weigh it again every two or three days, always at the same time of day.
With those figures you have three answers that no table can give you: how much water your pot holds, how much your plant consumes per day and how often it runs out. With a small pot the difference shows up in a single day; with a large one it can take five, and that’s where the controller comes in, so you don’t have to remember on your own.
What changes from one plant to another
| Plant type | Substrate reserve | Relative use | Sign it is short of water |
|---|---|---|---|
| Succulents and cacti | Very low | Very low | Substrate dries out entirely |
| Mediterranean herbs | Low | Medium | Soft leaves, faded colour |
| Leafy vegetables | Medium | High | Leaves drooping at midday |
| Houseplants | Medium | Medium | Dry tip, compacted soil |
| Ferns and shade plants | Medium | Medium | Brown, dry edges |
| Citrus and fruit trees | High outdoors | Medium-high | General wilting |
This ranking is deliberately qualitative. It helps you understand the order of magnitude, not calculate exact litres: for that there are the weight and the local climate.
Where the water is lost
Almost all the loss comes from three places, and all three can be corrected:
- Direct evaporation from the substrate. The darker and looser the mix, the less evaporates. A layer of mulch or gravel on the surface helps.
- Evaporation from the leaves. It’s proportional to the leaf area and to how dry the air is. That’s why a large plant on an exposed balcony consumes more than a small one in a corner.
- Water that runs out through the holes. If you water until soaked and don’t wait for the drainage, you’ve lost that water and created a salt problem on top. The details are in substrate and drainage in pots.
Signs you’ve gone too far and signs you’re short of water
You’ve gone too far if the plant leans with the substrate permanently soaked, if there’s a musty smell, if the underside of the leaves darkens and if insects appear around the substrate. You’re short of water if the leaves go limp and then dry out suddenly, if the tips turn brown, if the soil pulls away from the pot as it dries, and if the plant wilts in the hottest hours and recovers at night.
The reason why this happens is covered in dying plants and overwatering.
Why the leaves look fine and the root doesn’t
The plant lives on the water available in the root zone, not on the water sitting in the pot. A surface layer that is still damp hides substrate that is already dry underneath, which is why watering a little and every day produces a plant with limp foliage and roots in poor condition.
Moisture sensors exist precisely to see what’s happening at root level, which is why the position of the probe isn’t a minor detail: a badly placed probe measures a zone that doesn’t represent the plant.
How to water: once thoroughly or a little every day
Watering a little and daily leaves salts and the oxygen behind in the top layers, and turns the substrate into a sponge that holds water on the surface and dries out underneath. Watering every few days, until water comes out of the holes, keeps the substrate homogeneous. You set the frequency from the measured weight, not from a calendar.
If it rains and you can collect that water, it sounds absurd to waste it: rain water is the water with the fewest salts. For limescale problems, hard water has its own article.
Watering in the morning or at night
If you can only water once a day, do it in the morning: the substrate evaporates during the day, the plant transpires with a properly hydrated root and the excess is gone before nightfall.
Watering at night isn’t forbidden, but it has two drawbacks:
- Foliage wet for hours encourages fungi and leaf spots on susceptible plants.
- More water is lost to evaporation, because the temperature drops and the air saturates, but the plant is no longer transpiring: the water just sits there unused.
The exception is houseplants in winter: with cold soil and artificial lighting, watering at night is more convenient and causes no problems, because the foliage doesn’t get wet if you water directly onto the substrate.
Outdoors and in the heat, the answer is simple: morning, whenever you can. The time of day changes irrigation efficiency; the volume is decided by the weight of the pot.
A practical method to start with
- Weigh the pot dry and saturated: you already have the reserve.
- Note the weight after two days of sun: you already have the consumption.
- Divide reserve by consumption and you have your real interval, which may not be “every three days”.
- Schedule that interval and use a soil sensor to correct it when the season changes.
- Repeat the weighing once a month. Substrate ages and the reserve changes.
This method doesn’t need a manual: it uses your own pot and your own climate, which is precisely what no generic table can give you.
Sources and next step
The methodological reference is FAO-56, which defines the evapotranspiration calculation, and you can get local temperature and rainfall data from AEMET OpenData. For substrate behaviour in pots, the University of Michigan guide to growing in containers explains the relationship between pot size, mix and watering frequency.
External sources
- FAO: Crop evapotranspiration, Paper 56 — reference evapotranspiration and crop coefficients.
- MSU Extension: planting in containers — substrate and watering behaviour in pots.
- AEMET OpenData — local climate data to adjust water demand.