A garden loses water in two ways: it evaporates from the soil and it is given off by the leaves. The sum of the two is called evapotranspiration. It is calculated from temperature, wind, humidity and radiation, and is expressed in millimetres, like rain.
Why this indicator matters
This equivalence makes the balance simple: one millimetre of rain brings one litre per square metre, and four millimetres of evapotranspiration takes four litres away. If the coming days' rain covers the need, watering is a waste; if it does not, you have to make up the difference, and do it at the right time.
The right time is early morning: the air is cool, the wind light, and the water has time to soak in before the sun draws it back out. Watering in full sun loses a good part to evaporation, and watering in the evening encourages leaf diseases, as the foliage stays wet all night.
How to read it
The box adds up the rain of the next forty-eight hours and compares it with the need for the next two days, given by the forecast's reference evapotranspiration. It ends with one of three cases: water, wait for the rain, or top the rain up with a small amount.
It gives the morning slot, from sunrise, and the missing amount in litres per square metre. It stays silent when the garden is dormant, with a high below 8 °C, when the need is low, or when a shower is likely.
The thresholds used on this site
Every sentence in the “Today” box comes from a numerical rule. Here are the rules for this indicator, exactly as they are applied in the code.
- Wait: at least 5 mm of rain forecast over 48 h.
- Water: less than 2 mm over 48 h and a need of at least 6 mm over 2 days; top up in between.
- A probability of rain of at least 60 % postpones watering: the indicator stays silent.
- Below a high of 8 °C, the garden uses almost nothing: the indicator stays silent.
- With no evapotranspiration available: 0.2 mm per day and per degree above 5 °C, capped at 6 mm.
A worked example: Toronto
In Toronto, with the forecasts of 2026-11-02 08:13, this indicator stays silent: none of the conditions above is met at the moment. The rain expected within 48 h is 2.9 mm.
Its limits
Reference evapotranspiration describes a well-fed lawn: a vegetable patch in full growth, a young hedge or potted plants have different needs, and a sandy soil empties faster than a clay soil. The calculation does not allow for mulching, which reduces the need, nor for local water restrictions, which must be respected.
Frequently asked questions
How many litres per square metre should I give?
The box gives the missing amount: the two days' need minus the forecast rain. A generous, infrequent watering is better than small daily amounts, which keep the roots near the surface.
Why not in the evening?
Evening suits a scorching summer, but it leaves the leaves damp overnight and encourages downy mildew and powdery mildew. Morning remains the best compromise.
Does 2 mm of rain count?
Barely: it wets the foliage and the soil surface, but does not reach the roots. It takes several millimetres to moisten the soil in depth.