What's actually happening?
Many substrates are hygroscopic – meaning they can absorb moisture from the surrounding air and release it again. For outdoor plants, this happens in a natural daily cycle:
At night and in the morning, humidity levels are higher. The substrate absorbs moisture from the air – the measured value rises.
During the day, temperature and sunlight increase, the air dries out and the substrate releases moisture again – the value drops.
What the graph is showing you
The graph above shows exactly this interplay: at the top you can see the humidity – it rises regularly at night and in the morning and drops during the day. Below you can see the soil moisture – it follows the same rhythm, because the substrate responds to the humidity in the air. The waves at the top create the waves at the bottom. Both curves are not moving randomly – they are directly linked.
How are moisture, temperature and light connected?
The three parameters influence each other – and this is clearly visible in the graph:
High temperatures and strong sunlight during the day accelerate evaporation from the substrate → soil moisture drops.
Falling temperatures in the evening and rising humidity at night → soil moisture rises slightly again.
On overcast days or in low light, the effect is smaller – the curve runs flatter.
What really matters?
Not the individual peaks and dips, but the overall trend and the coloured zones in the graph:
Green = optimal range – your plant is well supplied.
Light blue = act soon – the value is approaching the critical range.
Grey = water now – the value is in the critical range.
As long as the curve stays mostly in the green zone – even if it moves up and down regularly – your plant is doing well. A wave-like pattern within the optimal range is not a problem. It's actually a sign that the sensor is precisely capturing the natural conditions around your plant.
When should I actually water?
For outdoor plants with natural daily fluctuations, the rule is: don't look at individual readings, but at the overall picture across the week. If the curve stays mostly in the green zone and recovers each day despite the fluctuations – everything is fine.
It's time to water when the lowest point of the daily fluctuation regularly drops below the green zone, or when the overall trend drifts noticeably downward over several days. That's the signal that the substrate is becoming too dry overall – regardless of what the night-time reading shows.