How to detect dew on grass?

I'm about to jump into the robot mower world. I'd like to mow at night, starting at like 1am and finishing around 6am, to keep the family from having to sidestep the mower during the day, but understand that might not be best for the health of the grass. In particular, my understanding is that mowing when the grass is "wet" from dew risks a subpar cut due to grass blades not standing straight up, and exposes grass blades to higher likelihood of disease. Maybe that's not as big of a risk when the robot mower is just taking off very small amounts (millimeters) every day. But either way, I might feel more responsible if I could avoid mowing on nights when (1) the grass is actually wet from dew; or (2) weather conditions are favorable for morning dew.

My question is this: how can I go about detecting (1) actual moisture on the grass from dew; and (2) favorable weather conditions for dew. Note this is different than detecting rain, as my irrigation rain sensor does not trigger when there's just dew.

Feel free to also chime in on your personal experience with whether or not robot mowing at night has proved problematic for your grass health :slight_smile:

I hope you don't have neighbors! I'm not sure about sensing dew. Mowing wet grass though will create dead spots if not raked up. Also if you have irrigation/sprinklers going on it's not ideal during those times. What about mid week late evening? Even during dinnertime when most people aren't in their yard? (Just a thought)

How about dew point from a weather API/station?
or you can attach a moisture sensor with probe to the lawnmower.


This, ideally from a weather station on-site.

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I found this about dew point :
"The dew point temperature indicates the temperature at which the air must be cooled in order for it to reach a relative humidity of 100% and become saturated with water vapor. When a surface is cooled beyond that point, the water in the air condenses, forming beads of liquid water – or dew.
When the dewpoint and temperature are relatively close, you will likely be waking up to a lawn covered in dew the following morning."

So, what kind of rule would you make?

If current temp is within 5 degrees of the dew point, consider dew present?
Or if current temp is below the dew point, consider dew present?

Does the amount of cloudiness matter? I also understand that dew is more likely to be present if it's a clear sky than if it's cloudy.

Interesting idea. I'll probably experiment with this to see how accurate it ends up being. Good thought.

To be honest, I don't really know how many degree you will set to have dew on your lawn.
I am in Canada West coast and dew is pretty much an everyday thing. I know I have dew last night and my temp is around 6C and dew point is around 3C.

Collectively these guys gave you most of what you need to know. Dew is coincident with the air temperature at the ground level reaching the dewpoint temp. In most areas (unless there is some local valley/mountain air current in play) the summertime temp low occurs within the hour and a half before sunrise.

I would bet you could cut between midnight and 3AM and be pretty safe without over sensor-ing the problem. However, this all does depend on where you are. Coastal weather is a whole different kettle of fish. Cold damp air masses, and rain, could come in with those evening sea breezes that are driven by the land thermals.

And about neighbors, if you are up North it is the case that a lot of folk don't have AC right? Annnnnd it might be the case that a lot of those folk sleep with open windows no? If you apply any sensor it might be a presence sensor so you know when someone runs over and grabs the thing to toss it in the local lake.

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I seem to recall that robot mowers just cut a bit at the top since they cut so often - would it be a problem in that case if the grass is wet?

Either dew point-temperature comparison or hi relative humidity should do it. Those may also correlate to rain reasonably well. You'll likely have to build in a long delay before resuming after a dew/rain event. On a calm, overcast day, it will take a longer for dew to disappear than it will with a bit of sun or breeze.

Tasmota with a bosch BME280 sensor will give dewpoint readings (along with temp, humidity & pressure) and can be imported to HE via node red or MQTT.

You might be able to use a leak detector (in a suitable enclosure) with a remote probe laid close to grass in flowerbed, etc as an actual detector.

Could you sit a leak sensor facing up near the the mower? Not sure if dew would produce enough moisture to establish a circuit or not, I've never used on myself...

Well, dew point alone doesn’t seem to predict morning dew for me. This morning there is morning dew even though the temperature was at least 10 degrees F above the dew point all night. Unless there’s some other variable to add to the rule along with dew point? Meanwhile I’ll test out a moisture probe to see if it will pick up dew.

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Is the dew point calculated from sensors right outside your home? Or a weather service?

This. Do you think dew point is hyper local to where it would deviate by as much as 10 degrees F? I guess you’re right though in the sense that perhaps the location of the dew point sensing is contributing another variable to the equation already….wonder if there’s a cheap PWS that I could give a try.

Honestly, I dunno. Sometimes temp and humidity conditions in your back yard can vary notably from nearby weather stations that supply data to cloud services, depending on geographic features, lakes etc.

To calculate dew point, I believe all you need is an outdoor temp and humidity sensor. I like Ecowitt devices, although you need their wifi bridge, which can integrate with Hubitat.

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Ah, that's true. Do you think a product like the Zooz ZSE44 would work well? It says to mount it under the eve, to avoid direct rain or direct sun. It would pair directly to HE without any gateway, so that would be nice...

I’m not familiar with that sensor specifically. But it says it’s rated for outdoor use, so probably a reasonable option (if it’s accurate). Zooz is a pretty well-liked brand among z-wave devices.

It could even be different at ground level vs. 12" off the ground. Soil moisture, wind conditions, etc. could all play significant roles.

I wonder a soil moisture sensor will be good enough by leaving it half exposed above ground.

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