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Subject:  ideal humidity for tomato plants

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Oak Grove, Mn

I am getting ahead of myself by a few weeks but my indoor controller has the option for humidity control. It can either increase or decrease but not both. Currently, I am at about 20 rh but with the addition of my deep water systems it should go up. After I know what it stabilizes at, I can adjust it. The problem, I don't have any idea what number to shoot for. What is the ideal humidity for tomato plants?

12/7/2019 8:02:25 PM


This is a quote from 1 site(hydroponic) and another site said 70% Don't know if it this any use to your set up.

"Temperature and humidity regime for tomatoes.
The ideal temperatures at which tomatoes grow are between 18°C at night and 28-30°C during the day. Pollination takes place between 8H00 and 11H00 in the morning. The grower must make sure that temperatures are not above 34°C during these periods otherwise yields will decrease due to a decrease in number of fruits developing. Growth also stops at temperatures above 34°C. The ideal relative humidity should be between 55% and 65%. These are ideal and cannot always be maintained, especially growing under shade cloth and conventional tunnels. There is always a trade-off between temperature and humidity."

12/7/2019 8:40:47 PM

G. Kins

Southwest WA

CO2 reduces stomata so that might be an extra variable for you. I would think to err low humidity would help prevent disease and improve pollination. Commercial greenhouses probably know whats optimal but they may utilize fungicides and bumblebees. Even if you mess up your grow still fun to watch.

12/8/2019 2:56:24 AM


Hertfordshire, UK

I'm away so not near my notes. But humidity target varies per life stage.

Also relative humidity changes with temperature. E.g. If you increase temp, and water in air remains constant, relative humidity drops. Warm air has a higher capacity for water.

There's much to be said for tracking VPD - which targets the differential between air and leaf temps and humidity. Targets here look to ensure that there is a humidity gradient between the two to ensure that transpiration takes place during sunlight hours.

This table useful with that in mind


12/8/2019 2:15:02 PM


Oak Grove, Mn

Thanks Doug, that link looks very useful. I am going to have to read up on some of the terms. Much for me to learn about humidity and growing. I can tell already, my room is too dry.

12/8/2019 5:21:05 PM

G. Kins

Southwest WA

LED's bnot... Save those high heat lamps for when you set a tomato.

12/10/2019 10:35:02 PM


Oak Grove, Mn

I have LED's but I do not like the purple light. It is difficult for me to see what is happening with the leaves. Today a humidifier will be delivered. Tomorrow a infrared laser thermometer should be coming. That will be used to calculate the VPD. This weekend, I should have decided on which dehumidifier I want. If I can figure out my lack of electricity in two weeks should have complete humidity control. I have plans to tap into two more household circuits for a total of 4 circuits tapped, drawing 8 amps per circuit. Should be just under 4000 watts available. 2000 watts is not enough for what I want to do.

12/11/2019 5:36:12 AM

G. Kins

Southwest WA

Awesome. As you may know mine is mostly getting the white spectrum, my colored lights mostly burned out. Anyhow I wouldn't blame how my plant looks on the spectrum, I think the white LED's (6000 K?) really do work. I forget the spectrum info, but you're probably gonna go your own route and you'll probably get a larger tomato. I did bump mine up with another 90 watts of mixed spectrum LED so hopefully I give you a little competition.

12/12/2019 3:17:46 PM

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