Hi, I'm hoping someone can help me narrow my latest greenhouse problem down. I have been attempting to grow herbs and lettuce since the beginning of march. After 3 months I found the sun was not enough to allow growth so I added T5 bulbs and fans inside (I prefer to keep the greenhouse closed...bugs).

Over the past 3 weeks or so growth has been tremendous! I was finally on a roll. Temps have occasionally gone over 100f but typically remained between 62 and 95 ish. Because I know that it will get a lot hotter (lettuce heat intolerant) I needed to defend against the heat, I installed an exhaust fan yesterday. It is set to come on when the greenhouse reaches 85f.

Today when I got home, lots of my lettuce had wilted as well as all of my immature cilantro. I left the inside fans on overnight last night for the first time (because I had installed the exhaust, I thought it would be ok to do this). So when I saw all of the cilantro wilted and a few of the older leaves of the lettuce but none of the immature lettuce had wilted, I assumed it may have gotten too cold overnight with the fans on. So I reverted to previous set up of turning inside fans off when the lights go off. I checked on it a few hours later tonight and my immature lettuce had begun to wilt. I turned the exhaust fan off all together until I can figure out the problem.

I have not yet created an intake vent, but the garden does not close very well so I thought that could wait a day or two.
The soil is moist, I have a moister reader and I use rice hulls as mulch which helps retain moister.
I have a light fertilizer on top of the rice hulls so it will slow release when watered.
Exhaust fan Airflow: 110 CFM
Fertilizer: Espoma Tomato-tone Granules Organic Plant Food

I don't want to loose all of my progress. I've already sowed more seeds but can anyone assist on what my issue may be, seeing as all this happened overnight. I cut most of the wilted leaves earlier before I decided to reach out of help so you won't see many wilted leaves, just less leaves on the larger lettuce. There was not enough cilantro yet to trim back the wilted portions so that was left as it was.

Thank you for your help in advance.

Hi @SunnyDayinGA and welcome to the forum

I have come back to your post several times since you first wrote it, but I have to admit I don't have a good answer for you. Is the fan drying out the soil faster than usual? Are you creating some type of negative effect by exhausting without an intake? As I said, I really don't have a good answer for you, but I wanted to reply so you didn't think we were ignoring your post. Perhaps someone with better knowledge and more experience than me will come along with a better answer.

If you don't have an intake and don't leave a door open, how can the exhaust fan do much? It can only draw from various leaks, and it will otherwise just churn the air in the vicinity of the fan, maybe not even that. Any hot air to be removed must allow for outside air to be admitted. I doubt very much if you're getting any real air flow. It may come on at 85, but if it can't get any outside air, it's just going to get hotter.

If bugs are a problem, install a screened intake opening or a screen door opposite the exhaust. If wild outside temperature extremes is a problem, perhaps look for a passive intake vent with louvers that can be pulled open by the fan and fall shut when it's off.

I can't imagine many places where it's going to get so cold inside a greenhouse that it can harm plants but where the house overheats badly. The Earth heat stored under plastic will see to the cold night, even if the outside temperature gets very low, which I doubt it will. Place a thermometer inside the greenhouse and see what it hits in the heat of the day. Turn on the exhaust with enough of an intake to allow free flow, and see what happens to the temperature.

Thank you both so much for your replies. After I didn't get any replies for a while, I decided to turn the fan back on and just let it be. I have also since added two intake vents (I was waiting for the screens to come because I had to measure before cutting). Everything is doing nicely now. I think the problem was, I changed the environment too quickly because once I re-sowed the seeds and cut back all the wilted leaves everything is growing wonderfully.

I have a pushsensor in the greenhouse and once I added the exhaust (even before adding the intake vents) my temps were tolerable. I could see an obvious average temp difference compared to before adding exhaust. The seams do not come together well at all so I figured before it started getting too hot, no intake would probably be ok. (I researched if negative air pressure would negatively effect and I didn't find that it would) But now that its getting a bit hotter I have even added a high output fan to the outside of the greenhouse to move the hot air sitting around it, so that the exhaust would be able to pull in cooler air as well...since one of the intake vents is right by a light.

Again, I really thank you both for taking the time to help me out because I was truly at a loss, I made my adjustments and threw my hands up in the air..."if I had to eat only what I grew, I would starve" lol

Let me know if you have any additional pointers....