I would suggest putting up a 50% shade cloth over your greenhouse to keep your tender tomatoes from too much heat until they get acclimated.

When is it safe to place tomato seedlings in a greenhouse on warm days?

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It depends HOW BIG and HOW AIR TIGHT your greenhouse is. The bigger and the taller the less chances of getting too hot.

You can install a thermostatically controlled exhaust fan to kick in when temps rise to a pre set level. This is a goof proof way.

As long as you can be there to closely monitor it at least hourly to keep the temps between 45 and 65 to70 max, fine. But you are months away from of your plant out date so I sure wouldn't rush them out into it.

Seedlings can't tolerate the temps of established plants and those units can climb to 90+ within minutes and fall just as quickly, Plus they have minimal air circulation in them. You'd come home to cooked and dead plants.

Check out the problem discussions with them over on the Greenhouses & Garden Structures forum here. They stress that they require very close attention when the weather is unstable as it is in the spring.

Dave

Agree that you want to closely monitor but sunlight is going to be more critical than heat initially. Cal mentioned shade cloth as a remedy but when you first move plants out in the structure you may want to set plants out of direct sunlight, or cold breeze, and mist with water if they begin to wilt.

In Denver I'd imagine your nights are still going to be dipping beloiw freezing and,as Seysonn mentioned, your small structure will add little temperature buffer. Of coarse your small unit will also be easier to heat so consider a back-up heat plan for cold nights. You could move the plants for cool nights, throw a thermal blanket over the structure, provide inside heat by various means or constantly spray plants with water throughout the freezing time of the night.

To answer when its safe to leave plants unattended is difficult to determine. If the sun is bright and your structure is closed tightly you could cook your plants before 9:00 A.M. If you work days you might be better to set plants outside the structure unless it is thermostatically or otherwise well ventilated.

Those little greenhouses are great but they can also be a pain in the rear. I've got one. The days that the weather stays the same are easy (for example, when it stays sunny or cloudy all day or when the temps are mild). It's the days that start out cool and overcast and then later transition to mild and sunny that you have to really watch out for. Or if it's cool and it keeps switching between cloudy and sunny. The temperature inside them climbs quickly. It can easily get well over 100 degrees if it's sunny and the greenhouse is closed. On more than one occasion I've come home to find an overheated greenhouse and dead plants. If the plants are watered well and you open the greenhouse quickly then most of them usually survive (you'll probably have some losses). If the plants are exposed to the high temps for any extended amount of time they are toast.

My point: If you know the weather is going to be cool and screwy on a particular day, it would probably be best to keep the plants indoors under lights (if it's possible). Unless you can keep a close eye on them and open the greenhouse when necessary.

Rodney

This post was edited by theforgottenone1013 on Wed, Mar 5, 14 at 9:29

I keep mine in the house under grow lamps on the cool days, and also at night. I put them out in the shade on sunny, and warm days.

Though I am farther south in Tx. So we have days with 75 to 80 degree weather. Then in a few hours it can drop to 35 and start raining. Then a few hours after that it drops into the teens. It did that this past week. Sat high was 80. By 8 am on Sun it was 34. Then the rain started pouring, then it froze.

It made me glad I put the plants in the house Sat evening.

Once the weather goes to night time lows over 50 for two weeks, with at least another week forecast to be similar I start to harden the plants to sunlight, and evening temps.

I use one of those small four shelf greenhouses inside my larger greenhouse that would be costly to heat. Got one at Big Lots for $30. At the bottom of the small greenhouse I put a small crock pot filled dry with lava rock and leave it on all night. It makes a nice cozy environment and it's only 45 watts.

I missed part of your description of your GREENHOUSE in my previous response.

You have already experimented and have measured the temps. That is the way to do it. That gives you a pretty good idea as how hot it can get inside that mini greenhouse. I can guess that as long as the temperature outside is under 50F, there in NO chance for the temperature inside that MGH to go anywhere near 85F. It also depends on HOW MANY HOURS of direct sun it gets, how windy it is. If you are not using it to full capacity, I would suggest NOT to use the top shelf, where there is a possibility of getting much warmer.