Well, close up one of your containers, place it in full sun on any day above 50 degrees, sit there and watch the thermometer and when the plants looked wilted and dead you'll have an estimate of that temperature.

Seriously, You don't want the soil to dry out. And if you haven't vented the tops the most seedlings will be stressed from the high heat and humidity--how high, hmmm, into the 90s in a closed container.

There's some things you can do to ward off heat stress. Adequately water the soil. Move the containers to part shade, or move part shade to the containers. On very hot days I run a sprinkler that keeps my containers cool and moist. I've moved them under trees which are leafing out. I've set lawn chairs over the containers to provide shade. If I had too, I could tie up a sheet to poles and make a temporary awning.

WS is all about being creative. Put on your thinking cap and give it your best shot.

T

I heard the rule of thumb is that it is ten degrees hotter inside your containers than it is outside. Most plants don't need more than seventy degrees to sprout do they?

A cheap white cotton bedsheet works great. Blocking the sun, it'll keep the seedlings cooler.

Thanks everyone
The temps inside my containers are running at about 78 degrees on 67 degree day.
I will move them if they get up into the 80-85 degrees or at least pop their tops.

That will work fine, don't forget to give the soil a good soaking too.

T