For your long weekend period, a seed starting tray, with plastic dome would work. The seed tray cells with media and seeds should be well watered Friday afternoon, and then the tray with sufficient water and dome cover should get you through to your Monday morning rescue, if sufficient tray temps. are maintained.....

Where are you and what conditions do you see in your greenhouse?

Consider clear plastic bags. Just pull one over each plant. I have newspapers delivered in long tubular bags and, when cut in half, they are *perfect* for pulling over a seedling cup. Zero evaporation with the bags in place. I keep my seedlings in bags until they are an inch or two tall, by which time their roots are deep. Once out of the bags (under lights at 75F), I only need to water every two or three days. In fact, after seeding, I never water at all until they come out of the bags.

If you feel the need to bag a plant for a few days, it's not too hard for a dozen plants. Harder if you have a hundred.

These Burpee trays work well for me but they do cost a little more than the above suggestions. They come in 32 cell and 72 cell. I use them every year and they work great. The second year I just put any kind of seed starting soil in the cells.

https://www.amazon.com/Burpee-Watering-Starting-Replacement-Water/dp/B00SNM24VU/ref=asc_df_B00SNM24VU/?tag=hyprod-20&linkCode=df0&hvadid=241883109355&hvpos=1o2&hvnetw=g&hvrand=18024724620639548374&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9010872&hvtargid=pla-415753948222&psc=1

I too would want to know more about the conditions in the greenhouse. I don't find it necessary to water either seedlings or plants every day and a weekend wouldn't faze them. Whereas leaving a dome or plastic bag on trays or pots in sunny conditions can literally cook the plants. All my seedlings are raised in a greenhouse. No lights.

Well, a closed greenhouse is like a big clear plastic bag over plants. So if your plants aren't getting cooked in your greenhouse, they likely won't get cooked in a clear plastic bag. Black plastic is another matter, but you won't be using that. Now, my seedlings are always under lights, and the art is keeping them warm enough, not cool enough. But care is in order.

I always recommend that indoor gardeners outfit themselves with some thermometers. Those sensor-on-a-cord digital thermometers you get from China are dirt cheap. Just a few bucks apiece. Get a bunch of those.

I disagree that a green house is the equivalent of a plastic bag. The volume of air is so much greater that the temperature is much more buffered and will not rise so fast or high. I grow in a small greenhouse. I also start cuttings and seed under bags and domes. The latter need constant monitoring. The greenhouse, at this time of year doesn't need half so much watching. Plus there are automatic opening systems and shading available for the hotter months. I am quite sure trays and pots could manage a weekend without water if they were well hydrated on Friday afternoon. If anything, I'd shade them to stop them drying out rather than putting them under plastic bags. That would keep them moist but could also boil them. Especially inside a greenhouse.

I put a thermometer in a clear plastic bag out on the lawn in full sun. Outside air temp is 62F. Inside the bag it is 82F. That's very comfortable for most seedlings. The ratio of air volume inside to area of sunlight admitted is pretty much the same in both plastic bag and greenhouse. Yes, a greenhouse has lots more air, but it gets lots more sunlight. My plastic bag over a cup is a pretty good small scale thermal model for a greenhouse.

Now, if the outside air temperature was hotter, it might be uncomfortable for plants inside the bag. Same for an unventilated greenhouse. That's why I say that care is in order, and that's why, in a warm climate when it is warm outside, unventilated greenhouses are death traps for plants. The OP still hasn't told us what the situation is with his greenhouse. Is it 20F outside, or is it 90F outside? Is the greenhouse ventilated to control the temperature in it?

Awesome. Thanks. I think I am gonna go the bottom watering route. I should have noted that if I do move it to school full -time it would be in a classroom and not in the greenhouse. Maybe at some point we will have that capability but I don't think the greenhouse stays warm enough. Having said that, at what point would it be ok to transfer some tomato plants into the greenhouse for good? Would that point be determined by the avg. temps outside or by the age of the plants? I guess what I mean is, do I need to wait until the frost date or is there any point where the plants are established enough to move them out before we end our freezes? I am in zone 7b which is northern MS.

Whether the greenhouse is warm enough depends on what you are growing and what your weather is like. Can you tell us the answers to those questions? Do you have enough light in a classroom? And it could be too warm, depending on what plants they are).

Josh The best way to know what a greenhouse is doing temperature wise is a mini/max thermometer. I wouldn't operate my greenhouse without one. They tell you how low or high the temperature has been over a certain period. I also use an electric heater with a thermostat that I can set to freeze guard or on 5 degrees. It only comes on if the temperature drops below freezing or the temperature it's set for. I use a heat mat under my trays where I'm starting seeds and I never lose plants from freezing.

I would suggest that you inspect the greenhouse for air leaks and calk them. I do this every spring