All your listed concerns are very valid. Lots of folks have killed the seedlings quickly by cooking them under a dome.

Such a set-up in no way equals the venting, air exchange, temp control and air circulation in a greenhouse.

There is still plenty of time in zone 4 to "bring them up to speed" normally.

There are good reasons for properly hardening off plants gradually. :)

Dave

I only keep them on at night.

Which concerns are "most concerning"? Cooking them?

My situation is not great, I currently have the flats in a basement with 2 verilux incandescent spotlight bultbs beaming down on them for 12 hours a day. So not sure how I'm going to get them up to speed with 4-6 weeks left. I have very limited SE facing window space and the wife goes buggy whenever a bug is seen in the area.

Am I able to successfully harden off younger plants? I have plants that have been growing in small cells under a dome since early Feb but weren't moved to bigger pots as soon as they could have been (just a day or two ago). Others have been growing in a 72 cell tray for only 4 weeks now (they are due to show second leaves any day now)

My other thought was to whip together a 2" X 2" Frame and hammer staple the clearest poly I can find to the outside, possibly also to the inner side of the lumber as well to make a double sided and more insulating frame. That would seem to keep them from getting a lot of cold damage at night here were the forecast still shows mid 30s and possible 30 overnights but maybe I'm being hopeful.

Which concerns are "most concerning"?

Let's back up a minute and discuss the role of domes in growing seedlings indoors. First, most experienced growers don't use them at all so domes sure aren't required for any reason. Rather they are a "crutch", a tool developed for newbies and inexperienced growers to make it easier for them to maintain the soil moisture levels until the seeds germinate. Once germination happens in the tray, the standard recommendation is to remove them and leave them off.

This is because the prevent adequate air circulation, trap stagnant air inside, and don't allow for proper moisture evaporation. That is the perfect recipe for damp-off, a fungus disease that attacks and kills seedlings. This is all discussed in much more detail in the FAQS on the Growing from Seed forum here.

But manufacturers market them to make more money and mislead the public by showing pictures of plants growing under them. They make it appear they are required when they aren't and when the potential harm from putting them on plants indoors far outweighs any possible benefits. Some even go so far as to call them "mini-greenhouses" when there is no comparison as any GH grower can tell you.

So, that said, there is no recommended role for dome covers when growing seedlings indoors once germination has happened and the risks associated with using them are several.

In addition to the damp-off, there is heat buildup which can kill the plants and trapped moisture which can lead to root rot and attracts insects such as fungus gnats and spider mites, stagnant air when fresh air is required, and reduced light exposure.

Your problems are not going to be solved by leaving a dome on your plants. You have left them under a dome since February and all that has done is stunt their growth and caused roots to rot. Plus you are using incandescent lights which produce lots of heat but very little light that is useable by the plants no matter how long you leave them on. Only cool white fluorescent bulbs are used in growing. seedlings and the standard recommendation for them is 16 hours on although 24 hours causes no harm.

Again, all of this is discussed in great detail in the FAQs and many discussions over on the Growing from Seed forum here.

Wheter you can harden them off or not I can't say. That is done outside, not inside so is a totally different discussion. But no, you cannot take them from growing indoors directly out into the direct sun.

Dave

Alright, you've thoroughly explained to me what's not needed and how not to screw things up.

Would you expand on how you would recommend I "bring them up to speed" normally?

It's so late in the season I could buy a light rig but we're down to weeks now and not months. As I mentioned the best light I have is SE facing.

If I could see them I could suggest some ideas. Definitely take the domes off.

How old are they, when were they started, how tall, any true leaves, if so how many? What are they growing in - soil and containers? Have you fed them anything?

Your plant out date for zone 4 tomatoes isn't until end of May at the earliest I think and peppers would be 2 weeks later. 6 weeks is plenty of time for tomatoes to be ready to go out and 8 weeks is the normal time for peppers too.

Do you have any fluorescent light fixtures in the house? if so can the plants be placed close to them?

And please do check out all the info on the Growing from Seed forum as it will answer at least some of your questions.

Dave

I think I got the zone wrong, we're kind of 4.5 here and usually go with May 15th.

I have true leaves on 2/3 of the stuff (both peppers and tomatoes). The other 1/3 are just tomatoes and think they'll be 3 weeks this weekend so should be getting their second leaves any minute.

I move them into some NothrupKing 20 count flats (probably 3" X 3") and filled with some Happy Frog potting soil full of humic acid, guano, worm castings and more. This would seem pretty rich, would you start supplementing with anything else in the water? I did pick up some "Tomato Maker" product that I could dilute down to 1/10 or 1/20.

Since all of these flats could fit in a 5X5 or 6X6' area, I'm thinking I should probably get a fluorescent fixture. Would a 4 ft, 2 bulb T5 fixture do the job, and is it worth the extra money for the high ouput variety?

Sounds like I should run the heck out of the light, maybe 24/7. I like experiments so I'd be interested in putting some of them out in a homemade cold frame to compare results. Possibly I'd shade them from direct sun for a bit but I have no experience with hardening off combined with coldframing.

Got a 4 tier "greenhouse" a few years ago from TSC- paid about 19.99 on sale for it. Works great and has held up well. I think its size keeps things from cooking in the sun and I can carry it in/out of the garage and use a bungee cord to secure it to a clothes line pole to keep the wind from knocking it over.