Do you want to over-winter the plants, or just get an early start this year?

Take a look at the other thread I linked to below and see if it gives you ideas.

Here is a link that might be useful: pictures to give you ideas

Winter is over, it's early spring i'm worried about. Your greenhouse appears to be polycarbonate. Mine is 6 mil plastic.

I just came in as it has turned cloudy. 50 degrees outside the greenhouse 59 within. It's not completely closed up. I need to tape, patch holes and sorta seal the base from drafts. Definitely not an ideal structure.

i'm about as early as you can get because these plants were started about 1 year ago. They are the suckered growth of a suckered tomato plant but i consider them to be 1 year old or more.

I have got away with a 6 month old tomato plant(same variety)I had grown in the house on Halloween because that year late blight wiped out everything. I wrapped it up in heavy row cover and set it outside April 1st. It got a bit frost bitten. That was 2 or 3 years ago. Last year I tried to get an early jump in March and the tomatoes all died. They were set in the ground though. no double greenhouse just a single one.

There's a pic of my plants in the red cups at the bottom of that thread. My covering is 6-mil poly. Last year I had an inflated double layer; this year just a single layer.

Consensus wisdom is that each layer of plastic will give you roughly 3-5 degrees of buffer from the outside temp. But that's affected by a lot of variables, like the amount of mass around the plants and mostly the outside temperature. If it stays below freezing for long enough, any number of layers of plastic will still freeze.

Container plants will grow faster than in-ground plants, until the soil warms up. Then it is the opposite. I put plants out in early April last year and kept them alive through two frosts, but nothing grew hardly at all until the nighttime lows rose into about the 60s and the soil warmed.

I have a hoop house with 6 mil plastic and I do exactly what you are suggesting; every year i construct a plastic tent inside my greenhouse w/ my seedlings in it. I hang a spot light at the top of the tent to provide heat and light. It works like a dream. I make my tent on top of a table in the center of the green house making sure it's completely closed by wrapping the plastic all the way around the table to hold in the heat. I put the light on a timer so it cuts off at dawn and cuts on at dusk. I make a "door" to my tent so i can water the babies. I have great success with this method. Good luck!

Oh I see Robbie, I stopped scrolling too soon. What temps are they experiencing?

Just got my greenhouse up to 85 degrees today. Been fixing the holes. I fear the ground cold will green right in though.

You've got me tempted JCrowder,

It's 85 in there right now. Think I'm gonna try it. I feel like spring is about to burst. The birds are singing. Yeah here goes nothing