Potting shed greenhouse and night time temperatures for seed starting

I have a combo shed/potting shed. The potting shed side has polycarbonate roof, single pain windows, and some regular 2x4 walls and siding. The floor is slightly elevated off the ground and has gaps between the wood floor (more like 2x6s with small gaps between).

My use case right now is really centered around seed starting for a large garden in zone 6b.

This is new for me, and I set up a temperature sensor recently and found that the potting shed area gets much hotter during the day with sun, but at night it tends to match the outdoor temperature pretty closely. I believe this will make it very difficult to do seed starting early like this and am curious what I must do to gain 10-20F higher temperatures at night (my guess for bare minimum!). I do have power but would like to use as little as possible. I also have an exhaust fan for the hot temp days.


My initial thoughts include stones around the perimeter to stop airflow under the floor - since it’s slightly elevated off the ground. Basically enclose under the shed, is that enough? I could add insulation to the walls if needed, worth it? The roof is polycarbonate and I can’t cover that so assume this is as good as it gets? I could do heating mats under my seeds if needed? If I must, and thermostat controlled oil or baseboard heater? Other thoughts?

Thanks!

Some folks build RMH aka Rocket Mass Heaters for their greenhouses or make a simple Rocket stove.

Another option would be heat mats and grow lights.

Your idea about using stones might work in a temperate warm weather region.

Maybe trial and error would give you some idea of what works best.

The two general ways of doing this is to make the heat last longer or add more heat.

Insulation would probably be a good option, as it would help the heat stay longer. Using some sort of heat storage like barrels of water or stones could also work. A combination of both would be really good too.

Or you could add heat, like the heat mat or a RMH like Anne suggested. I've used a heat tray before. I used those electric pipe heat tape and run it back and forth on an insulated table with a thin layer of sand to help spread the heat. There are probably better options, but it was what I had, and it worked fairly well.

One thing to remember is that soil temp is more important than air temp when it comes to seedlings. Obviously if it gets too cold they will die, but if their roots are warm, they can stand a lot colder temps on their leaves.

My best answer would be to recover the floor with plywood, sealing all edges to help stop the air leaks.  Also insulate the shed, (I would use foam board).

If you only need heat for a few weeks, use a dark color top on the bench ( to help hold the heat), also using some kind of "mass" to help hold the heat - a few stone tiles or broken countertop material, build an indoor "cold frame" over the seed trays, perhaps use a kerosene lantern under the bench with a hanging sheet metal barrier between heater and bench, or use the electric seed mats ( I once saw an old electric blanket double sealed in plastic used as a heat mat )

Best of luck, please keep us posted on what you try and how it worked!

I'm also in USDA zone 6 b, not that it really means all that much. The only reason I might mess with anything other just direct seeding my garden is if I want to produce plants to sell and for them, I use a very low tech, unheated cold frame. Low tech in that if it is warm and sunny, I might have to open it to vent and if it's going to drop into the low 20s or teens, I toss an old blanket over it.

Greenhouses are great for lots of things, and I would like to have a nice one, but to grow a garden in my climate they are completely unnecessary. The cold frames are really unnecessary for a garden too, but very useful for producing things to sell.

All good ideas here, thanks everyone.

Mark R - this is a good point and I have considered it, but I found that germination and seedling life outside has been really challenging.  I've attributed it most to my soil, and like to be able to grow more seedlings and then transplant the best ones/give some away to neighbors.  But to your point maybe a cold frame for seed starting directly outside is something to try too.  More to think through!

- this is a good point and I have considered it, but I found that germination and seedling life outside has been really challenging.  I've attributed it most to my soil, and like to be able to grow more seedlings and then transplant the best ones/give some away to neighbors.  But to your point maybe a cold frame for seed starting directly outside is something to try too.  More to think through!

More to think about ...

I am the one who likes to start my seeds outside the way Mother Nature does it.

I don't have success trying to plant inside, I have no idea why.

All I can say is do what works for you.

Welcome to the forum!

I have done a little research on heating a greenhouse without power. I'm in a warmer zone, but the suggestions should still apply. I would just combine them. Someone mentioned a heat sink, and that's a great idea. I took all my heavier jugs, like bleach, and laundry soap container. I cleaned the outside and painted them black. I filled them with water and placed them between the plants/seedlings I want to protect. I also had a gap around the bottom of my greenhouse. I made a simple frame and filled it with concrete. It was easy and pretty cheap. I would think anything that seals the gap would work. I have a gravel floor, because I was inexpensive, but brick, or pavers will work too.  If you can get some tile, brick, I would think even terracotta saucers would work. Paint them black. Basically anything you can find that you can paint black, and will hold the heat it soaks up during the day will help. You can also use milk jugs like the winter sow method. It adds another layer.  There's lots of videos on how to make a heater with terracotta pots stacked on top of each other, using a jar full of crisco and a wick. It's supposed to keep the space warm for about 8 hours. I want to try it, but haven't yet.
I start my warm weather seeds in the house. Some on a heat mat, and humidity dome. As soon as they sprout I transplant them in a small pot under a grow light until they have two true leaves. Then I put them in there own pot and they go out to the greenhouse.  My greenhouse is new to me as of last year, so I'm still experimenting. I did try to sprout seed in it last year. It did work, maybe even better seedlings then in the house, but they took so much longer, and I really had to be on the soil moisture, because it would easily dry out during the day, so inside is easier for me.
Good luck, let us know how it goes.