when is the greenhouse warm enough?

Hi,
Complete newbie here with a greenhouse.  I'm going to build a cattle panel greenhouse this winter.  I have researched and plan to put bricks and water barrels in my greenhouse to absorb the daytime heat to keep it warm enough at night.  My question is, about when (month, temp wise) in the spring does it get warm enough during the day to accomplish keeping the temps high enough at night to keep seedlings in there?  Also, would a wood chip floor help keep any heat in?
Northcentral Montana Zone 4

Thank you!

My greenhouse (set into a hill, glazing rather than plastic) has a first frost date late November, and last frost of March 1. This is zone 6-7. It stays cold much longer, but it's never frozen inside after that date. I don't open it up permanently until the daytime temperatures are consistently in the 60's because it can easily hold 30-40 degrees higher than the outside during the day.

I have a waterwall and two 50 gallon barrels, so a total of about 200 gallons.

Look at your average temperatures. If your greenhouse maintains a night-time temperature 10-15 degrees above the outside (mine runs 12-15, entirely passive), then find the date when your greenhouse nights will be consistently above freezing. So if your outside average on March 1st is 15 degrees, you can expect the temperature in your greenhouse to be in the high 20's F. If your average on April 1st is 25, you're probably good. If you're growing in-ground, remember that your floor temperature is usually going to be higher than the air, so if your air inside is 35 F the floor will probably be in the mid forties.

Daytime temps can fluctuate, as long as they stay below about 100 F. Night-time temps are going to be your killer during the winter.

I start my in-ground greenhouse seedlings mid March. Other seedlings are transferred out into the greenhouse about the same time.

I have a woodchip floor, but I can't say whether it makes a difference as I've never had anything else.

Good question so I’m following. I’ve never had a greenhouse and will need one so I’d like to know myself. I’ve been playing with the idea of having the compost bin in the greenhouse could that be done in a way to put off enough heat? Anyone have advice or thoughts on this plan?

Deborah Matt wrote:Good question so I’m following. I’ve never had a greenhouse and will need one so I’d like to know myself. I’ve been playing with the idea of having the compost bin in the greenhouse could that be done in a way to put off enough heat? Anyone have advice or thoughts on this plan?

It can be done that way. Some people have said that it created too much gas and smell, others have done it apparently without that problem.

Another did the compost pile against the back wall, piled between the greenhouse and a secondary wall. Same deal, in a sense.

I tried the compost inside my greenhouse for two winters and never got it to work.  But it did make gasses and mold so maybe it was working.  My pile was enclosed with a fan to aerate it so maybe an open pile would cook better.  But offgas more.  I would advise against it.

Here is an related thread that made me think carefully about using water and other thermal mass in a greenhouse https://permies.com/t/132290/Gallon-Drums

The gist is that water can work against you on cloudy days in spring and fall unless you have another way to heat it when the sun isn't there to do the work.

Matt Todd wrote:The gist is that water can work against you on cloudy days in spring and fall unless you have another way to heat it when the sun isn't there to do the work.

It works for me. I haven't noticed a significant difference in the spring after three years of record keeping. In fall as the sun starts to drop is the hottest part of the year in the greenhouse, but that's not the water mass.

Much depends upon the individual kinds of plants.  I have a high tunnel.  I am in the process of building a concrete block addition (4×4) on the N side to house a small wood stove.  I am not interesting in heating it all winter. I just want to be able to deal with heavy frosts.

John F Dean wrote:Much depends upon the individual kinds of plants.  I have a high tunnel.  I am in the process of building a concrete block addition (4×4) on the N side to house a small wood stove.  I am not interesting in heating it all winter. I just want to be able to deal with heavy frosts.

Yes, that is what I would be dealing with also, heavy frosts in the spring.