Overwintering in a greenhouse?

[Eckhoffw]

Ok y’all. I know this has been discussed to some extent, but I’m still needing some guidance on wether or not my greenhouse will make a good home for my plants overwinter.

So, Minnesota 4b or 5a. Not sure exactly these days. 8x12 Unheated greenhouse.
I was thinking of mulching in my plants under the benches I made inside.
My worry is that temperature fluctuations are great!early this fall I had this.Now I know the sun intensity and longevity is greatly reduce in the winter, but I still think I’m going to have problems with spikes in temp on warm days.

I thought about also completely shading out the plants under the benches with something, but I’m still worried about heat fluctuations.

Does anyone have any experience with this in my climate?
Any thoughts are welcome
Thank you.

 

[Paradox]

Which trees are you thinking of putting in there?

Most temperate trees that need dormancy need below 40 deg F for an extended period.
A couple of months at least and some species like longer and colder (scotts pines, mugo, Japanese White Pines}
If you cant keep them below 40 deg F then they wont go dormant for long enough or at all.

Fluctuating above and below 40 is dangerous

A coldframe or mulched in along the foundation of your house on the north or east side is your safest bet

 

[Deep Sea Diver]

I really like your greenhouse!

Not in your climate, yet we do have two greenhouses. One is exposed to the sun, the other is shaded. The coldest temperatures we had were 14F.

We leave the door open when the temperature is looking like it will be above 40F. This action, coupled with temperature actuated vents, roof and base and a good fan, have never been an issue with either greenhouse during cold temperatures. Both temperature actuators and vents can be obtained as add ons.

(Summertime is a whole different story. Even with the vents full open, the door open, 70% shade cloth and the fan going, the temperatures get into the 90’s. In this situation we water down the floor and roof a couple times a day to lower the temps.)

Fluctuating above 40F is not usually an issue unless it’s high enough temperature for a long enough time to break dormancy. Maybe 3 days or more. This won’t happen until the trees have accumulated enough chilling hours to meet their species quota. At that point it’s an issue and one would have to protect the trees from freeze damage.

This is explained in this resource in the dormancy section.

Good luck!

cheers
DSD sends

 

[Dav4]

I'd love a greenhouse like that to get a headstart on the growing season! As others have mentioned, heat is your enemy during winter. You want your cold hardy trees to stay consistently cold through winter and into spring when environmental factors are more amenable to growth. With that in mind, you either need to increase ventilation capacity- probably limited here- or you've got to blck the sun passing through the panels and creating heat gain. One option is whitewashing the exterior of the greenhouse verses covering the roof with white plastic sheeting... I think both can be sourced via greenhouse supplies. Once the trees have broken dormancy, you need to keep the temps in the greenhouse above freezing. That'll require a heat source and thermostat. Anyway, just some thoughts off the top of my head while I drink my morning coffee, wishing I had your problems.

 

[rockm]

You are going to have problems with heat, no way around it. Opening the doors can help, as can throwing something over the top of the greenhouse to block sunlight. If you really want to control things, however, a ventilation system is a must. I have long stored some trees of mine in a cold greenhouse at a nursery. That greenhouse is about ten times bigger than yours. The mass of the gravel floor and its covering can lag temps a lot, as can large containers of water kept inside...but your greenhouse has less mass to hold onto colder temps so fluctuations will be faster...

The nursery owners have a huge ventilation fan tied to a thermostat. That vents heat on warmer days (and warmer is a relative thing, as a 39 degree day with full sun can spike temps into the 80's inside the structure if left unvented). They also have the reverse "bonsai two step" occasionally in the late fall, bringing trees outside in the day, and back in after dark to avoid spiking temperatures inside.

 

[Robertji]

Hi there, this is my setup in Ontario Canada zone 5. I had the same concern about temperature but was wondering about the chill hour interruptions. A friend who studied horticulture assured me that chill hours were not affected by short interruptions as DSD pointed out. This is my 3rd winter with the greenhouse, under the tarp is single pane glass.

Year one I did nothing to manage heat, I just had a heater on a thermostat to keep things between 2 and 5 degrees C. Everything started leafing out in February which was fun and I could go out on sunny days without a coat at -25C!

Year 2 I put white tarp inside the roofline to little effect since the angle of the sun is so low it comes in the front.

This year I have blocked the front but put a mini blind on the door that I can open to let in the sun when I want to play.

As Dav4 points out the whole idea is to extend the growing season and I’m not willing to give that up so I’ll find the right compromise between heat and cold eventually. Last year I had flowers on my Fuji cherry at the end of February!
Cheers, Jim.

 

[jimib]

I have a similar greenhouse. The sun is blocked in the back by arborvitae’s, The rest of it I covered with a breathable cotton canvas that I got at the paint store. It’s basically just a dropcloth. I do that to keep the sun out. Shading those particular trees inside the greenhouse won’t help with the temperature fluctuations inside it. Also make sure you have a fan to keep air circulating.

 

[rockm]

This is a pic of the front of the cold greenhouse I store my live oak in. Note the big louvered vents and white and black coverings on the top of the structure. Inside, the floor is four inches of gravel.

 

[Eckhoffw]

Wow! First I just want to say thank you guys so much for all your insight!
I really appreciate it!
I’ll admit, when I got the greenhouse kit I wasn’t to sure what it would entail to actually store plants in it!
It has 4 vents (not automatic) and the sliding front doors, but as summer showed me just how hot it gets in there, I started realizing many mods/add-ons were going to be necessary.

I think this year I will mulch most of my plants outside per usual, and perhaps experiment with the greenhouse over winter to get a better idea what I’m up against.

I will document my findings, for any other newb thinking about trying a similar setup.

Thanks again!!!!

 

[Scorpius]

You want an automatic ventilation system with louvers to let air in and out automatically. If you can do this and find a dual heating/cooling thermostat that can be set to at least 15 degrees Fahrenheit you'll be set.

 

[Deep Sea Diver]

Personally I wouldn’t be scared off using your greenhouse this year, at least on a trial basis. Taking the mass of those trees out, which act as a heat and cold sink, will change the temperatures you’ll see anyways. And ambient air temperature isn’t the end all. Recall most sensors measure air temperature not media temperature, which is much slower to change. I know through last year’s temperature study. The air temperature will rise quickly, yet the media temperature is much slower to change.

Small greenhouses operate very differently than larger ones. Both on the positive and negative side. Each owner has to actually put their materials in to accurately figure out each greenhouse’s pattern with respect to heat retention and loss qualities based upon the size, materials used and the location. For example I might have to put a shade cloth on earlier, or later, in the year then you would. You might have to take your trees out earlier.

The upside is we’ve seen growth gaining a month in either side of the winter.

There are inexpensive remote air temperature/humidity sensors one can get to help gather data. We use Gouvee units in each greenhouse and one outside the house at the same height above the ground as in the greenhouses. Just as a very poor example as you don not live here, this is the past week’s results with door open, small fan on and vents on their own..

Greenhouse One. Greenhouse Two. Outside House


Again, not the best example as this is a ‘bridge time of the year’ it’s been alternately raining and partly sunny all week… but it is a very good example of remote monitoring tools.

Anyways, these are only my thoughts and techniques, it’s your greenhouse and trees that matter here.

cheers
DSD sends

 

[rockm]

Personally I wouldn’t be scared off using your greenhouse this year, at least on a trial basis. Taking the mass of those trees out, which act as a heat and cold sink, will change the temperatures you’ll see anyways. And ambient air temperature isn’t the end all. Recall most sensors measure air temperature not media temperature, which is much slower to change. I know through last year’s temperature study. The air temperature will rise quickly, yet the media temperature is much slower to change.

Small greenhouses operate very differently than larger ones. Both on the positive and negative side. Each owner has to actually put their materials in to accurately figure out each greenhouse’s pattern with respect to heat retention and loss qualities based upon the size, materials used and the location. For example I might have to put a shade cloth on earlier, or later, in the year then you would. You might have to take your trees out earlier.

The upside is we’ve seen growth gaining a month in either side of the winter.

There are inexpensive remote air temperature/humidity sensors one can get to help gather data. We use Gouvee units in each greenhouse and one outside the house at the same height above the ground as in the greenhouses. Just as a very poor example as you don not live here, this is the past week’s results with door open, small fan on and vents on their own..

Greenhouse One. Greenhouse Two. Outside House
View attachment 461400 View attachment 461403 View attachment 461401

Again, not the best example as this is a ‘bridge time of the year’ it’s been alternately raining and partly sunny all week… but it is a very good example of remote monitoring tools.

Anyways, these are only my thoughts and techniques, it’s your greenhouse and trees that matter here.

cheers
DSD sends

FWIW, gaining a month(a month earlier bud break) in Washington state is a lot different than having trees leafing out a month early (Like early Feb.) in a small greenhouse in Minnesota. I have amur maples that leaf out in Feb. and I leave them on the bench all winter. When they leaf out, it's a complete hassle and has set me back. I have to bring those trees inside to keep them alive through Feb. and half of March.

IMO, Unless the OP plans on beginning to heat his greenhouse in Feb. (A tall expensive order in a Minnesota winter), then it's really not a great overwintering facility. It's at cross purposes--either it's an overwintering (cold) greenhouse, or it's a heated greenhouse meant to give a jump on growing seasons. Trying to both will result in mostly failure to do either...

 

[Deep Sea Diver]

Personally, being the eternal optimist, I see absolutely no reason this little gem shouldn’t do a good job for OP.

It’s got lots of vents and a huge door, potential for excellent ventilation. While the greenhouse may need some doable mods, that’s the norm for new construction.

Looking forward to seeing how things progress

cheers
DSD sends

 

[Eckhoffw]

Personally, being the eternal optimist, I see absolutely no reason this little gem shouldn’t do a good job for OP.

It’s got lots of vents and a huge door, potential for excellent ventilation. While the greenhouse may need some doable mods, that’s the norm for new construction.

Looking forward to seeing how things progress

cheers
DSD sends

Thanks so much for your help.
I’m going to give it a go.
I’ve decided to invest in a nicer thermostat to monitor inside the greenhouse.
I would love for it to be used through the winter in a way that will be beneficial to my trees. Even if I can just adequately keep them cold in there, I will be happy.
If I need to do more than leave the vents and doors open during the sunny days, I will do more.
I guess I will see soon enough!

Thanks again!

 

[Balbs]

I’m in massachusetts, either 5b or 6a. I have a small greenhouse, a little smaller than yours. The photo above shows my entire winter temp fluctuations. Like mentioned above, this is air temp and the ground-pot-soil temp will be some moving-average of these values. What’s important to note with these small greenhouses, though, is that the temperature fluctuations with height off the ground pretty significantly, especially if you don’t have adequate circulation. On a sunny winter day, the temp at the ground and peak will vary quite a bit.

 

[Frozentreehugger]

In 4a Ontario. I have tried the regular options . Cold frame buried in snow and heeled in . Currently I use a insulated room non heated in a non attached garage or shed if you like . It’s fairly large 8 x15 . There is complete dark . I feel strongly about the success of this is trees freeze on the bench . Then go in the room stay dormant . I leave them long enough I. The spring to be sure they come out and never re freeze . I have discussed this on other threads . At some time I plan a green house . But am aware as pointed out here . That . Small green houses have serious fluctuations compared to a large commercial green house . I point this all out because I think a hybrid system . Had merit . This is my ultimate idea. . Winter the trees in a more stable temp . Like my shed or even buried in the snow but use the green house to shorten . The winter delaying freezing the trees and get them going earlier . But keep them more stable cold in the heart of the winter . This I hope will improve what can be grown . A lot of people further south are not aware that warmer climate trees . That are cold hardy . Are not equipped for the length of winter here . People here’s struggle with . Japanese maple is a example . I gave up my first crack at bonsai . Winter is just to long . Duficukt to keep them dormant and or break dormancy without the capabilities to keep them warm . Once they start .

 

[bwaynef]

I had to move my greenhouse a few years back when I had some trees removed. Being indecisive (and lazy?), I took my time in setting it back up when the tree guys left. Along comes a tropical storm and my greenhouse was in tatters. I finally got in contact with the company after assessing the damage and got the requisite parts to rebuild. The one thing I did differently was to put a base of 4 4"x4" treated wood on a concrete paver base. The result is that it's 14" (...4x4's aren't actually"4" in any direction) taller than it was before. That 14" allows the heat to be further away from my plants. I have some vents in the base and run a fan that pulls air from the top and sends it out through the door. It still gets hot in there, but I've got a lot of useable space.

Shade cloth, automatic vents, and plant volume (makes it easier to keep humidity up when its full of plants) are your friend.

SC is NOT MN, so make of this what you will.

 

[Frozentreehugger]

The solar capabilities of the sun . In the north can be misunderstood by those that don’t live here . At least 50 percent of the time in winter . January February . We have perfectly clear skies . Add the reflective quality of the white snow . It might be -20 c outside. But you can get a suntan on your face spending the afternoon outside . The effect this can have . On a small greenhouse is not easy to deal with

 

[Pinenut]

Which trees are you thinking of putting in there?

Most temperate trees that need dormancy need below 40 deg F for an extended period.
A couple of months at least and some species like longer and colder (scotts pines, mugo, Japanese White Pines}
If you cant keep them below 40 deg F then they wont go dormant for long enough or at all.

Fluctuating above and below 40 is dangerous

A coldframe or mulched in along the foundation of your house on the north or east side is your safest bet

Yep!....went the same way with my trees...but be aware on sunny winter days. The sun will heat up that green house like a sauna if you don't have automatic ventilation system installed. I had to on mine, but with a bright sunny day in winter, even with the auto venting system you should be very cautious especially if you can't monitor the greenhouse 24/7. Also,venting in the winter time will dry out your trees if not monitored. I had so many problems with the winter sun heating up my greenhouse I had to do something. So, I turned my greenhouse into a full winter only storage structure for my bonsai. I installed 1" foil faced polyiso foam board insulation inside with white outside which is all permanent. The inside is completely dark with zero light sunlight. I also covered the outside of the greenhouse with a white heavy duty white tarp across the roof and two sides (sun facing). Now I have zero problems trying to control the temperature and sunlight. The auto venting system works great on those days when we get unusually warm days through winter and spring. I know it's not a greenhouse anymore but it now does what I intended it to, and that is to keep my bonsai safe through dormancy. My bonsai are on the outside benches and in the sun as soon as the growing season starts.
My winter cost to heat the greenhouse now is 1/10th the cost I was paying before the retrofit... and no more premature waking up from dormancy. I now can maintain the inside temperature between 32-34 degrees with
a favorable humidity level throughout the winter, sun or no sun. Best thing I ever did for my bonsai collection!!!

 

[Frozentreehugger]

Yep!....went the same way with my trees...but be aware on sunny winter days. The sun will heat up that green house like a sauna if you don't have automatic ventilation system installed. I had to on mine, but with a bright sunny day in winter, even with the auto venting system you should be very cautious especially if you can't monitor the greenhouse 24/7. Also,venting in the winter time will dry out your trees if not monitored. I had so many problems with the winter sun heating up my greenhouse I had to do something. So, I turned my greenhouse into a full winter only storage structure for my bonsai. I installed 1" foil faced polyiso foam board insulation inside with white outside which is all permanent. The inside is completely dark with zero light sunlight. I also covered the outside of the greenhouse with a white heavy duty white tarp across the roof and two sides (sun facing). Now I have zero problems trying to control the temperature and sunlight. The auto venting system works great on those days when we get unusually warm days through winter and spring. I know it's not a greenhouse anymore but it now does what I intended it to, and that is to keep my bonsai safe through dormancy. My bonsai are on the outside benches and in the sun as soon as the growing season starts.
My winter cost to heat the greenhouse now is 1/10th the cost I was paying before the retrofit... and no more premature waking up from dormancy. I now can maintain the inside temperature between 32-34 degrees with
a favorable humidity level throughout the winter, sun or no sun. Best thing I ever did for my bonsai collection!!!

You and I have essentially evolved to the same system . For the same reasons . See my posts above . Also in equal climates . My only experiments since using a un heated shed . Is delay to max time before going in storage . So everything is good and dormant froze . At first I retrieved them out into the light as early as possible . Thinking they need light to get going . This I have learned is not best . I delay removing until I am sure they will thaw and never freeze again . Everything gets instant full sun . Even shade liking trees . To jump start . The conifers have to say lose some colour slightly in the dark and . Dicid had delayed budding . Compared to outside trees . But both explode to life and catch up in 2 weeks . Solves a lot of problems