Greenhouse heating question

[Chester B]

I set up a small greenhouse this fall (6x4 footprint) to protect all my small seedlings while I'm growing them out to plant in the garden eventually.  These are all pretty cold hardy species (Trachycarpus, Sabal, Butia, Jubaea, etc) but my question revolves around supplemental heat.  I have insulated the greenhouse with 1/2" bubble wrap and have installed one string of C7 lights (I have extra strings) that are plugged into a thermocube that turns on at 35F and off at 45F.  We don't have too many nights a year that drop below 35F and the greenhouse on its own is running 2-3F warmer at night since I've set it up.  So is there any benefit of running the lights on a timer overnight?  Or better to let the thermocube do it's job and keep it above 35F?

 

[Chester B]

[Fusca]

When I lived in zone 8b I've allowed similar cold-hardy strap-leafed seedlings remain outside under a covered patio into the mid-twenties (for short duration) with no issues so I'd say you're good to go with your thermocube.  Looks like a nice set-up!  Unless you're wanting more light for the seedlings to grow I wouldn't bother leaving them on all night.

[jimmyt]

That GH looks well insulated.  The main issues that I have run into are actually when you get below 32 F and how long the temps stays there.  Obviously 15 F is much worse than 32 F., but the duration of exposure to any freeze will be a killer as well.   The seedlings will be most susceptible from the start too.   You should be ok if your temps do not drop below 35 F, and if below 32 F for a short time.  Most of the species you listed I also have and they do well even with temps into the mid 20's.  Mine are also in pots.  Do you monitor the temps in your GH at night to see how effective your heating is?   What does the temps climb to in the daytime?   I have a 30' X 48' GH that is heated and vented but will have temps climb 20-30 degree higher than the outside temp. during the day.  Is is shaded with shade cloth and Vari-shade paint.  For me If the outside temps get near 50 F that means the GH temp will get to 70 on cloudy days and 85 F on sunny days.  I am in Zn 8b.  Winter temps get down into the mid teens at times.  Brrr!

[Chester B]

Usually when we drop below freezing its short duration like 2-4 hours overnight.  This may only happen a couple of times per year, BUT we can have days that do not reach above freezing every few years.  3 years ago we had close to 3 days in a row that did not break freezing, although that was very unusual.  Yearly lows in the last 3 have been 25, 25 and 18F for very short durations.  

I have many of these species (larger specimens) planted out with no protection but the majority of these are small plants in small pots that can freeze up and damage the roots.  I have a temp/humidity sensor that does trend the data so I am able to see how it performs overnight.  We haven't had any cold nights so far this year so I have no idea how well my set up is going to do, as the lights haven't turned on yet.  I think the coldest I've seen since I had this set up is 42F.  Tomorrow night looks like we should be going to near freezing so I hope this will give me some good info.  I ran the lights yesterday for a few hours yesterday and saw 7F warmer than outside, so it's got me thinking I should add another string of lights.  In our cold months daytime highs are usually in the mid 40's with some days in the upper 30's.  We tend to have a lot of cloud cover this time of year so the greenhouse has only been marginally warmer than outside, and due to the position of the sun in the sky and my position on a hill covered in large trees it doesn't get a lot of direct sun this time of year.  

The main purpose of the greenhouse was to protect small seedlings in the absolute worst few days per year, speed the growth of my Sabal seedlings and to get a good start for my vegetables.

Thanks for the info so far guys.

[RyManUtah]

@Chester B how has this worked for you? Has it been cold enough to use yet? I asked because I’ve built a little greenhouse for my brahea (too heavy to move inside) and I don’t want the pot to freeze. Heating with a 2litre of water has sufficed, but it’s gets too humid in there overnight for its liking. Was thinking of trying the lights instead. 
been getting into the low twenties more frequently, so I don’t want the pot to freeze the roots either.

[Chester B]

We don’t usually get that cold, the lowest this year so far has been 26. I swapped out the thermocube for an inkbird temperature controller. I have it set to come on at 35 and as soon as the lights come I see a rapid rise in temp. So far the greenhouse itself keeps the temps at least 3f above ambient and the lights raise it about 7-8f more. I added another string of c7s because I wanted to make sure I have enough heat. The main issue I’m having is humidity. It’s pegged at 99% and it’s hard to drop when ambient humidity is over 90%. I have a fan in there but noticed some fungus so I’ve decided to run one string continuously and the other on temp control. When the lights are on I’ve noticed the humidity drops. 
 

Ive also noticed the plants are still actively growing so I guess that’s a bonus. 

Edited December 16, 2019 by Chester B

[RyManUtah]

51 minutes ago, Chester B said:

We don’t usually get that cold, the lowest this year so far has been 26. I swapped out the thermocube for an inkbird temperature controller. I have it set to come on at 35 and as soon as the lights come I see a rapid rise in temp. So far the greenhouse itself keeps the temps at least 3f above ambient and the lights raise it about 7-8f more. I added another string of c7s because I wanted to make sure I have enough heat. The main issue I’m having is humidity. It’s pegged at 99% and it’s hard to drop when ambient humidity is over 90%. I have a fan in there but noticed some fungus so I’ve decided to run one string continuously and the other on temp control. When the lights are on I’ve noticed the humidity drops. 
 

Ive also noticed the plants are still actively growing so I guess that’s a bonus. 

Thanks for the info. I’m gonna try to whip up something similar with a fan. I can however, take it off during the daytime, so that does help. But if I can keep it covered and humorist down, like you said it will keep actively growing Til I can plant it in February 

[Ittai Baratz]

I have a greenhouse in the same size, but very filmsy without any insolutation. I am in Zone 10A and so far our lowest temp' was 40F. 
I use smart sockets as well as a 250W heat bulb that turns on every day at 5:30am for a couple of hours, as well as at 5pm. I found that it would up the temps by about 10F, depends on the wind and if the flap is closed all the way or half (Terrified of fungal issues as the humidity levels rarely go below 80% due to our wet winter in North Cali).
I also have two USB fans at the top of the greenhouse which are times to run 4 x 1 hour per day.

This is the bulb I use:
https://www.amazon.com/gp/product/B0066L0ZRU/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

And these are the temps over the last week. The spikes are when the bulb goes on and off - I am curious if that is beneficial for the plants or not as I realize they need to "harden" themselves to the cold and I am worried if this is somewhat confusing them? 




Is that a Guava? I have one in the yard and the leaves are already going purple due to the cold - Yours looks happy in the greenhouse.

[Chester B]

If you’re not planting out this winter then no need to harden off.   Really the cold hardiness of the plants I think is genetically ingrained. 

I run a full sized fan 24/7 to help with fungus but the humidity is just so high here in the winter.  In the summer it can go into the teens  it’s so crazy. 

Yes the plants are the common guava. The only ones we can reliably grow outside here year round are pineapple guava. I have three of these that are going into their second winter.