Greenhouse experience? (flowers, growing, lemon tree, vegetables)

[HB2HSV]

The winter is coming (or is here). I have a few citrus trees (lemon, lime, orange and Kaffir lime) that I moved inside the garage for the winter. They are in big pots like 1/2 whiskey barrels and they take up valuable spaces. I am thinking seriously about building a greenhouse in my backyard to over-winter these plants. I am considering from hoop house (simple) to HF greenhouse kit (more complex assembly required) to build my own shed/ greenhouse (cost & complexity).

My biggest question is heating & cooling the greenhouse. I was not sure if hoop greenhouse or the HF GH using polycarbonate will require heating at night for my zone (7a), so I did an experiment today. I placed my outdoor wireless thermometer under the 6 mil greenhouse plastic sheet and read its temperature throughout the day. During the day when the sun is out, the temperature under the plastic rose significantly upto 30 degrees higher than ambient. But after the sun goes down, temperature drops quickly. It was no warmer than the outside air air temperature.

So this simple experiment tells me that, if I go with simple hoop house with plastic covers or the HFGH with polycarbonate Windows, I will need the either significantly insulate them or be prepared to pay significant electric heating bills for the GH in the winter. I read one lady in zone 5 pays over $190/mo electric bill for her HF 6x8 greenhouse. It sounds like keeping them in the garage is the cheapest option.

Is my observation consistent with your greenhouse experience? Can you share what you do to reduce your GH heating cost?

A 3rd option is to build a greenhouse/ shed with 2x4s, OSB, lots of windows, and Corning pink insulation. I estimate that will cost $3~5K but an excuse to buy lots of power tools!

[Hemlock140]

I have a commercial made/put together hoop style greenhouse that I use for a longer growing season here in zone 8b. It can be as much as 15 degrees warmer in spring-fall, but at night maybe 3-4 higher without heat. I have a smaller one for my less hardy bonsai, and it's heated, in fact the heater has been on the last few days at night since it's been down to 25-29F. Being small, I can get by with a small 2-heat oil filled radiator, usually on the 700 watt setting, then 1500 watts when it gets to about 12F. It's arranged to that the heater will not get wet when the drip system kicks on. If it ran 24/7 it would cost about $40/month in electricity. I would say that you might be able to handle maximum 5'x10' greenhouse in your zone with a 1,500 watt heater. My Meyer Lemon is in the family room next to a south window, with 3 almost ripe, and since I brought it in probably 40 blossoms. What a great aroma! In an unheated garage it will stay alive but not grow, and may die if no window or supplemental light since they are evergreen. My vegetable garden greenhouse was only $100 with free shipping, 6'x12' and lasted 4 years, I'll replace it next spring. Similar to this:

[jacqueg]

It all depends on your winter temps and how warm you want to keep your greenhouse. I think most greenhouse suppliers will be able to give you estimates of what it would take. Also, sunlight is a factor - where I live, zone 8b, there's very little sun from november through february, which is hard on plants.

There are ways to increase the thermal mass of your greenhouse, which will help.

Another option that might work - oranges and lemons can withstand temp drops to freezing. I was once contemplating a greenhouse (never built, alas), and had the idea of planting my 2 meyer lemons in a large insulated planter (4x6 feet), running a heating cable through the planter, and, during the coldest part of the year, erecting a greenhouse-within-a-greenhouse over the planter box.

I was going on these ideas -

heating the soil yields more bang for buck than heating the air - warmed soil protects the roots and is itself a thermal mass that releases some heat to the air

a rule of thumb is that each layer of plastic "moves" your plants about 100 miles or one USDA zone south (this is a very rough rule of thumb)

I heard a presentation once from a grad student in michigan who produced berries during winter in an unheated greenhouse by using a greenhouse-within-a-greenhouse

You might want to look at this - Bioshelter Market Garden | New Society Publishers

Then there's this book, which I haven't read yet - Chelsea Green Publishing - The Forest Garden Greenhouse

[HB2HSV]

Hi Hemlock,

Thanks for your feedback. I think I did not close off the bottom of plastic cover so mine reading is essententially ambient temperature after the sun goes down. If it's only 3~4 degrees warmer without heat, it would take significant heating to stay above freezing. How big is your bonsai greenhouse if you don't mind me asking?

Thankfully we have relatively mild winter here (zone 7a) that only occasionally go down to 10 degree F, but mostly in the 20s. Last night it went down to 28 degree F. I am planning a 8x12 greenhouse. The 6x8 is just too small (lemon tree gets big). So I need probably 15~20 degree temperature rise above ambient. If I assume 1,500 Watt heat per hour @ $0.11/KW-hr, it will be ~$40/mo if it's only on for 8 hrs per day, or $240/mo if it's on 24/7.

[Hemlock140]

Originally Posted by HB2HSV

Hi Hemlock,

Thanks for your feedback. I think I did not close off the bottom of plastic cover so mine reading is essententially ambient temperature after the sun goes down. If it's only 3~4 degrees warmer without heat, it would take significant heating to stay above freezing. How big is your bonsai greenhouse if you don't mind me asking?

Thankfully we have relatively mild winter here (zone 7a) that only occasionally go down to 10 degree F, but mostly in the 20s. Last night it went down to 28 degree F. I am planning a 8x12 greenhouse. The 6x8 is just too small (lemon tree gets big). So I need probably 15~20 degree temperature rise above ambient. If I assume 1,500 Watt heat per hour @ $0.11/KW-hr, it will be ~$40/mo if it's only on for 8 hrs per day, or $240/mo if it's on 24/7.

The bonsai greenhouse is a metal rack with 4 shelves, and clear plastic cover that zips up in the front. It's about 6' tall, 3' wide and 2' deep. I can fit quite a few in that space, since they are all pretty small.

My monthly bill for gas and electric averages about $250 in winter, it's hard to imagine spending that much just on heating a greenhouse, so I hope you don't have to run it 24/7 much.

[HB2HSV]

I am going to another experiment today. I have a low tunnel hoophouse set up over my raised bed, I will pull the plastic tarp over it with wireless thermometer inside and take readings. I will also put a 100W light bulb inside and turn it on later to see if that makes enough of difference. The weather is warmer for the rest of week but this gives me the temperature difference data.

Jacgueg - yes I too have read about putting an inside layer as double insulation. Each layer will increase your effective USDA zone by 1.5. Trouble is my lemon tree is tall & wide (6 ft by 4 ft) and lime trees are smaller but not by much.

What gave me hope is Elliot Coleman's writings & YouTube videos. He lives up in Maine and claimed his hoophouse with no heating gives him all winter production of vegetables. Maine is a lot colder than where I am so I am hoping I can do this without incurring significant heating bills.

[CraigCreek]

Timely thread - currently, I am putting my old small attached glass and wood pit greenhouse to use again to winter over houseplants and create a small green oasis for myself. But it's in less than ideal condition, leaks badly, and needs some obvious repairs. Some heat enters it from the house, but it gets limited sunlight, so a supplemental light and heat sources are needed. I have a heavy translucent plastic tarp over the roof which seems to have addressed the leakage (at least temporarily) and have also covered the swing window which allows access to outside, and have read that keeping jugs of water inside (in the sunny areas) can help with retaining daytime heat - has anyone tried this? I do have electricity in the greenhouse still, so may use a space heater but don't want to leave it running when I am two floors away. Most of my plants can handle low to bright light rather than requiring direct sunlight.

Good luck with your project - wish me luck, too!

[grampaTom]

I used to have a neighbor with a greenhouse. In the winter he would put 5 gallon buckets of water in the greenhouse and they would absorb heat during the day and then keep the greenhouse warmer at night. I think you could also throw old blankets over the plastic at night to help.

[Moth]

I have one like Hemlock's, only smaller. I heat it with an oil heater. No huge bump in my electricity bill.

It works fine. The only thing is that you have to turn off the heat and vent it (open the door) on sunny days lest your plants get cooked. And if its cloudy and cold, keep the heat on.

It is great for Citrus. They will keep growing and flower much earlier.

[jacqueg]

Originally Posted by Moth

I have one like Hemlock's, only smaller. I heat it with an oil heater. No huge bump in my electricity bill.

It works fine. The only thing is that you have to turn off the heat and vent it (open the door) on sunny days lest your plants get cooked. And if its cloudy and cold, keep the heat on.

It is great for Citrus. They will keep growing and flower much earlier.

Yup, all they crave is a little sun.

For a couple of winters, I lived in an old house with an unheated sun porch. Got down into the low 30s at night. My Meyers liked it just fine, started flowering in Feb IIRC. Winters were a USDA zone colder there than they are here, but a lot less rainy. They survived last winter here OK, but didn't put out any flowers until April or so, due, I am sure, to the lack of sun.