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Discussion forum for the various methods and structures used for getting an early start on your growing season, extending it for several weeks or even year 'round.

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Old February 7, 2018   #1
FourOaks
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Default Second layer of plastic and frost cover?

Alrighty, I have a question for you fellow High Tunnel/Greenhouse growers.

Before I ask my question, let me explain what I did last year, so you can get a better idea. Then, we will go from there.

Last year to keep my plants comfy cozy, I installed a small propane fired furnace in the end wall. It came out of a camper. I dont recall, but I want to say 18000 BTU? But im not 100% for sure on that. The furnace is controlled by a thermostat. I usually kept it at about 55 degrees. The lowest setting.

I ran 4 inch duct from it, up to the benches. I suspended a piece of plastic from the hoops, on the inside. I basically created a heated tunnel, within the High Tunnel.

This worked. I also installed a switchover valve so 2 tanks were attached, allowing me to never run out of propane. Im fairly pleased, but I feel like something better could be had.

Being that I am doing a lot more plants this year, I need to maximize what I have. My thoughts are to move the benches into the middle of the High Tunnel. I have enough bench space to cover a 4 foot by 24 foot area. Might have to make more.

Then, bend 10 foot PVC into an arch over the benches, do maybe 3 sections.

Previously covered with additional plastic. But now im wondering. Is there something better? Does anyone know the R value of Frost Cover, or Row Cover?

Maybe combine the 2? Drape light weight row cover over the plants in the evening then lower the additional plastic down? Or put row cover OVER TOP of the plastic layer to act like a blanket?

Any thoughts on this? In the end, im trying to keep my propane use in check, so I need to find some way to insulate the plants.

Right now, Im ok as all my seedlings are indoors under lights. But we all know how quickly you can run out of room.

This is the setup from last year. Not the prettiest, but it was spur of the moment, and it worked.

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Old February 7, 2018   #2
KarenO
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Another way would be to use plastic sheeting to wall off a full height narrow room just the back of your high tunnel and heat just that portion. It would hold more plants, you can go in and tend your plants without uncovering them and I think they would get more light
However if what you had worked for you last year maybe just go with that again
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Old February 8, 2018   #3
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I divided my greenhouse in half by tucking left over greenhouse plastic between an arch and the covering, dropping it clear to the floor. keeping it heated with an electric heater. I was doing that in Feb up here.
how cold is it there?
do you keep a remote sensor to monitor or measure the nighttime temps?
I have my thermostat set to run the furnace at 40° and remote sensors to watch the temps. in the greenhouses.
We put arches in the high tunnel and cover those crops with row cover to protect them. I have to uncover each morning but that is just a part of trying to run on a shoestring budget. as a matter of fact... my 30x96 has a row of lettuce transplants in it that I planted in the Fall along with a packet of seeds at the same time. there were a few below 0° nights.... I know it was below 0° but by how much? no idea. I was in texas at the time... but it was COLD! under the cover the lettuce survived and the seeds sprouted believe it or not! I was totally shocked to see how well it did. so... a tunnel inside a tunnel works very well, too. that said... a larger structure has a larger heat mass, too.
I don't use plastic as a row cover inside the tunnels. I use row cover, plastic tends to keep too high of humidity. (see Cole-Robbies thread on " it gets colder inside than outside on his hightunnel"... I think it has to do with humidity) grow molds. then you are battling diseased plants before you even get started. if you chose to go that route make sure it is ventilated some. just my thoughts and observations on what has worked or happened here. (Kevin does most of my worrying )
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Old February 8, 2018   #4
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Quote:
Originally Posted by KarenO View Post
Another way would be to use plastic sheeting to wall off a full height narrow room just the back of your high tunnel and heat just that portion. It would hold more plants, you can go in and tend your plants without uncovering them and I think they would get more light
However if what you had worked for you last year maybe just go with that again
Thought about that. But I would rather have an insulating layer (whatever that might end up being) closer to the plants, since heat rises.

Light was never an issue as the plastic is/was rolled up each morning.

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I divided my greenhouse in half by tucking left over greenhouse plastic between an arch and the covering, dropping it clear to the floor. keeping it heated with an electric heater. I was doing that in Feb up here.
how cold is it there?
The weather can flucuate something-else here. But in general our average low in Feb. is 32, Mar. 38, Apr. 47..

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do you keep a remote sensor to monitor or measure the nighttime temps?
Yep, sure do.

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Originally Posted by clkeiper View Post
I have my thermostat set to run the furnace at 40° and remote sensors to watch the temps. in the greenhouses.
Dang, 40 degrees? I might have to look for a thermostat that can go lower then.

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We put arches in the high tunnel and cover those crops with row cover to protect them. I have to uncover each morning but that is just a part of trying to run on a shoestring budget.
I understand that. And yes, here in the morning, early, you have to uncover. Otherwise, you will cook some plants.

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as a matter of fact... my 30x96 has a row of lettuce transplants in it that I planted in the Fall along with a packet of seeds at the same time. there were a few below 0° nights.... I know it was below 0° but by how much? no idea. I was in texas at the time... but it was COLD! under the cover the lettuce survived and the seeds sprouted believe it or not! I was totally shocked to see how well it did. so... a tunnel inside a tunnel works very well, too. that said... a larger structure has a larger heat mass, too.
Well, that worked out nicely.

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I don't use plastic as a row cover inside the tunnels. I use row cover, plastic tends to keep too high of humidity. (see Cole-Robbies thread on " it gets colder inside than outside on his hightunnel"... I think it has to do with humidity) grow molds. then you are battling diseased plants before you even get started. if you chose to go that route make sure it is ventilated some. just my thoughts and observations on what has worked or happened here. (Kevin does most of my worrying )
Each morning before rolling up the plastic, there was definitely a film of moisture, thats for sure. Luckily, never had any disease issues.
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Old February 8, 2018   #5
BigVanVader
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When I eventually build a seedling house I plan to build benches with lids like the ones in this video. https://www.youtube.com/watch?v=rZS15J2w9gc Double plastic, cheap and easy, and easy access. Probably wouldn't even need heat but I plan to have a heated water system using a water heater. Tubes run under a thin layer of sand the plants sit on. This would keep it warm and moist and cost very little (in my climate).
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Old February 8, 2018   #6
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When I eventually build a seedling house I plan to build benches with lids like the ones in this video. https://www.youtube.com/watch?v=rZS15J2w9gc Double plastic, cheap and easy, and easy access. Probably wouldn't even need heat but I plan to have a heated water system using a water heater. Tubes run under a thin layer of sand the plants sit on. This would keep it warm and moist and cost very little (in my climate).
Im always waiting for his Cat to try to climb up the plastic, every time I watch his videos.

I have thought of exactly the same thing for heated benches. I was planning on a propane fired hot water heater, myself. I have a very small one, in the same camper the furnace came out of, but Im not sure if it even still runs. I just havent had the motivation to remove it, to test it out. Theres also a water pump in there, already designed for hot water.

I even have a roll of pex pipe, that would be perfect.

Maybe I need to rethink my strategy. Invest in some heavy weight row cover, and really harden off the seedlings. Thinking back to when I worked at the Nursery, the outside plants were only covered when a cold snap was protected. These included a lot of annuals. The covering was mighty thick though.

Well, the sun is shining, its about 80 degrees in my Larger High Tunnel/Seedling House, time to go do some transplants..
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Old February 8, 2018   #7
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We are using a lot of row cover in the field here and can be kept covered for months at a time without any worry. Most crops it is no hoop just tuck it in and let them push it up as they grow. In the farm greenhouse, we also use row cover over seedlings on a cold night, and yes it's best to remove it in the day time if it gets hot. But it isn't too deadly because it does help to retain moisture too, and yet it breathes, unlike plastic which will really cook your seedlings if the sun strikes in.

I've tried plastic vs row cover for early tomatoes in my greenhouse... row cover was better. The worst feature of plastic was the tendency for condensation to form on the surface and make the plants wet and more prone to frost damage. There is a tradeoff with the weight of the cover - light cover allows more light but provides fewer extra degrees of warmth, vs heavier cover which is warmer but blocks too much light, especially in our climate where it's often overcast as well as cold.

For any cover, the downside is the labor involved in covering and uncovering. I've tried some different approaches - hoops over young plants; or attaching and draping over cages. Then I tried a different thing - hanging row cover around the perimeter of my rather small greenhouse and using a different piece to cover the top of the plants at night. The perimeter cover could be left up during the day, and honestly, it did continue to make a difference to the temperature without seriously blocking the light. I could use the warmer stuff around the sides and the lighter stuff on top which could be left on if we had several cold days in a row. Using heavy row cover around the perimeter would likely help to reduce your heating costs. I used clothespins to hang mine from a wire, hanging at the point where about 6- 7 ft height to the ceiling about the width of the row cover roll and a few feet back from the glazing.
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Old February 8, 2018   #8
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Originally Posted by bower View Post
We are using a lot of row cover in the field here and can be kept covered for months at a time without any worry. Most crops it is no hoop just tuck it in and let them push it up as they grow. In the farm greenhouse, we also use row cover over seedlings on a cold night, and yes it's best to remove it in the day time if it gets hot. But it isn't too deadly because it does help to retain moisture too, and yet it breathes, unlike plastic which will really cook your seedlings if the sun strikes in.
Yep, been there with the accidentally cooked plants.

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I've tried plastic vs row cover for early tomatoes in my greenhouse... row cover was better. The worst feature of plastic was the tendency for condensation to form on the surface and make the plants wet and more prone to frost damage. There is a tradeoff with the weight of the cover - light cover allows more light but provides fewer extra degrees of warmth, vs heavier cover which is warmer but blocks too much light, especially in our climate where it's often overcast as well as cold.
Luckily here, its usually sunny, unless we have a rainy spell. Im starting to gather at this point from the responses, that the heavier cloth would be the way to go.

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For any cover, the downside is the labor involved in covering and uncovering. I've tried some different approaches - hoops over young plants; or attaching and draping over cages. Then I tried a different thing - hanging row cover around the perimeter of my rather small greenhouse and using a different piece to cover the top of the plants at night. The perimeter cover could be left up during the day, and honestly, it did continue to make a difference to the temperature without seriously blocking the light. I could use the warmer stuff around the sides and the lighter stuff on top which could be left on if we had several cold days in a row. Using heavy row cover around the perimeter would likely help to reduce your heating costs. I used clothespins to hang mine from a wire, hanging at the point where about 6- 7 ft height to the ceiling about the width of the row cover roll and a few feet back from the glazing.
Thank you for the detailed description.

Im thinking a Heavy Cover might just be the way to go. Then, a propane space heater, or the propane furnace only when it is either:

A. Unusually cold at night.
B. Cloudy and cold during the day.
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Old February 8, 2018   #9
FourOaks
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Ok, so I looked around and think I found what I need. Amazon has:

https://www.amazon.com/Agfabric-Ultr...pSrc=srch&th=1

A 2 oz row cover. Several different sizes.
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Old February 8, 2018   #10
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Yeah that one only has 30-50% light transmittance so it is mostly for dormant perennials afaik. Would be okay for side walls or a side perimeter but it would not allow enough light for seedlings to grow.

The manufacturers will tell you both light transmittance and number of degrees of protection for the different weights. It would be abnormal to go below 80% light transmittance for anything actively growing (unless you use it at night only).

The stuff I used for my perimeter sides was extra heavy - came from Dollarama with no transmittance value and I think pretty low. The stuff was no good outdoors, torn up in no time by wind and basically disintegrating at the drop of a hat, but it worked fine for inside the greenhouse and was only $2 for I believe a 20 ft by 7 ft piece. I had a friend used them as 'bunting' for her wedding then gave em to me afterwards.

Okay just checked amazon for 'agribon' that is the professional brand we use at the farm - my mistake! The ag-50 is also only 50% light transmittance with 8 degrees of frost protection. And more expensive.
The ag-30 has 70% transmittance and 6 degrees of protection.
Agribon or the dealers of their product should be happy to discuss your needs and recommend one or the other weight - whether you buy their product or not.

Last edited by bower; February 8, 2018 at 12:42 PM.
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Old February 8, 2018   #11
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My understanding of row cover is that it is meant to be placed against the ground, so as to conserve the slightly above freezing air coming off the unfrozen earth. It is still not much of an insulator. I don't even think they give you an r-value for it, so the value would probably be about the same as plastic, or layers of plastic, which is what I would suggest. One layer of 6-mil is about r=.6 to .8 and a double layer is about r= 1.6. I don't think it even matters that much that the plastic be 6-mil. I would just buy a bunch of painter's plastic drop cloths and use those in layers. If they hardly ever see the sun, it won't matter that they lack UV inhibitors.

And oh yeah, don't let the leaves of the plants touch the plastic. They really don't like that.
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Old February 8, 2018   #12
FourOaks
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Bower, thanks for the input. Keeping in mind, I would only use the product during dark hours.

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My understanding of row cover is that it is meant to be placed against the ground, so as to conserve the slightly above freezing air coming off the unfrozen earth. It is still not much of an insulator. I don't even think they give you an r-value for it, so the value would probably be about the same as plastic, or layers of plastic, which is what I would suggest. One layer of 6-mil is about r=.6 to .8 and a double layer is about r= 1.6. I don't think it even matters that much that the plastic be 6-mil. I would just buy a bunch of painter's plastic drop cloths and use those in layers. If they hardly ever see the sun, it won't matter that they lack UV inhibitors.

And oh yeah, don't let the leaves of the plants touch the plastic. They really don't like that.
Thank you for your input on the use. Something I have never thought of. It may just be more effective at ground level.

Painters plastic is exactly what I used in that photo up above.
Yeah, I had a few leaves get damaged when they touched the plastic during a spell of extreme cold that we had.

Maye a combo of the two would be better then just one or the other. Row cover against the plants, then plastic on top.

There has to be some better way for those of us who do this, then to run heat. Or run excessive heat. I didnt bother last year to keep track of my propane receipts, I really wish I would have.

I did do a google search and came across several threads here where it was discussed, about what temp to protect seedlings. The opinions and line of thinking is all over on that one.

I agree in general though, that the seedlings need to be hardened off. Vigorously.

Might just have to put in the extra effort this year. At least with a High Tunnel structure, then extra row cover or plastic (whichever) the chilly "wind factor" is removed. Adding in the removal of humidity buildup might just be a winner.
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Old February 8, 2018   #13
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If you want r-value, make all but the south-facing wall out of other building materials such as styrofoam, or even 2x4 framed walls with fiberglass. The south and top can be clear, which will minimize the area you have to cover up at night. I have seen pictures of a Chinese design for a heated greenhouse built like that. I think they rolled a big blanket down over the clear part each night and then removed it at dawn. I am still working on my design of a blanket, and how it handles elements such as rain. It could also be insulated shutters instead of a blanket.
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Old February 8, 2018   #14
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Hmm.. why not put the blanket on the inside?
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Old February 8, 2018   #15
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I think it's just easier to lay it over the top than to suspend it up against the ceiling. It might get wet from condensation if it were inside as well.
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