Low Tunnel & Mild Winters

[tlintx]

I've been thinking about building a hoophouse -- low tunnel? -- to overwinter some less hardy plants, mainly peppers, tomatoes, and some flowers. I'm not expecting a whole lot, just experimenting, so it has to be pretty inexpensive or multi-functional.

My tentative plan is to make low hoops (3') from 9 gauge wire and cover it with floating row cover, then put a second, larger set of hoops (4') over that with plastic once it gets much cooler. The plants are all in SWCs in a spot that is protected from the wind.

Is it correct that if I use standard construction or painting plastic sheeting from Home Depot, it won't hold any heat in at night because it's transparent to IR? Can I offset this by covering it with an old blanket or something? In this application, would IR insulated greenhouse film be of any real benefit?

Will I be able to use the setup above to put my plants out early in the spring? Is there much benefit to doing so, or will the plants set out later catch up to the ones set out earlier pretty quickly?

Any and all advice would be welcomed! Haven't been able to find much about this sort of thing near Houston, so I'm wondering if it's not done much for a reason?

[joseph]

A sheet of plastic has about the same R value as a pane of glass. A hoop house made from plastic will trap some of the heat of the day in the ground for release at night, so the plants will stay warmer. It will also trap moisture, so if your area is prone to fungal diseases a hoop house could make things worse.

Common plastic is OK to use on a hoop house, but it is not resistant to UV light, so it decomposes sooner than plastics that have a UV stabilizer added.

[tlintx]

Everything decomposes faster in the Texas sun except pine needles. I suspect even real greenhouse film won't last long out there, but I only need it up for a couple of months at a time.

Elliot Coleman suggests this method "moves you 500 miles south" for every layer. I'm assuming this only relates to harsh winter climates.

What's confusing me is that I've read that radiant cooling may remove the heat right away, and that in some tests it ends up actually cooler underneath than ambient. Anecdotal evidence is all over the place, from "30 degrees higher at night" to "a few degrees of protection" to "no difference". And nobody specifies what kind of plastic, they just say "plastic"!

It's an inexpensive experiment, I'm definitely going to try it. Just would like to know where to set my expectations - I'm thinking "a few degrees of protection" is about it.

[Doug9345]

Originally Posted by tlintx

Everything decomposes faster in the Texas sun except pine needles. I suspect even real greenhouse film won't last long out there, but I only need it up for a couple of months at a time.

Elliot Coleman suggests this method "moves you 500 miles south" for every layer. I'm assuming this only relates to harsh winter climates.

What's confusing me is that I've read that radiant cooling may remove the heat right away, and that in some tests it ends up actually cooler underneath than ambient. Anecdotal evidence is all over the place, from "30 degrees higher at night" to "a few degrees of protection" to "no difference". And nobody specifies what kind of plastic, they just say "plastic"!

It's an inexpensive experiment, I'm definitely going to try it. Just would like to know where to set my expectations - I'm thinking "a few degrees of protection" is about it.

The only plastic I've ever messed with is covering a plant for frost protection over night. I count on it to give me about 4 degrees for a couple of hours.


I think you are going to find that peoples experiences are all over the place because the variables are all over the place. Humidity, precipitation, wind, altitude, ground moisture and many others. I think the real gain will be from the higher temperatures during the day.

[joseph]

Sometimes I cover my tomatoes in the fall with clear plastic to protect them from our intense radiant cooling due to the crystal clear skies. It seems to me like most of the radiant cooling happens at the topmost surface of the plastic. If a layer of dew or frost forms on the plastic, then that is an IR shield. Hmmm. clouds expected here the next few nights. Perhaps I'll test this when things clear up.

[MrBig46]

The losses of the heat are incomparably higher in the plastic tunnel than in the greenhouse, because plastic foil don´t blockade heating radiate as well as glass. Only polycarbonate has the same heat atributes as glass.
Vladimír

[tlintx]

Originally Posted by Doug9345

I think you are going to find that peoples experiences are all over the place because the variables are all over the place. Humidity, precipitation, wind, altitude, ground moisture and many others. I think the real gain will be from the higher temperatures during the day.

This is my thinking too, which may be why people don't bother around here.

Further research suggests thermal mass will be of limited value in such a small space (we're talking a 6x3 area). And I think compost heating will be outside my scope this year with a toddler underfoot (although I may experiment with a container or two).

I also found some research on using space blankets to keep the heat in over plastic, so that's an option too. And inexpensive.

Sadly, polycarbonate sheets are outside of my budget this year, along with greenhouse plastic and, well, anything over about $50.

[Doug9345]

Let's do a little math here.

Assumptions.
You cover the bed with a plastic box that is 6'x 3' x 4' high.
The R value of plastic is 1.
That you can put 6 1 gallon jugs of water inside for thermal mass.
We'll Ignore any other thermal mass in there such as the plants and ground.
That the water in the jugs starts at 70°F and can cool no further than 35°F
That you are trying to gain a 10° difference

The box has 90 square foot of area.
That means it loses 90 btu per hour per degree difference between inside and outside and that it will lose 900 btu/hour.

A gallon of water weight 8 lbs and it takes 1 btu to change a pound of water 1°F.
Therefore 8 x 6 x35 = 1680 BTU. That thermal mass will buy you around 1.5 hours with a large number of simplifications and assumptions.

I think once you start talking about adding thermal mass and such you are drifting more into a solar green house and away from a tunnel.

Covering it up with some kinf of insulation at night will definitely help. I'd do it about a houe BEFORE sunset.

[clkeiper]

Up here it is different, but you can use the same concept I would think. We use a high/low tunnel as a refrigerator for the Winter. We grow the "cold crop" plant and "hold it" until it is needed (this doesn't work for the tomatoes or peppers, though. It is too cold nor do we have enough sunshine to grow them during the winter. Your temps may be warm enough to do them though.). We cover the plants inside the tunnel each night to protect them from frost/freeze and uncover during the day.

[tlintx]

Thank you for the math breakdown, it makes sense to me now. I'll admit to skimming all the BTU talk I've read elsewhere because it seemed too complicated. I can't imagine the SWC reservoirs would make much difference, and I don't have room for lots of gallons of water.

I'm thinking most of the winter crops in my climate will be fine over the winter without any protection? Shouldn't be too hard to put hoops over my raised beds if not -- I'm hoping not to have to, though. Trying not to bite off more than I can chew!

I actually wonder if the main reason more folks don't do tunnels here (unless they do and just aren't mentioning it) is the difficulty of regulating the temperature during the day. That and it's only a couple of months downtime anyway.

[Cole_Robbie]

Here is a link to calculate heat loss from a structure:
http://www.builditsolar.com/Referenc...s/HeatLoss.htm

The r-value of 6-mil greenhouse plastic is .6

I'm getting ready to order a bender for EMT conduit for my first low tunnels, probably from lostcreek.net - I bought their top rail bender last year and built a high tunnel with it.

Both low and high tunnels are very poor at holding heat and will be close to the outside temperature by dawn. But they do protect from frost, and a killing frost can occur on a night as warm as about 40 degrees F. They are also obviously much warmer during the day, and provide wind protection as well.

[tlintx]

Sadly, at around ~$80 for greenhouse plastic, it's going to be Home Depot construction grade for me this year. Having a hard time justifying the $30 expense for wire, plastic, and a row cover as it is, since I'm pretty sure I won't get much return (except maybe some gardening therapy in January).

Have you used the 9 gauge wire? Any opinions on that versus the EMT conduit? Or on layering row cover and plastic ala Coleman?

I was thinking of using the "California system" where the plastic/row cover is fastened all around the edges but split and clipped along a top guide wire. That way I could drop the sides most of the time and just pull them up when it gets cold or a frost threatens.

http://edis.ifas.ufl.edu/cv201