Low tunnel for planting early

[Salaam]

Hello all,

I would like to have a go at planting about three weeks early this season. Instead of May 21, May 1, give or take a few days. This would mean facing at least one to three nights of 28F to 35F, worst to best case.

In order to tackle this, I plan to install a hoop house / low tunnel over the bed(s).

The beds will have been covered by black ground cover plastic (https://www.agriculturesolutions.com/de ... oz-3-x-300) two weeks before planting, and the tomatoes will be planted in holes made in this ground cover. I assume this will help alleviate the soil temperature problem.

The question I have is what row cover or greenhouse plastic to use to ensure frost protection. And generally, is this a good plan?

[Cole_Robbie]

Keeping the plants from cooking if it gets warm and sunny is the other challenge besides frost. I have not used it myself, but I have seen slitted row covers that are supposed to vent the heat through the slots in it. They open up when hot.

You will get frost protection from the plastic, and probably some freeze protection. On a 28 degree night, you would probably want an extra blanket or tarp over the row cover just for the nighttime.

[maxjohnson]

You can use 6mil greenhouse UV treated plastic. And/or garden fleece.

Some examples:
https://amazon.com/dp/B077GQTQWK
https://amazon.com/dp/B07WBX6F76

For the overheating issue, I build my own low tunnel frame that act like a lid that I can lift, it just depends on how much work you want. It's easy enough to just use clips and lift the plastic when you need.

[Salaam]

Boy, I'm having a hard time finding either 4 or 6mm plastic or Agribon type row cover now here in Ontario, Canada. At reasonable sizes - my beds are 4x20. I'll keep searching on line.

[bower]

Could be tough to get it in time for May 1 this time! You have my sympathy.
Dubois Ag in Quebec sells the agribon. You may even have a chance of getting something shipped from Quebec, if Ontario businesses are shut down.

[GoDawgs]

Does Canadian Tire sell rolls of 6 mil plastic?

[Salaam]

I found and https://www.veseys.com/ca/floating-row-covers.html!

But I can't decide between the plastic and the fabric. As far as I understand, the plastic will give more frost protection and will extend the season more, but requires much more intervention taking it off nearly every day of moderate weather to avoid excessive heat and for air circulation and for watering. Whereas the fabric has less protection but can be left on more of the time because it doesn't make the tunnel as hot, it allows air circulation, and lets in rain.

I'm also wondering how much season extension is worthwhile because while the tunnel fixes the temperature problem, the sunshine problem remains. As we get into November and December, because there is so little sun, there may not be enough production to warrant maintaining the tunnel.

What do you all think?

[bower]

Here's a useful link for your daylength and similar: Your days in Ottawa drop below 10 hours of daylight on November 3, so just a couple of days more than we get here. Below ten hours of daylight you cannot expect crop growth. Realistically, you may keep plants alive for awhile into the dark days but it's doubtful if the temperatures will even be enough to promote further ripening. Any shade structures or geographical features can cut your growth time back as well due to the low angle of the sun. Your maximum would be an unshaded hill top, but the risk of temperature inversion there is also high.
https://www.timeanddate.com/sun/canada/ottawa
Greenhouse plastic does not really give more frost protection. It heats the interior more when the sun is shining, but you can also get temperature inversions and frost inside, in certain weather conditions. So it can drop colder than under row cover fabric. Secondly I have found that a plastic cover collects condensation on a frosty night, and if tomato leaves touch the plastic they will suffer some damage. This would not be an issue with your small transplants, but could be an impact at the end of season.
Row cover gives reliable frost protection. But it will only raise your air temperature by a couple of degrees. IDK if that will be enough to jump start your tomato season 3 weeks early. Since row cover admits moisture it is fantastic for cool weather crops that like a high relative humidity - lettuce, green vegs, carrots etc. and decreases the need to water. Tomatoes OTOH tend to develop leaf diseases in higher RH especially if temps are lower than their ideal. So the success of row cover alone would depend on your weather, how much rain vs drier conditions and sunshine, and ambient temperatures of 10 C or higher. For tomato growth, even the cold tolerant varieties don't do much growing at temperatures below 15 C. The tolerant varieties start to be stressed if you have 3 or more consecutive days that don't go above 10-12 C. In a humid environment produced by row cover and rain, the outcome could be worse. (I doubt it would work for us here, with a wetter and colder spring than you probably have)
So as Cole pointed out, the project of season extension for tomatoes really requires multiple interventions to guarantee success. Warming the ground, protecting from excess moisture with a plastic ceiling, venting when too hot, and adding row cover when too cold.
But if your spring weather is not too wet or cold, and your time/cash budgets don't allow the juggling of both, then roll the dice and see what happens. Let us know.

[Salaam]

20210501_191127.jpg

Well, we assembled the low tunnel yesterday evening. 6mm greenhouse plastic over 10 foot diameter hoops. Bed is about 22 feet long by four wide. It's had black ag cover over it for two weeks now.

This morning at about 11am I checked the thermometer I placed on the surface of the bed. The temperature read 23C while it was 9C outside. The low temperature in the greenhouse read 3.5C while the low outside was 2C. It was a cold and rainy night. So not much night temperature protection so far. I'll monitor for a couple of days then hang incandescent Christmas lights on the hoops and see how that does.

[bower]

That looks great.
Containers of water is another way to keep it a few degrees warmer. Clear containers are the best - my favorite is the Rougemont type apple juice bottles. 2L, easy to move around, stay tightly closed, and they are flat sided so they stay where you put them. 2L of water will solar heat up pretty quickly if it's lying on top of the black soil cover inside your hoop house. They will release all of their heat at night as the temperature drops, and warm the air around your plants.

[Growing Coastal]

Containers of water or buckets of rocks! I had some pots full of rocks out there that I had not moved yet and left them by the potted tomato plants under cover, for night heat.

[Salaam]

Around 2pm, I checked again. The temperature outside was 13C and clear and sunny. Inside the tunnel it was a shocking 51C with the smell of hot plastic! I opened the two ends of the tunnel, and when I checked a couple of hours later, it had gone down to 30C. By 7pm it was down to 17C (the bed doesn't get much sun after 5:30pm). It seems that if daytime highs are above 5C or so, the ends of the tunnel must be opened, then closed in the evening some time.

Thanks all, I've noted your water/rocks recommendations.

[bower]

Yes, I will second the rocks! I had some fist sized rocks in black nonwoven shopping bags one year, which I draped around the ends of the containers. I was amazed how well they took up heat and moderated the temperature.
Salaam, it is not so much the outdoor temperature as the amount of sunshine. The hoophouse will stay cooler if it's a cloudy or rainy day. Full sunny blast and it becomes a cooker.

[JRinPA]

I'm curious how this worked out. Good benefits? Any disasters?

I have been bit more than a few times using plastic. It just gets too hot, often with no warning. A cool day, but with clear morning sun, can get incredibly hot inside a small tunnel. And that is just with 0.9 mil and perforated. Gets too hot in the day, and doesn't keep that warm at night. Unfortunately my brother bought a "small" roll of it (2000 ft) so I keep on trying to find a use for it. This year I tried to use it for seed starting outside. Between damping off and cooked plants, I may be ready to write off that plastic.

Overall I have been much better off with AG19.

[MrBig46]

I would like to ask if anyone has experience with hotbed heating. In the spring, I would like to put 70 liters of freshly chopped grass, dry leaves, some compost in a plastic bag and pour it over warm water. I close the bag, make holes in it for air access and place it on the side of the frame. Do you think that compost set in a bag in this way will help to increase the night temperature in a cold frame?
Vladimír

[bower]

I think it could, Vladimir. As long as it's warm enough for organisms to work breaking down the grass and leaves, it will give off heat due to their respiration process during the day. That means it will continue to be warm at night and continue to give off its heat until it is cold.
The only problem would be if it gets too cold at night or cools too quickly vs how quickly it can warm up and start to work in the daytime. Maybe try a double bag, with the holes for air access on the inside one?

[Whwoz]

Theory is sound Vladimir, only problem I can see it that the volume is not large enough for self heating of compost to occur, if that is what you are looking for you need a pile about 1meter cubed.

If on the other hand you want it as a heat bank, give it a try.

One other alternative that I am aware of, we used it 40+ years ago, is to layer fresh, as in straight out the cow yard, about 20 cm thick under about the same depth of soil. Needs lots of fresh manure to do this.

[Salaam]

I'm curious how this worked out. Good benefits? Any disasters?

I have been bit more than a few times using plastic. It just gets too hot, often with no warning. A cool day, but with clear morning sun, can get incredibly hot inside a small tunnel. And that is just with 0.9 mil and perforated. Gets too hot in the day, and doesn't keep that warm at night. Unfortunately my brother bought a "small" roll of it (2000 ft) so I keep on trying to find a use for it. This year I tried to use it for seed starting outside. Between damping off and cooked plants, I may be ready to write off that plastic.

Overall I have been much better off with AG19.

Well, we got our first fully ripened tomato about June 28. Normally it's mid to late July. So I'd call it a success.

But, the weather this year was very unseasonably warm and sunny in May, with only a couple of minor frosts that the combination of Ag19, poly, and concrete pavers for heat sinks took care of. We had two harder frosts end of May after "official planting date", but I had already taken down the covers and the plants were strong enough to survive.

Operations...I would open both ends of the tunnel if it was sunny. I didn't even have to look at the thermometer once I got used to it. Sunny and cool - both ends open. Sunny and warm - both ends plus one side slightly open. Overcast - can leave it closed. To minimize the work, just open in the morning a couple of hours after sunrise and close at sunset. This turns sunny 40° days to 60-80° and cloudy 40° days to 50° or so if I remember correctly.

At night, 32° outside was about 38° or so in the tunnel. I had concrete pavers (brick size) and fabric pots full of soil in the tunnel and these worked as heat sinks. I also put Ag19 on the hoops under the poly later on, but I don't think it made a difference. I think Ag19 works when laid close to the plants not high up on the hoops. So I think that for night time low temperatures, heat sinks will do the job to prevent frost.

Next year, I'll try kale in the low tunnel. Plant seedlings Apr 15 or so six weeks before last frost date. That should get us kale harvests by May 15.

[JRinPA]

Thanks for the update. I may well try that next year with a few Moskvich. I can't do it at the house as there is not enough springtime sun, but could get an early start at the full sun comm garden.

[MrBig46]

Bower I don't understand how to do this: "Maybe try a double bag, with the holes for air access on the inside one?"
Vladimír