How much warmer are they?
 

I don't have any greenhouses or cold frames at all. What I am curious about, is how much warmer it is to grow under cover, without added heat, than it is outdoors.

Some folks talk about cold frames here....how much of a headstart do they give you? 10 degrees? 20 degrees? What about unheated hoop houses or greenhouses? Does it depend on the materials (say glass or plastic)? I know there are fancy setups some use to "heat" their structures or retain heat, but without those, I'm just wondering how much earlier you can plant out.

I've got one of those multi-tiered little greenhouses that you can start seeds in, but only have used it in April, inside a protected porch with a plastic cover on it so I was curious :)

It will probably depend on materials and design, but my two small cold frames give me a pretty consistent 10 degree boost, and 20-30 on warm sunny days. 10 degrees makes a big difference when the temp is hitting the danger zone for certain plants in the spring.

Eliot Coleman, who's done quite a lot of research into "four season" gardening, says that that growing under plastic covering provides the equivalent growing conditions of one zone farther south. So, for example, if you're in Zone 6, growing under quick hoops would put you in Zone 7. Adding an additional layer of row covering (e.g. Agribon) provides an additional "zone" of warmth. There are obviously other variables involved including wind chill, snow covering, exposure to winter sunlight, etc., but these are general guidelines.

With a simple high tunnel without supplemental heat, you will only get a 1-2° F added protection against frost. The time and cost of installing such a structure certainly would not be worth it if your objective is to protect against freezing temperatures.

As indicated, addition of a second layer of row covering can get you another 10° of protection.

The real benefit of a hoop house (or a related design) is increased daytime temperatures and soil temperature. Tomatoes, for example, grow best at temperatures of 70-85°. Even a nice, sunny, "warm" Spring day with 60° high temperature will not do you much good as far as getting tomato plants to grow, flower, set and ripen fruit.

With a plastic (or glass) cover, on a sunny day, you can get tempertures in the 70+ range by 10 a.m. and keep them there until 5 p.m. or so, even in the dead of winter. On sunny days, venting will be required to keep the temperatures from rising too high.

I saw amazing results with my little (7'X16') high tunnel last year. It sure made me want to install one over my entire garden!

Info. about benefits of high tunnels for strawberry production at:
[URL]http://extension.usu.edu/files/publications/publication/Horticulture_HighTunnels_2010-01pr.pdf[/URL]

I can't comment on the cold frame, but I can on my little 6 by 8 green house. All most as cold at night as outside. Other materials might insulate better. How hot? WAY hotter. You won't believe how hot it can get in there when the sun shines without a ventilation fan.

I agree - you have to insulate at night to hold the heat ( I use a reflective bubble wrap), and heat build up on sunny days is a real threat.

An old gardening method for raising temperature in cold frames is to dig a trench and fill it with mix of stale manure and straw. Cover with soil and in few days temperature raises.

My greenhouse is about 8'x12' and is made of a thick corrugated plastic. I keep a small heater in there that turns on when it gets cool. There is also a layer of rocks on the ground that I assume was put there to absorb heat during the day and release it at night.

There are some areas that are not air tight, but my overwintering peppers appear to be doing pretty well, so I know they haven't gotten below freezing, at least not for long.

I really want one of those climate sensors that tells you what the temp, moisture, ect. is. Then I would know how long the heat holds on cold nights and if I need to add another heater, just to be safe. We haven't had any long cold spells yet this year, so I'm still a little concerned.

For Spring, there are two built in fans, one a large automatic type in the peak over the door. (Right now it's turned off and covered so that all the heat doesn't escape) I can set that one to turn on at different temps. But even with both of the fans going, I don't think many things would survive in there past about June in our area. By that time, it's usually over one hundred degrees in there during the day. Even if the plants could stand it, I couldn't!

Another way to think about a cold frame for early planting is for mitigating weather risk.

Too cold, put a blanket on top or add a light bulb inside at night.

Too hot, add a shade screen, open the top fully or take the top completely off.

Too much wind, rain or hail, keep the top closed or open just enough for ventilation.

If you want to go "hi-tech" you can add a vent opener, thermostat, heater, fan or irrigation to do most of this automatically.

A cold frame should allow you a 2-6 week head start for seedlings and transplants.

Rick

When cheating spring and picking ripe maters on May 5th I'll let you be the judge! Zone 5 here. The key is to have a smaller heated area inside of the hoophouse to nuture your babies along. Who needs to heat a 7-8' high gh when your plants are only 6-12" or so. By mid-June my plants are pushing 5-6ft tall and never stop. Otherwise, a coldframe averages 7-10 degrees warmer (nightly) overheats by day and the hoop house is equal to overnight outside lows. A few light bulbs in an insulated coldframe/low tunnel brings amazing results.

Just built a new cold frame and I am consistantly getting 15-20 degrees extra in the day and 10-15 at night. I actually planted tomatoes out in the cool weather to slow them down. They were getting too big sitting in that cold frame all day.

Is there any way to add extra light? I have low light in the area my high tunnel will be this winter, due to trees and walls.
Reflective mulch, anything?

Here is a photo of my shelving greenhouse I purchased before the season for around 40.00. It keeps my tomato plants at 10 to 20 degrees warmer than outside.
If it becomes too hot I simply unzip the front and tie it back.
It works great. :)


Julia

Any ideas to increase lighting? I'm thinking next winter that light will be my limiting factor. The bed gets about 6 hours of sun right now but there's a big pine tree and a wall which shade it in the afternoons.
Has anybody added lights to a high tunnel?

I am considering a cold frame to start seedlings in instead of indoors under lights.
If it would keep enough warmth to work starting about mid March, and it can get below freezing in mid March in southern VA, I think I could do it.

Anyone use theirs for that purpose?