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High Tunnel Hoop House - Extending the Season Without Heat
Extending the season in a high tunnel hoop house without heat: the cover, the sides, the doors and the timing that buy extra weeks. .
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  • Features
Poly tunnels standing side by side with end wall doors, seen on an overcast day
A long row of poly tunnels along a farm track in cold weather

What Extra Weeks Are Worth:

The money in a high tunnel hoop house is not made in midsummer. It is made in the two or three weeks at each end of the season when the field has nothing to sell and the market pays a premium for anything fresh. That is the whole case for the structure, and it changes how you should think about the tunnel: not as a place to grow better crops than the field, but as a place to grow the same crops a few weeks earlier and a few weeks later. Everything below is about buying those weeks without installing a heating system - because for most growers, heat is what turns a cheap tunnel into an expensive one.
 

Layer One: The Cover:

The film is the first and biggest layer, and on a cold site it is worth spending on. A thermal grade formulated to hold more heat overnight is the obvious upgrade, and a double layer of film with air between it holds considerably more warmth again - that is what the tunnels in the photos above are doing in cold weather, running as a closed house rather than an open one. A well-sealed tunnel with a good thermal cover will keep a crop through light frosts on its own, which is the point at which the extra weeks start to pay. What the cover cannot do is beat a hard freeze, so the honest position is that a tunnel buys you the shoulders of the season, not the middle of winter.
 

Layer Two: Sides, Doors and Closing Up:

The second layer costs nothing to install, only discipline. A tunnel that is opened at nine and closed at four keeps far more warmth than one left open all day, and in the shoulder seasons the difference is measured in days of growth, not degrees. The roll-up sides want to be closed in the late afternoon before the temperature drops, and both doors shut at the same time - a tunnel with one door left open loses most of what the cover gained. On a cold night the crop is protected by the air the house has stored, and that stored air is exactly what a careless afternoon throws away. Growers who get the extra weeks usually get them from timing rather than from hardware.
 

Layer Three: What You Can Add Cheaply:

If you need more than the cover and the doors give, the next steps are cheap and proven. A floating row cover over the crop inside the tunnel adds a further few degrees for the cost of the fabric, and it is the single best value item in a cold-season tunnel. Low mini tunnels over individual beds do the same job with a bit more structure, and black or clear mulch film on the soil warms the root zone and gives the crop a start in spring that the air temperature alone will not. An internal thermal screen under the roof is the next step up and is still far cheaper than heating. What none of these do is replace heat on a genuinely cold night, and it is worth saying plainly: a tunnel should be sold as a season extender, not as a heated house.
 

Timing, and What Comes With the Tunnel:

The weeks you gain in spring are set by how early you can plant without the crop sitting still in cold soil, which is why mulch and row cover matter more than they look. In autumn the limit is light rather than temperature - the crop stops growing long before it freezes - so an autumn tunnel extends the harvest of an established crop rather than starting a new one. Tell us the coldest nights on your site and what you plan to grow at each end of the season, and we will specify the film grade, the double layer option, the doors and the roll-up sides to suit. Normal shipping time is 4 to 6 weeks after receipt of order, plus transit from the factory; 30% of the basic greenhouse price is charged on receipt of order and the balance before shipment. Look through the tunnel greenhouse models and the film greenhouse range they belong to, add greenhouse irrigation at the same time, or see the wider commercial greenhouse range and read more about our greenhouse factory, call +86 191 5068 3942 or send an E-mail to sales@cngreenhouses.com.
 
We have supplied growers and project contractors across Africa, South Asia, the Middle East and Latin America for many years. Notes on season extension and high tunnel production are published by organisations such as the USDA, and international greenhouse trade news is reported by HortiDaily.
 

Frequently Asked Questions:

Q: How many weeks can a high tunnel add to the season?
A: It depends on the site and the crop rather than on a fixed number. A well-sealed tunnel with a thermal cover will carry a crop through light frosts and gives the shoulders of the season; the hard freeze in the middle of winter is beyond what an unheated tunnel should be expected to handle.
Q: Do I need heating?
A: For season extension, no - and avoiding heat is what keeps a tunnel cheap. The cover, disciplined opening and closing, row cover inside and mulch on the soil will get you a long way. Heating turns a tunnel into a different and much more expensive proposition.
Q: What is the cheapest thing that actually helps?
A: Floating row cover laid over the crop inside the tunnel. It adds a few degrees for the price of the fabric and is the best value item in a cold-season tunnel. Mini tunnels over the beds and mulch film on the soil are the next steps.
Q: Is a double layer of film worth it?
A: On a cold site, yes - a double layer with air between it holds considerably more warmth overnight than a single sheet. It costs more at the outset and it is the difference between a tunnel that only keeps frost off and one that carries a crop through a cold week.
Q: Why does my autumn tunnel stop producing before it gets cold?
A: Because in autumn the limit is light rather than temperature - the crop stops growing well before it freezes. An autumn tunnel extends the harvest of an established crop rather than starting a new one, so plan for a crop that is already mature going in.
Q: What do you need from me to specify a tunnel for season extension?
A: The coldest nights on your site and what you plan to grow at each end of the season. From that we specify the film grade, whether a double layer is worth it, and the door and roll-up side arrangement.

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