Editing Polytunnel (section)

{{Short description|Type of greenhouse}}
{{Use dmy dates|date=October 2020}}
{{more citations needed|date=April 2023}}
[[File:Polytunnel Balhungie.jpg|right|thumb|250px|Polytunnels on Balhungie Farm, [[Angus, Scotland|Angus]]]]

A '''polytunnel''' (also known as a '''polyhouse''', '''hoop greenhouse''' or '''hoophouse''', '''grow tunnel''' or '''high tunnel''') is a tunnel typically made from steel and covered in [[polyethylene]], usually semi-circular, square or elongated in shape. The interior heats up because incoming [[solar radiation]] from the sun warms plants, soil, and other things inside the building faster than heat can escape the structure. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. [[Temperature]], [[humidity]] and [[ventilation (architecture)|ventilation]] can be controlled by equipment fixed in the polytunnel or by manual opening and closing of vents. Polytunnels are mainly used in temperate regions in similar ways to glass [[greenhouse]]s and [[row cover]]s. Besides the [[passive solar]] heating that every polytunnel provides, every variation of auxiliary heating (from hothouse heating through minimal heating to unheated houses) is represented in current practice. The nesting of row covers and low tunnels inside high tunnels is also common.<ref>{{Cite web|last=Blomgren|first=Ted|last2=Frisch|first2=Tracy|date=May 2007|title=High Tunnels: Using Low-Cost Technology to Increase Yields, Improve Quality and Extend the Season|url=https://www.sare.org/wp-content/uploads/HighTunnels_UVM.pdf|access-date=26 October 2020|website=SARE (Sustainable Agriculture Research and Education)}}</ref>

[[File:Superbay Polytunnel.jpg|thumb|Superbay Polytunnel at Van Arnham Nurseries]]

Polytunnels can be used to provide a higher temperature and/or humidity than that which is available in the environment but can also protect crops from intense heat, bright sunlight, [[wind]]s, [[hail]]stones, and [[cold wave]]s. This allows [[fruit]]s and [[vegetable]]s to be grown at times usually considered off season; [[market garden]]ers commonly use polytunnels for [[season extension]].<ref>{{Cite web|title=Winter Production in the High Tunnel {{!}} Johnny's Winter Growing Guide|url=https://www.johnnyseeds.com/growers-library/vegetables/winter-growing-guide-high-tunnel-production.html|access-date=26 October 2020|website=www.johnnyseeds.com}}</ref> Beyond season extension, polytunnels are also used to allow cold-hardy crops to [[overwintering|overwinter]] in regions where their [[hardiness (plants)|hardiness]] is not quite strong enough for them to survive outdoors. Temperature increases of only {{convert|5|to|15|C-change|0}} above outdoor ambient, coupled with protection from the [[drying]] effect of [[wind]], are enough to let selected plant varieties grow slowly but healthily instead of dying. The effect is to create a [[microclimate]] that simulates the temperatures of a location several hardiness zones closer to the equator (and protects from wind as well).

[[File:Bespoke Superbay Polytunnel.jpg|thumb|Superbay Polytunnel – Otter Nurseries]]

Every factor influencing a crop can be controlled in a polytunnel. Polytunnels are often used in [[floriculture]] and [[plant nursery|plant nurseries]], as the [[revenue]] value of the plants can justify the expense.

In recent years the true adaptability of polytunnel structures has been realised by adapting them to suit livestock housing. It is now commonplace in the UK to see polytunnels used for housing [[sheep]], [[alpacas]], [[goats]], [[Calf (animal)|calves]] and [[poultry]].