A high tunnel greenhouse is a specialized, unheated structure used in agriculture primarily for season extension. It functions as a tool for passive cultivation, allowing growers to plant earlier in the spring and harvest later into the fall and winter. The structure is essentially a large, walk-in hoop house covered in a single layer of plastic film, providing a protective shell for crops. High tunnels help mitigate the impact of unpredictable weather, increasing the reliability and quality of harvests for farmers.
Defining the High Tunnel Structure
The framework typically consists of bowed components, often made from galvanized steel tubing, anchored directly into the ground without a permanent concrete foundation. These arched frames, known as bows, are held together by horizontal structural members called purlins. Purlins run the length of the tunnel, providing stability against wind and snow loads.
The covering material is most often a single layer of greenhouse-grade polyethylene film, usually rated at six-mil thickness for durability and longevity. This plastic sheeting is designed to be UV-resistant and allows high light transmission, typically between 85% to 90%, which is necessary for photosynthesis. Inside the structure, plants are grown directly in the native soil, distinguishing the high tunnel from systems that use fixed benches or containers.
Ventilation for temperature and humidity control is achieved through passive means. Most high tunnels feature roll-up sides, which are manually raised or lowered to allow cooler air to enter near the ground level. End walls often include large doors or removable panels that facilitate cross-breeze ventilation and allow access for farm equipment. The structure’s height, ranging from 6 to over 17 feet, allows for comfortable working conditions and the growth of tall, trellised crops.
How High Tunnels Differ from Greenhouses
The primary difference between a high tunnel and a conventional greenhouse is the degree of environmental control. High tunnels are characterized by their passive systems, meaning they rely on solar gain and manual ventilation to manage the internal climate. They generally lack automated heating, cooling, or complex electrical systems, which keeps their initial construction and operational costs significantly lower than those of a greenhouse.
A traditional greenhouse is an active structure, often built on a permanent foundation and equipped with automated systems. These systems include forced-air fans, mechanical ventilation, and dedicated heating units to maintain a precise temperature range, enabling year-round production regardless of outside conditions. Greenhouses typically use rigid glazing materials like glass or polycarbonate panels, offering greater durability and insulation than the single-layer polyethylene film of a high tunnel.
The financial and regulatory status also differs, making high tunnels an attractive choice for many growers. High tunnels are often considered semi-permanent agricultural tools, which can result in different property tax classifications or make them eligible for government conservation programs. Greenhouses, due to their permanence and extensive infrastructure, are subject to more rigorous building codes and higher investment costs.
Cultivation Benefits and Uses
The structure’s most significant function is season extension, enabling a longer harvest window for crops. By trapping solar heat, the structure can raise internal air and soil temperatures by approximately 4 to 7 degrees Fahrenheit compared to the outside air, allowing planting to begin weeks earlier. The structure also protects crops from early frost events, extending the growing period into the late fall or early winter.
High tunnels also serve as a protective barrier against various environmental stressors. They shield crops from excessive rainfall, which prevents soil erosion and reduces the risk of foliar diseases like blight and mildew. The physical enclosure also protects sensitive plants from high winds, hail, and certain insect pests, leading to cleaner, higher-quality produce and more reliable yields.
This controlled environment supports the intensive production of high-value crops, such as tomatoes, peppers, leafy greens, and bramble fruits.