Investigating best practices for efficient minimal heating of high tunnels with modular heaters and row covers.

Project Overview

Project Type: Farmer/Rancher
Funds awarded in 2023: $29,784.00
Projected End Date: 01/31/2025
Grant Recipient: Millsap Farms LLC
Region: North Central
State: Missouri
Project Coordinator:
James Millsap
Millsap Farms LLC


  • Vegetables: greens (leafy), greens (lettuces), leeks, onions, cut flowers


  • Crop Production: continuous cropping, greenhouses, high tunnels or hoop houses
  • Education and Training: demonstration, farmer to farmer, networking, on-farm/ranch research
  • Energy: energy conservation/efficiency, energy use
  • Farm Business Management: budgets/cost and returns, risk management
  • Pest Management: weather monitoring
  • Sustainable Communities: sustainability measures

    Proposal summary:

    As our farms and many others have increased vegetable production in winter with unheated high tunnels, we’ve also increased our risk and liability. Even with the use of multiple layers of frost blankets within tunnels, crops are still damaged or killed by increasingly erratic weather. Poor air circulation and moisture buildup under the frost blankets also lead to crop diseases. It is no longer unusual in our region to experience temperature swings of 60 degrees or more in a 48 hour period, or a polar vortex that plunges temperatures to record lows (for example, the 16-18th of October we went from an 81 degree day to a 17 degree night 48 hours later). To insure against these losses, many farmers add heaters for emergencies, but because they are adding heat to high tunnels not set up for heating, and not interested in converting their tunnels to full greenhouses, the back up heat is inefficient. To farmers dedicated to sustainable farming, these inefficiencies are totally unacceptable. Using large amounts of fossil fuels to grow vegetables out of season is antithetical to their ideals. Many of us feel stuck between unacceptable crop losses in our tunnels and converting to full greenhouse production.

    Project objectives from proposal:


    Some commercial greenhouses make use of an automated heat retention curtain that is deployed as an extra layer of insulation. This equipment is far too expensive for soil based high tunnel farmer, whose square foot sales are much lower. However, similarly, everyone farming with unheated tunnels is using row covers as frost blankets. On freezing nights, the blankets are pulled over the crops. The blankets trap some of the heat stored in the soil to help protect the crops during winter, a low-tech but high labor method.

    We propose combining these blankets with portable, modular, unvented (hence extremely efficient) greenhouse heaters, to increase the effectiveness of the freeze protection and the efficiency of the heater in a high tunnel. Convection tubing is an inexpensive ducting made from clear, perforated 4mil plastic used to distribute heat in greenhouses. It is typically hung from the greenhouse trusses or laid under the canopy of the plants. This project would make use of this tubing to deliver heat from an LB White direct fired 170,000 BTU propane greenhouse forced air heater. Two convection tubes will be used in each house and will be set near the outer edges of the tunnel. Two farms are participating and each will install a heating system in two of their tunnels. One tunnel will use a frost blanket over the convection tubes and crop with the heater’s thermostat installed under the cover. The other tunnel will use the convection tubes and no frost blanket with its thermostat near the crop. Thermostats will be set at 35 degrees F. Temperature sensors will be placed throughout the tunnels.

    The goal is to keep the crop above freezing temperatures as fuel efficiently as possible, and determine if the combination of the heat distributed under a frost blanket is more efficient than without. On freezing winter days, the routine of the farmer is the same; we go out in the afternoon and pull a frost blanket over our high tunnel crops before dark, and then remove the cover in the morning as the sun warms the structures. The covers used in this experiment will be Agribon Pro-50. One large piece will cover the entire growing space. At night when temperatures under the blanket drop below 35 degrees, a thermostat under the blanket will turn on the heater, blowing warm air under the cover and down the length of the tunnel via the convection tubing, ensuring there is no crop loss. The anticipated fuel savings is better for the environment, better for the farmer’s budget, and makes this crop insurance more palatable to the sustainable farmer. 

    To make this whole system easily reproducible, we propose to use all off the shelf components, and heaters which are meant to be moved around as the need arises.  These heaters need no combustion air intake, and no exhaust, meaning that installation is as easy as wheeling them into place, and connecting electric, gas, and ductwork.  This should make the system very adaptable and practical. 


     1. Design and prototype two effective, reasonably priced systems for minimal heating (35degrees F) of 30x96 high tunnels.

    2. Measure and quantify fuel and labor requirements of two different systems, one with row cover, one without. Produce useable data to guide other farmers who might want to adopt similar systems. 

    3. Determine economic viability of these approaches, sharing that data in several field days, conference presentations, farm tours, and a regional farm gathering in spring of 2024. 

    4. Produce and post to YouTube a video describing the systems an results, to further the quest for climate resilient farming practices.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.