High tunnels consist of a single layer of plastic stretched over metal or PVC hoops. Tunnel designs can vary and be modified or adapted to fit a growers’ needs.

The plastic covering protects crops from damaging weather events, such as rain or hail. Temperature modification, however, is what high tunnels are most known and used for. Air and soil temperatures inside a tunnel warm more quickly in the spring and remain warmer longer in the fall. This allows for earlier plantings in the spring, and extended production into the fall and winter months.

Temperature control is achieved through manual or passive methods. Temperatures are raised by solar radiation while cooling and humidity control is done manually by opening doors, rolling up/down sidewalls and opening vents.

 When managed correctly high tunnels offer growers several new opportunities and advantages over field production: 

• Longer growing season  
• Protection from adverse weather that could damage or kill plants 
• Moderate control over a crops climate 
• Better crop quality and yields 
• Improve farm revenue
• Reduced disease pressure (in some cases) 
• Improved irrigation and fertility management

Inside a tunnel, plants are most often grown directly in the existing soil and irrigated using drip systems. The incorporation of fertigation into a drip system can give growers a targeted nutrient delivery system that reduces waste. For information on how to install or design a fertigation system see Additional Resources below. Various plastic and organic mulches are commonly used to help control weeds and improve soil health.

 

High tunnel production often requires more detailed attention to management strategies when compared to field production.

The modified environment that protects crops and promotes faster growth rates also attracts insects, rodents, and other pests. The absence of rainfall, in addition to the optimal climate conditions inside a tunnel can allow insect populations to reach damaging levels more quickly.

Without the implementation of effective integrated pest management (IPM) practices the need for insecticide applications may occur more often in high tunnel production than under field conditions. Improper climate management, or poor tunnel design can lead to increased incidences of diseases and the need for pesticide intervention.

For information on IPM strategies for high tunnel production see IPM Practices and Strategies below. 

Whether buying a manufactured kit, or designing your own tunnel there are a few things that should be considered to ensure your tunnel design will support your production goals. A poorly designed tunnel will cause management frustrations and increase insect and disease pressures. Consider the following questions when selecting a tunnel design:  

1.   What is the climate like where the tunnel will be located?                    

Not all tunnel designs will produce well in Arkansas. Even with a shade cloth temperature within a tunnel will be warmer than the temperature outside. Because of this many high tunnel producers in Arkansas often struggle with excess heat and humidity within the tunnel, particularly when using the tunnel thoughout the summer. Among the most important design considerations is good ventilation. 

Look for designs with large doors, or end walls that can be removed for increased air flow. Additional roof or sidewall vents will increase the cost of the tunnel, but the improved air flow could prevent heat damage stress and disease hotspots. See the section on ventilation.

Even in northern parts of Arkansas where temperatures are colder, adequate ventilation is important for disease management. However, additional considerations may be necessary to protect plants during the shoulder seasons and the cold winter months, if year-round production is desired. 

2. What crops will be grown in the tunnel? 

While all tunnels can easily support shorter plants, such as lettuce or strawberries, canopy management for taller crops could become frustrating if the tunnel design does not accommodate the canopy height. Crop quality and yields could also suffer. While more expensive, including taller sidewalls in a tunnel design will provide more usable canopy space and accommodate taller crops more easily. Increasing sidewall height will not only accommodate taller crops, but also improve airflow within the tunnel.

3. What size tunnel can the land support?

A high tunnel should be in a sunny spot, on level ground for good drainage. Tunnels need to be placed an adequate distance from nearby structures or tree lines to prevent shading within the tunnel. Reductions in sunlight will negatively impact plant growth. Poor drainage can cause flooding within a tunnel which will compromise production success and cause disease and insect problems.

Year-round access to irrigation and labor should also be considered when selecting a tunnel size. Remember, tunnels often require additional labor hours for climate management, pest monitoring and out of season production.

4. What equipment will be used inside the tunnel? 

Select doors or end walls that will allow desired equipment to enter the tunnel easily. Sidewall height will also influence what equipment can operate safely within the tunnel.  

5. What common adverse weather conditions will the tunnel encounter? 

Additional structural support may be necessary in areas that experience high winds or snowfall. While many single bay tunnels are better able to withstand adverse weather conditions when compared to multibay tunnels, keep in mind that not all single bay tunnels have the same level of support or sturdiness. Gothic styled tunnels, for example, shed snow loads more easily than their rounded counterparts. Tunnels with a sturdier design, or the inclusion of additional supportive structures will cost more upfront but could prevent costly repairs in the future.

Crop selection will depend on the purpose of the tunnel. If profitability is not a concern, then just about any common fruit or vegetable crop can be grown inside a tunnel, if the gardener is willing to put in the needed effort.

For growers using a high tunnel to increase farm revenue, crop selection should include market research to determine what crops can be grown profitably in their area.

Nationally, vegetables are the most common high tunnel crops, with tomatoes easily being the most popular and profitable, followed by bell peppers, cucumbers, lettuce, summer squash and eggplant. Strawberries and other small fruits have also been profitably produced within a tunnel. In more recent years production of larger fruiting crops, such as raspberries, blueberries and even some tree fruits have been trialed in high tunnels in many states.

Just like in field production, planting date is important for high tunnel production. Planting date is largely determined by a crops’ specific growing requirements and the temperatures that can be maintained inside the high tunnel.

Generally, if no supplemental heating is provided, more hardy warm season crops, such as tomatoes and peppers, can be transplanted a month to a month and a half before the areas last frost-free day in the spring, while more tender crops, such as cucumber or summer squashes, can be transplanted about two weeks later. Frost cloths may still be necessary to protect plants on very cold nights.

Cool season crops that are intended to be harvested as full-sized heads or plants during the winter months, should be planted early enough for plants to reach nearly full size before temperatures and light levels become too low to support growth. Later plantings are possible for crops harvested as individual leaves or as smaller plants. Keep in mind that growth rates will be markedly slower for these later plantings and having multiple plantings spaced a few weeks apart could produce a supply of fresh young leaves for a longer period. Cool season crops can be transplanted in late December or January if spring harvests are desired.

The generalizations outlined above are a good place to start when determining planting dates for high tunnel production. However, adjustments may be necessary for unique micro-climates and market goals. Take the time to become familiar with your climate and how well your tunnel design can modify that climate for crop growth. This knowledge and understanding will help narrow down optimal planting dates.   

Ventilation:

The hot and humid summers in Arkansas could become limiting for high tunnel production if heat and humidity buildup cannot be sufficiently vented. Poor air circulation can lead to heat stressed plants and cause disease and insect "hotspots" to form within the tunnel. This means that adequate ventilation will heavily impact the success of a high tunnel. Even in cooler months, when heat capture is desired, ventilation will still be necessary for humidity and disease control.  If secondary covers are being used for additional frost protection, these should also be vented to prevent high humidity levels.  

Tunnel design considerations that can improve ventilation:

1. Sidewall height: taller sidewalls will allow more airflow through the tunnel and improve the tunnel's ability to cool.  
2. Endwall design: Endwalls with larger openings will improve airflow throughout the tunnel. 
3. Tunnel orientation: Orienting a tunnel so vents are perpendicular to the prevailing wind direction will improve airflow.
4. Tunnel height and width: Narrower tunnels will cool more easily while a taller wider tunnel will retain heat longer on a cold night. 

Taller crops will reduce air movement within a tunnel. This can lead to pockets of warm wet air within the tunnel that should be monitored closely for disease outbreaks.

 

Common Types of High Tunnel Ventilation:

Shade cloth:

In addition to adequate ventilation, a shade cloth will be necessary in the long, hot, and sunny Arkansas summers to avoid heat related plant disorders and stress. A shade cloth reduces the amount of sunlight entering a tunnel which helps reduce the internal temperature. Shade cloth is made from a loosely woven polypropylene product and comes in a range of shade ratings (10% to 80%) and colors. Keep in mind that in addition to reducing heat a shade cloth will also reduce the amount of light reaching plants which, if reduced too much, can negatively impact plant growth.

Secondary Covers:

Plants may need additional frost protection during the spring, fall and winter seasons, particularly on cold nights when temperatures drop below freezing. Many growers use secondary covers inside a high tunnel to give plants a few more degrees of protection. While a variety of secondary covers have successfully been used to protect plants, the most common is a woven fabric, often referred to as a floating row cover or frost cloth, that is placed either directly on plants like a blanket or suspended over plants using hoops. Frost cloths can come in a range of thicknesses that offer varying degrees of protection. Humidity levels can build beneath secondary covers and it is important to remove or open them to discourage disease growth and spread. Often, secondary covers can be removed during the day to vent humidity and be replaced in the late afternoon or early evening.

 

The tunnels' protected environment, which does such a great job promoting faster plant growth and earlier harvests also favors the rapid reproduction of many soft bodied insect pests. The absence of natural mortality events, such as rainfall, will further quicken population growth. This is especially true for insects that overwinter in the soil or may reproduce on non-crop plants growing in or around the tunnel. The lack of rainfall can also decrease the impact of entomopathogenic pathogens (pathogens that grow on or within insects often killing them), which help manage soft-bodied insects in field production. Consequently, without intervention, small populations can reach damaging numbers within days. Insect pests that are becoming common high tunnel pests are two-spotted spider mites, aphids, white flies, and thrips

While high tunnels can help reduce pressure from diseases that rely on rainfall or water splashes to infect plant material, other diseases will experience a favorable environment inside a tunnel. However, when the tunnel climate is managed effectively, disease pressure inside the tunnel can be much lower than under field conditions. Effective management includes venting to keep humidity levels and temperatures within optimal ranges while maintaining good air circulation around the plant canopy. A crop rotation schedule will help prevent the buildup of soil borne disorders. Diseases seen most often in Arkansas high tunnel production include powdery mildew and botrytis.     

An integrated pest management, or IPM, program is a sustainable decision-making approach that uses a combination of pest management practices to manage pests while considering the economic and ecological impact of the management tactics.

A good IPM program begins with effective cultural and physical practices that minimize the introduction or spread of a pest inside a production system. This is of particular importance for high tunnel production due to how quickly pest outbreaks can happen once a pest is introduced to the system. The isolated nature of a high tunnel makes biological methods, such as the introduction of a natural predator, more effective than under field conditions. However, timing is paramount for a biological control practice to work successfully. Chemical control methods, both organic and conventional, are generally available for use within a high tunnel, however, label restrictions may exist. Before applying any chemical, read the product’s label to understand all safety precautions and restrictions. When applying a chemical inside a closed tunnel it may be advisable to follow greenhouse application restrictions.

Below is an introduction to some IPM practices for high tunnel production. 

Have an Action Plan Ready to Go: Whatever pest issue may arise, be prepared to react fast. Before planting, have a plan of action in place. Tunnel environments can cause rapid population growth and issues can get out of control quickly. A fast response will prevent severe outbreaks and crop loss. When possible, have any necessary supplies on hand.

Check Transplants Before Planting:  Carefully inspect transplants for signs of disease or harmful insects before bringing them into the tunnel. When possible, select cultivars that exhibit some resistance to expected pest issues. If purchasing transplants, acquire them from a nursery that has a reputation for supplying disease and insect free plants.

Sanitation: Good sanitation can reduce favorable environments that foster the growth of insect populations and disease development as well as remove sources of inoculum and improve air flow within the tunnel.

Good sanitation practices include:

• the disinfection of any equipment, such as trellising materials or clippers, used within the structure
• the removal of any infected plants or plant debris.

Rotate Crops Grown in a Tunnel: A good, well-thought-out crop rotation plan will disrupt disease and insect cycles, preventing pest pressure from building with each new planting. Avoid successive plantings of a crop, or crops in the same family, in the same high tunnel.   

Proper Environment Management: Many diseases and insects prefer a humid or moist environment. Inadequate ventilation within a high tunnel can cause pockets of warm humid air, creating “hot spots” for diseases and insects. Ensure proper ventilation and monitor potential hotspots closely.

Scouting and Monitoring for Diseases and Insect Pests: Prevention and early detection of pest issues is the foundation of a successful IPM program. Scouting should occur multiple times a week. If that is not possible, scout at least once a week.

Good Record Keeping: Good records can be used to indicate when annual pests are likely to appear each season. This will allow growers to be prepared with needed supplies and act before outbreaks occur. Records can also indicate where “hotspots” are within a tunnel.

Pesticides in a High Tunnel: When applying a pesticide (organic or conventional) keep an eye on chemical temperature restrictions as it is often warmer inside the tunnel. When applying, be safe and follow all safety recommendations on the label. Most organic pesticides will work best as a preventive spray and will have little impact on a severe outbreak.  Be wary of applying any oil-based pesticide or EC formulations when temperatures reach 80-90 degrees, especially on the west side of tunnels in the evening hours.

Organic Producers: High tunnels offer organic growers alternative pest control options including avoidance (such as growing outside the normal season), natural enemies, and biocontrol. Keep in mind that organic options work best as preventive methods and will do little against an insect or disease outbreak. Sanitation and other preventive methods should be done rigorously to avoid possible outbreaks.

This list of control practices is not meant to be an exhaustive description of IPM strategies for high tunnel production, but instead is intended to give a grower a place to start. Contact your local Extension office for more information or further assistance.

Additional IPM Resources:

Insecticide Recommendations for Arkansas - MP144

2023 Southeast Vegetable Crop Handbook

The Arkansas Plant Health Clinic

Quick Facts about the NRCS High Tunnel Grant:

---

WHO IS ELIGIBLE FOR THE FINANCIAL ASSISTANCE?

Applicates must own or have control over eligible land for the lifespan of the program’s contract and must develop an NRCS EQIP plan of operation. Additional eligibility information can be found at your local NRCS field office.

TUNNEL SIZES:

While the contract does not restrict tunnel length, the tunnels’ width cannot exceed 30ft and the tunnel must be a minimum of 6ft in height (at its peak) to qualify for financial assistance. While the amount of funding received is based on tunnel size, beginning in 2024 new regulations will introduce a cap, and will only cover a maximum square footage of 2,160. Underserved producers may qualify for additional financial assistance. Visit your local NRCS agent for more exact financial information.

PRE- MANUFACTURED KITS ARE REQUIRED:

To be eligible for financial assistance, tunnels must be purchased as a kit from a reputable manufacturer. All manufacturer instructions and specifications must be followed when preparing for and constructing the tunnel. Additionally, applicants must select a tunnel design that the manufacturers specifications indicate can handle environmental conditions common in the area, such as snow or high winds.   

APPLICATES ARE RESPONSIBLE FOR THE INITIAL COST:

Growers will initially be responsible for the full cost of the tunnel. Once the tunnel is constructed, they will be reimbursed. Advance payment options are available for applicants that meet specific eligibility requirements. For more information on reimbursement timelines and payment options visit your local NRCS office. Some high tunnel suppliers are willing to work out payment plans with growers to reduce the size of initial payments while they wait for reimbursement.

Qualifications for advancement payment:

TUNNEL MODIFICATIONS AND IMPROVEMENTS:

Modifications or improvements can be made but must meet all the specifications of the manufacturer. Improvements include (but are not limited to) electricity, heating systems and additional/automated venting systems. Growers are responsible for the cost of any modifications or improvements.  

WHAT CAN BE GROWN IN AN NRCS HIGH TUNNEL:

Any annual or perennial crop can be grown in an NRCS funded high tunnel but must be grown in the natural soil. Mounding soil without any structural sides is allowed but must not exceed 12 inches in height. Raised beds with structural sides are not permitted (there may be some exceptions to this, visit your NRCS office for more information). Under the current guidelines, growing crops in pots or on benches is not allowed. Tunnels cannot be used as a storage shed.

OTHER NRCS PROGRAMS THAT CAN IMPROVE HIGH TUNNEL PRODUCTION:

• Water management (irrigation and microirrigation system design)
• Drainage management systems
• Nutrient and salinity management
• Integrated pest management
• Underground outlets
• Mulching
• Critical Area Planting
• Crop rotation
• Cover crops
• Rainwater catchment system
• Compost application (soil carbon amendment)

ADDITIONAL RESOURCES:

Find your local NRCS county office

Criteria for an NRCS high tunnel

Information on additional NRCS practices and programs

Series of videos on preparing, selecting, and constructing a high tunnel - PennState Extension

Basics of Drip Irrigation and Fertigation for Specialty Crops

Considerations for Subsurface Drip Irrigation Applications in Humid and Sub-humid Areas

Managing soil surface salinity with subsurface drip irrigation

 

How to recover a high tunnel: