Written by Mary Hathaway, OFRF’s Research and Education Program Manager, and based on eOrganic Webinar by Dr. Zhao on Adapting and Expanding High Tunnel Organic Vegetable Production for the Southeast

I am excited to share some new research findings on high tunnel production for organic vegetables in the Southeast region. But before we get into those findings, let’s answer a couple of FAQs: what exactly are high tunnels, and why are they important? High tunnels (HTs), sometimes referred to as protected culture systems, are commonly used for season extension and/or production of high-value crops. High tunnels are increasingly being utilized by organic growers for their many benefits, such as crop protection and improved crop quality. While HTs are generally regarded as a tool for production management, they can also aid farmers with risk management and act as a resource conservation practice. 

One reason why high tunnel use may be growing in popularity is the availability of funding from the USDA-NRCS EQIP High Tunnel System Initiative. This NRCS program has been pivotal in providing the funding and support needed to access this season-extension tool that can be a real game changer for farm operations. As someone who has gone through the process first-hand of receiving and building a high tunnel, I can attest that the program is worth any of the front-end paperwork, and helps growers improve their infrastructure while gaining many positive ecological benefits. High tunnels can provide many benefits, including protecting your crops from extreme fluctuations in weather. 

Reasons to consider a high tunnel for your operation:

✓ Extend the growing season

✓ Improve plant quality and soil quality

✓ Reduce nutrient and pesticide transportation

✓ Improve air quality through reduced transportation inputs

✓ Reduce energy use by providing consumers with a local source of fresh produce

Unique Challenges in the Southeast

While high tunnels may provide positive benefits, growers in the Southeast face several challenges when implementing HT in their growing systems. These challenges are typically related to airflow and managing excessive heat stress, humidity, pests, and diseases. I have experienced some of these challenges first-hand at my small-scale vegetable production farm in Central Florida. It is no easy thing to deal with the excess humidity and fluctuating pest and disease pressures, so I was excited to see new research led by Dr. Xin Zhao focused on “Adapting and Expanding High Tunnel Organic Vegetable Production for the Southeast” (more specifically, Florida and Georgia). This body of work is thanks to a collaboration by researchers from the University of Florida, the University of Georgia, Florida A&M University, and the USDA-Agricultural Research Service.

Check out the eOrganic Webinar presented by Dr. Xin Zhao:

Adapting and Expanding High Tunnel Organic Vegetable Production for the Southeast

Findings

Temperature

Anyone who has ever stepped inside a high tunnel knows that air temperatures are higher inside HTs than outside. One part of this research study entailed comparing different types of shade cloth to determine their effect on mean air temperature within HTs. Interestingly, when comparing silver cloth with black cloth, the mean air temperature was higher in HTs with silver shade cloth. In externally mounted shade cloths with the same shade factor, dry nets provide less cooling than wet nets.  

If it gets too hot, expect plants to die. High root zone temperature (RZT) can influence plant growth and function. Increased air temperature and RZT in the unshaded HTs likely increased plant mortality and reduced plant growth, particularly in plants grown with black mulch. 

Shade cloth brings down the temperature inside your high tunnel, but is still hotter in the day than average outdoor temperatures. HTs with shade cloth had similar soil temperature as outdoor soil temperatures, but shade cloth provided reduced soil temperatures during day and night when compared to non-shaded HTs. No air temperature difference was recorded in the night hours between the two types of HTs. During night hours, soil volumetric water content (VWC) was greater in the shaded tunnel than in the non-shaded tunnel. Soil VWC increased in the shaded tunnel between day and night. Shading did not affect average relative humidity (RH) levels during the day or night. However, relative humidity levels were lower outside during the night than in either of the high tunnels.

Insect pressure

Researchers also looked at the relationships between pests, temperature, and shade in high tunnels. They found that in pac choi trials, the major pest problems were whiteflies, thrips, aphids, and Southern armyworms. Though the number of thrips, whiteflies, aphids, leafhoppers, and predatory flies were similar among shade cloth treatments, they found that both black and silver shade cloths harbored more beneficial parasitoids than unshaded conditions in one study season. The high light conditions in the unshaded treatment may have negatively impacted the parasitoid wasps. Temperatures in HTs may not have an impact on insect pressure. Thrips were the only pest that decreased in number with increasing HT air temperatures. Other insect numbers were variable among shade treatments in increasing air temperatures. 

Researchers also found success in attracting beneficial insects by planting Sweet Alyssum, buckwheat, or marigold at the front and back of leafy green beds and corners of each high tunnel. The push-pull system, including companion planting and refuge planting, is worth further research in high tunnel systems.  

Image credit – ICIPE

The ‘push-pull’ strategy,  uses a combination of behavior-modifying stimuli to manipulate the distribution and abundance of insect pests and/or natural enemies. Pests are repelled or deterred away from the main crop (push) by placement of plants that mask the host or are repellent. The pests are simultaneously attracted (pull), using attractive plantings, to other areas such as traps or trap crops where they are concentrated, facilitating their control.

Photo Credit – UGA Extension

Fogging 

Like shade cloth, water fogging is a technique used to reduce air temperatures in high tunnels and provide evaporative cooling. Fogging systems differ from misting or sprinklers in that they produce a much smaller droplet, and operate at a higher psi. Fogging systems operating at 700-1000 psi produce droplets around 25 microns – as fine as the point of a needle. The smaller the droplet the more quickly it can evaporate. In this study, fogging did not affect average soil temperatures or average air temperatures during the day or night. Plots that received water fogging did show greater soil volumetric water content than those without fogging. Even though fogging systems did not run during night hours, fogged plots also had significantly greater leaf wetness than non-fogged plots in evening hours, which may be a major consideration for disease management in humid climates. Fogging had no impact on average photosynthetic photon flux density (PPFD), (a measurement of the amount of light in the portion of the light spectrum utilized by plants for photosynthesis that actually reaches your plants). 

Shade and Light

Shade cloth reduced air temperature, root zone temperature (RZT), and photosynthetic photon flux density (PPFD). Tomato plants in HTs need to receive sufficient PPFD to maximize fruit yields, and under the shade cloth the PPFD was below the requirements for tomato leaf photosynthesis, decreasing the marketable tomato yield. To avoid this reduction in necessary PPFD, researchers recommend that externally mounted shade cloths are removed once high air temperatures are no longer a limiting factor, or when daylengths are short and irradiation is low. 

The plastic films that cover HTs do age, and with age the level of light transmittance can change, depending on the quality and plastic film composition. In this study, the HT plastic without any shade cloth reduced PPFD by 36%. It is recommended that the plastic film is washed to prevent excessive light reduction in your HT. 

Finally, when selecting your shade cloth, remember that the net shade factor provided by the manufacturer is based on the photosynthetically active radiation (PAR) transmissivity rather than the solar transmissivity. 

There is more to explore in these research papers, and I am looking forward to more findings and recommendations on the effects of a combined use of compost, cover crops and fertilization to improve long-term soil fertility while supporting the immediate nutrient availability needs of the current crop; more on effective and economical push-pull systems; and how shade and moisture can help expand our growing season. 

This blog used the following sources: 

Díaz-Pérez, Juan Carlos, Sudeep Bag, Timothy Coolong, Xuelin Luo, Amanda Hodges, Mamata Bashyal, Hayley Milner, Naga Charan Konakalla, and Adam Pitcher. “Plant Growth, Fruit Yield, and Tomato Leaf Curl Disease of High Tunnel Organic Tomato Affected by Shade Net and Plastic Mulch Color”. HortScience 59.3 (2024): 323-331. < https://doi.org/10.21273/HORTSCI17516-23>. Web. 5 Mar. 2024.

Laur S, da Silva ALBR, Díaz-Pérez JC, Coolong T. Impact of Shade and Fogging on High Tunnel Production and Mineral Content of Organically Grown Lettuce, Basil, and Arugula in Georgia. Agriculture. 2021; 11(7):625. https://doi.org/10.3390/agriculture11070625

Tian, Shufang, Jeffrey K. Brecht, Bala Rathinasabapathi, and Xin Zhao. “Influence of Soil and Nutrient Management Practices on Crop Productivity and Quality in High Tunnel Organic Leafy Green Production”. HortScience 58.12 (2023): 1610-1621. < https://doi.org/10.21273/HORTSCI17327-23>. Web. 5 Mar. 2024.