Vegetable Production under High Tunnels on Crystal Organic Farm

eOrganic authors:

Emily Marriott, University of Illinois

Michelle Wander, University of Illinois


Nicolas Donck, Crystal Organic Farm

This farm description is one of several developed for Ecological Soil Management, a course taught by Michelle Wander at the University of Illinois in the Fall of 2015 with support from Organic Valley. Additional farm descriptions from this course are Borgerding Dairy Farm: Organic Dairy Case Study and Grazing Acres Farm: Organic Dairy Case Study.


Intensive year-round vegetable production under high tunnels is a cornerstone of Crystal Organic Farm. Farming in the Deep South, Nicolas Donck gravitated toward high-tunnel production because of its many benefits including less soil erosion from storm events, ability to achieve lasting soil quality improvements, and greater yields compared to his outside fields. He uses practices such as soil solarization and compost tea to manage pests and diseases and to improve high-tunnel vegetable production.

Overview of Farm

Nicolas Donck has been growing organic vegetables at Crystal Organic Farm in Newborn, Georgia (Newton County) for over 20 years. The farm encompasses a total of 175 acres, with 25 acres in field production and 1.5 acres under high tunnels. The rest of the farm consists of pasture, woodlands, and wetlands. Products include a wide assortment of vegetables, cut flowers, eggs, and some fruits, sold to a variety of direct and wholesale markets.

Climate and Soils

Located in north-central Georgia, about an hour southeast of Atlanta, Crystal Organic Farm is in a humid subtropical climate, classified as USDA plant hardiness zone 8a. The winters are mild, and summers are hot and humid. Annual precipitation, mostly falling as rain, totals about 47 inches spread evenly throughout the year. Cotton production was prevalent in this region during the late 1800's and early 1900's. Significant topsoil erosion occurred during this time, which still affects soil quality in the area. On Crystal Organic Farm, soils are generally classified as well-drained sandy loams and sandy clay loams with deep to very deep rooting depth, low natural fertility, and low organic matter.

Type of System

Year-round high-tunnel production takes center stage in this intensive mixed vegetable operation. Although high tunnels make up a small portion of the overall land in production, over half of farm labor is devoted to high-tunnel production, generating over half of the farm's revenue. With year-round production in the high tunnels, Nicolas is able to support a year-round staff of 15 people. He considers the high tunnels to be high-end real estate and spends more time and energy managing, monitoring, and scouting high-tunnel crops compared to those planted in the open fields.


Crystal Organic Farm sells to diverse markets with short value chains. Their biggest market, accounting for about 60% of sales, is the Morningside Farmers Market in Atlanta—a year-round market selling only certified organic produce. Nicolas was a founding member when the market started in 1995 and serves on their board of directors. Additionally, Nicolas sells directly to a handful of restaurants and to a small local distributor, which helps him limit the amount of time devoted to making deliveries. Crystal Organic Farm sells to Whole Foods Market on a small-scale basis, and also has a small year-round CSA with approximately 35 subscribers.

High-Tunnel Infrastructure

Nicolas says, “I can't praise high tunnels enough because they've really allowed us to grow our business. I'd advise people to build them.” Nineteen of Nicolas's 25 high tunnels are used in multi-bay systems comprised of 26 ft x 100 ft quonset-style tunnels that are connected along the long wall (Fig. 1). Nicolas also uses three long high tunnels (24 ft x 154 ft) that are not attached, but are situated about 2 feet apart. Finally, he has two 16 ft x 100 ft Atlas high tunnels, one of which is used for propagation. Additional structures include two shade houses (20 ft x 120 ft) made from pipe that was bent on-farm. Nicolas prefers wider tunnels over longer ones to increase airflow. Depending on orientation, long, narrow high tunnels tend to retain more humidity due to less air movement, which can lead to disease problems.

Figure 1. Crystal Organic Farm's multi-bay high tunnels consist of 26 ft x 100 ft quonset-shaped tunnels attached along the long walls. Photo credit: Nicolas Donck, Crystal Organic Farm.

End walls and side walls are removed in the summer months to provide ventilation and are replaced in the winter. On warm winter days, end walls can be opened like a curtain and then closed again at night (Fig. 2). According to Nicolas, “If it gets really cold we'll put a row cover inside to protect the more tender crops.”


Figure 2. Crops under high tunnel with end walls open. Photo credit: Nicolas Donck, Crystal Organic Farm.

With so many high tunnels, considerable work is involved in making sure everything is watered properly. Crystal Organic Farm uses good-quality well water for irrigation, and has not had problems with salt buildup in the soil. Nicolas recently installed an automatic drip irrigation system in the high tunnels that he can control from the barn. This system has greatly reduced labor and improved the timing of irrigation.

Nicolas uses the same size beds and space configuration in each of the tunnels, with 5-foot wide beds that run the length of the tunnel. A 4-row pinpoint seeder is used for direct seeding arugula, radishes, and baby turnips. A single-row seeder is used for carrots, beets, cilantro, and beans. All other high-tunnel crops are transplanted. Because high tunnels can be very humid, especially when they are closed up in the winter, Nicolas maintains adequate row spacing to discourage disease.

Roof plastic is replaced on the tunnels every four to six years. After six years, Nicolas notes that the plastic is quite dirty and light penetration is reduced. When deciding to replace roof plastic, Nicolas aims to strike a balance between maximizing the use to reduce costs and waste, and minimizing impacts on productivity from the increased shading.

Key Practices

Drainage Around High Tunnels

Water running off of high tunnels must be managed to prevent erosion, water pollution, and flooding. Drainage can be an especially important management consideration with multi-bay or gutter-connected structures, and is most efficiently addressed at the construction phase prior to the installation of the tunnels. At Crystal Organic Farm, during new high-tunnel construction, trenches are dug along the outside edges and between attached high tunnels to provide drainage and keep the tunnels from flooding during large rain events (Fig. 3). The trenches are lined with landscape fabric. Nicolas notes, “A lot of water sheds off of those greenhouses so you want to plan for that. We have a fairly nice trenching system around the high tunnels to guide the water to a kind of retaining pond we have that's naturally there. We worked pretty extensively in trying to make sure that the water didn't flood the high tunnels.”

Figure 3. Trenches are dug between connected high tunnels and lined with landscape fabric to provide drainage and prevent flooding. Photo credit: Nicolas Donck, Crystal Organic Farm.

Crop Rotation

Nicolas doesn't follow a set crop rotation in the high tunnels, but instead adaptively manages based on how quickly the crops mature and any particular management or disease problems that might occur. He rotates crops by botanical family, with his primary goal being to keep two years between crops in the Solanaceae family (also called nightshades, which includes tomatoes, peppers, potatoes, and eggplants), and a year between other botanical families (Fig. 4). He keeps detailed notes specific to each bed, including crop, disease, and pest scouting information. The high tunnels are kept in production for as much of the year as possible. Once a crop is finished, the crop residues are removed and a new crop is planted right away, often in the same day. This leaves no time for cover crops, so fertility is maintained through the use of composts, commercial fertilizers like alfalfa meal, and compost teas (discussed in detail in the “fertilizer” section below).

Figure 4. An example of two years of crop production in a 5 ft x 100 ft high-tunnel bed at Crystal Organic Farm. Crop plant families are given in parentheses.

Cover Crops

Although cover crops are not used under the high tunnels, Nicolas uses them extensively in the fields. Commonly used summer covers are buckwheat, sudangrass, and cowpea. Sometimes the sudangrass and cowpeas are planted as a mixture. Nicolas likes buckwheat as a fast growing, flowering cover crop that provides nectar for bees and other pollinators, but it can quickly become a weed if it's allowed to drop seed. Winter cover crops are usually a mix of winter rye, spring oats, and Austrian winter peas. Winter cover crops are used on all fields that aren't growing a winter crop. Nicolas is planning to participate in a multi-state collaborative research project on using cover crops under high tunnels with the Universities of Kentucky, Tennessee, and Georgia.


High tunnels are usually installed over existing fields where the soil has already been improved by management. Local compost used to be the primary source of fertility, and the soil under new high tunnels would be amended with compost for the first two to three years.

Very often, fertility sources shift with changes in access to materials and costs. This is true for Nicolas who shared that “when I first started building high tunnels there was this company that made great compost. We used a tractor with a bucket-loader. In a typical high tunnel [we would use] two to three of those bucket loads in there. The company went out of business. Now I can get compost from further away, but [delivery] costs are so expensive that I'll buy maybe half a load and we'll use it around the farm, in pots and stuff, unless a bed is really bad… but it's site-specific, it's not like I'd do the whole [high tunnel].”

Currently, alfalfa meal (3-2-2) and compost tea are the primary soil amendments used under the high tunnels. Alfalfa meal is applied before each crop at a rate of about 2.5 lb/100 ft2 (33 lb/ac N, 22 lb/ac P2O5, and 22 lb/ac K2O) for each crop. Assuming five crops are grown each year, about 160 lb/ac N, 110 lb/ac P2O5, and 110 lb/ac K2O are applied each year under the high tunnels.

Nicolas also produces some compost on-farm, but does not manage it intensively enough to meet organic certification requirements for vegetable use, so it is used on fig trees or perennial crops. Outside fields receive a commercial pelletized poultry litter-based fertilizer (3-2-3).

Compost Tea

Compost tea is brewed on-farm and Nicolas's brewing methods have evolved over time. After starting with a 5-gallon bucket and an aquarium pump, he now uses a 15-gallon vortex brewer (Fig. 5) which provides increased aeration, an important factor in compost tea production. Nicolas uses only OMRI-approved ingredients in his compost tea. In a sturdy fabric sock, Nicolas combines 2-3 pounds of worm castings with 4 ounces of a mineral salt (SEA-90). The sock is dropped into the water with 4 ounces of liquid or powdered kelp (a good source of micronutrients) and 2 ounces of organic black molasses. The brewer runs overnight and the compost tea is used the next day. It should be noted that adding a sugar source when brewing compost tea needs should be done with care as it has the potential to result in the growth of any pathogens that may be present in the compost used. To prevent growth of pathogens, Nicolas relies on an OMRI-listed, non-manure-based, high-quality commercial worm casting product when brewing compost tea. Note: producers making teas need to use NOP approved practices and confirm that their methods are acceptable to their certifier because interpretations of the rules can vary. (For more information on compost tea, see the webinar, Making and Using Compost Teas).

Compost tea is applied through the drip tape irrigation system using a fertilizer injector. About two gallons of compost tea are diluted with three gallons of water. This 5-gallon dilution is applied to a single high tunnel (four 5 ft x 100 ft beds). Usually, each crop receives one application of compost tea, and the timing of application varies. Nicolas says, “I don't have a set schedule. It depends more on the crop. I used to do it once or twice or sometimes more if I felt the crop needed it, but now mostly once per crop. Tomatoes maybe we'd do a second time. It's more about how it all looks. If it looks like the plants need a little love, I'll give them some compost tea. But if they look strong and healthy and growing well I do not.” Outside fields also receive compost tea through drip tape or as a foliar application using a tractor-based pull-behind sprayer.

Figure 5. Compost tea brewing in a Vortex brewer at Crystal Organic Farm. Photo credit: Nicolas Donck, Crystal Organic Farm.

Soil Testing

Soil testing is not a regular practice on Crystal Organic Farm. When crops are growing well and he has few concerns about high-tunnel productivity, Nicolas relies on plant response and past practices rather than soil tests to inform his decisions about inputs. When he was relying more heavily on compost under the high tunnels, soil test results were instrumental in informing his decision to reduce compost application and turn to alfalfa meal. For soils under high tunnels that are kept covered year-round and thus not exposed to leaching rains, monitoring soluble salts on a regular basis by testing electrical conductivity is generally recommended. 

Soil tests taken in the spring of 2015 (Table 1) show distinct differences between the high tunnels and outside fields due to differences in management, particularly water management, and inputs. High-tunnel soils tend to have higher pH and organic matter than open fields. The buildup of Ca, Mg, and Zn in the high tunnels compared to the fields is likely due to the absence of leaching rains under the high tunnels. The high to excessively high P values under the high tunnels may be a legacy of past compost use. All soils tested medium to low for K. Based on these soil test results (Table 1), Nicolas is considering using a lower P fertility source under the high tunnels and adding additional K fertilizer.

Table 1. Spring 2015 soil tests (8” sample depth) for selected high tunnels and outside fields at Crystal Organic Farm.

      Mehlich I Extractant
Sample pH OM P Ca K Mg Mn Zn
    % ------------------------------- ppm ------------------------------
high tunnel 1 6.5 5.7 473 2777 54 194 20 14.6
high tunnel 2 6.4 5 480 2869 95 215 21 14.8
high tunnel 3 6.5 6.2 223 2274 76 190 20 7.6
high tunnel 4 6.6 5.4 260 2750 76 204 21 7.5
high tunnel 5 7.2 3.9 246 2172 47 160 19 6.4
high tunnel 6 6.8 3.5 218 1586 126 141 22 6.6
field 1 5.2 4.1 12 258 94 36 68 0.9
field 2 5 2.9 11 184 67 30 36 0.7
field 3 5.3 4.1 16 367 118 41 79 1.2


Soil Solarization

The hot summer months are used to solarize the soil in the high tunnels for four to six weeks (Fig. 6). Tunnels are solarized every two years—generally in the year before Solanaceous crops are grown. This is a key component of their weed control strategy. Nicolas also thinks solarization helps control disease, as problems that he has in field-grown crops seem to slowly go away under the high tunnels. 

Solarization is a process in which soil is covered with plastic to create high soil temperatures for four to six weeks. Benefits of soil solarization can include improved control of annual and perennial weeds, nematodes, and some soilborne diseases such as Southern blight (Chellemi et al., 1997; Ristaino et al., 1991). For more information, see Soil Solarization for Gardens and Landscapes and the University of California's website on soil solarization.

Figure 6. The ground under high tunnels is covered with clear plastic sheeting (right and left high tunnels) when undergoing soil solarization in the hot summer months. Photo credit: Nicolas Donck, Crystal Organic Farm.

Nicolas's soil solarization process starts with thoroughly cleaning out the beds and removing the weeds and crop biomass. Fertilizers such as alfalfa meal or compost are tilled in, then drip irrigation is put down. He makes sure that the irrigation is working and then puts the plastic down. Nicolas purchases a fairly thin (0.5 mm) plastic from a local hardware store. This is thinner than the 1 mm plastic recommended for solarization, but Nicolas prefers to minimize cost and waste, as this plastic is only used once. A strip of plastic covers two beds. The plastic is placed on the walkways as well, so it covers all of the soil in the high tunnel, not just the beds. Soil is then watered thoroughly. Water is essential to allow the system to heat and become anaerobic, although too much water has resulted in caky soil and algal growth under the plastic. He leaves the plastic on for four to six weeks, depending on the weather. The goal is to achieve 110-125°F daily maximum soil temperature in the top six inches. 

Weed Control Tactics and Tillage

Hand weeding is used in the high tunnels, and weeds are prevented from flowering as much as possible. Keeping low weed pressure in the high tunnels also helps to control disease. Even with landscape fabric lining, the drainage channels need to be weeded regularly to maintain performance and keep perennial weeds like Bermudagrass in check. In the high tunnels, a rototiller and sometimes a chisel plow is used to cultivate planting beds.

Pest and Disease Control

Nicolas does not use a regular spraying schedule in the high tunnels. He relies on frequent scouting for pests. Organic pesticides are used as a last resort after beneficial insects have been given a chance to control problems. Nicolas uses a variety of OMRI-approved products. Solarization, adequate row spacing, and low weed populations are all part of the disease-control strategy under the high tunnels.

Case Study Takeaway Points

  • Nicolas reports that production is better and easier under the high tunnels due to the ability to manage water, solarize the soil, prevent erosion, and better control diseases and weeds.
  • High tunnel production requires a significant investment of labor but also generates the majority of the revenue on Crystal Organic Farm. 
  • Nicolas takes advantage of the summer months when the high tunnels are too hot for many crops to solarize the soil, greatly reducing weed pressure under the tunnels.
  • Quick turnaround of crops, ventilation management, creative approaches to crop fertility, reducing fertilizer inputs over time, and careful observation are key strategies to successful high-tunnel production on this Georgia vegetable farm.

References and Citations

Additional Resources

Published February 21, 2017

This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.