This article was reprinted with permission from Penn State Extension Vegetable and Small Fruit Gazette.
Every good reason to grow tomatoes and other high return vegetables and small fruits in high tunnels has a compelling argument to counter it. These potential pitfalls of tunnel culture are seldom mentioned in the rush to put a high tunnel on every farm.
While there are many excellent reasons to grow tomatoes and other high dollar return vegetables and small fruit in high tunnels, there are some very compelling arguments against them too. These are seldom mentioned in the rush to put a tunnel on every farm. These are some of the more important negative aspects of tunnel culture.
Return on Investment
Simply put, input costs are substantially higher in a tunnel versus the field. After amortizing the costs of the structure, the plastic film and other specialized equipment that are required to effectively produce in a tunnel, the field planting has much lower input costs. Yes, increased yield, quality or market window can justify the increased costs of tunnels. However, we have observed in some situations poorly managed tunnels yield equal or less than field plantings. Consider carefully what every square foot of production space is worth and manage with the goal of justifying the increased input costs.
Greenhouses and High Tunnels are typically engineered carefully to balance the environment versus the need to keep material costs low. Metal tube structures do fail, as do crops. Between your investment in the tunnel itself, the costs incurred in producing a crop and your anticipated return on investment, growing in protected culture requires a greater attention to details and pest management in order to realize economic goals.
Increased Pressure from Insects and Arthropods
While tunnel culture brings with it the opportunity for higher crop quality, Aphids, Whiteflies, Western Flower Thrips, Spider mites and Broad mites all thrive under tunnel conditions. The dry foliage, stems and fruit grown under intense irrigation and fertigation are ideal environments for these pests to flourish. Scout regularly for pests and consider a proactive pest management that includes banker plants such as Black Pearl and Purple Flash peppers hosting Minute pirate bugs (Orius). Every crop reacts differently under tunnel conditions versus field grown. With no rainfall, Spider mites can increase populations incredibly rapidly unless carefully managed.
Irrigation management requires greater care vs. field grown
This is especially so on the margins of the season when there are often days with little sunshine. Learning to grow dry(er) during the early and late season will reduce root-borne diseases. Growers must adapt to rapidly changing conditions and know when to increase irrigation flow in order to maximize plant growth, reduce cracks, and Blossom end rot. Consider investing in tensiometers or irrometers in order to monitor root zone moisture levels. Tunnels will require more irrigation than field plantings. Farms with less than adequate water supply should look at tunnels with caution.
Increased Disease Pressure
While tunnels do reduce diseases such as Early Blight and Septoria Leaf Spot, other diseases are accentuated. Brown leaf mold, Powdery mildew, and Botrytis are occasionally seen in field-grown vegetables, but are standard fare under high tunnel conditions. Increasing air flow, reducing humidity, and using disease resistant varieties will all help to manage these diseases. Brown Leaf Mold in tomatoes is almost exclusive to tunnel-grown tomatoes. We are finally seeing the release of the first tomato varieties with strong resistance to leaf molds. Check with your Extension Specialist/Horticulture Educator for the latest on recommended tunnel varieties.
Tunnels Perpetuate Viruses
Tunnel tomatoes are a very ‘hands-on’ crop as many growers have come to appreciate the benefits of greenhouse methods of pruning to improve yields. However, viruses such as Tobacco Mosaic (TMV) are spread mechanically. Workers move the virus down the row with weekly suckering or pruning. Field tomatoes see much less handling and thus are generally at lower risk. High technology greenhouses on the other hand have disinfection protocols in place to reduce the spread. The soil-based system is also more difficult to disinfect than a concrete floor. The in-between greenhouse field nature of a high tunnel perpetuates TMV. So what can we do to prevent TMV in tunnels? Buy only from reputable seed sources and consider seed treatment. The use of disposable gloves, regular hand washing and tool disinfection will reduce the spread of viruses and other systemic diseases. There are now commercial pruning tools with reservoirs to disinfect the blade continually during usage. Remove suspect plants immediately.
Soil Health and Nutrition
Tomatoes are the single most popular high tunnel crop due to their high return on investment and high market demand. There is considerable pressure not to rotate tunnel crops as you would fields. This can result in steadily increasing soil-borne diseases such as Fusariums and Verticilliums. Using only the best quality plants from known and trusted sources along with inoculating plant roots with Actinovate AG, RootShield Plus, Companion, Cease….(there are an increasing number of biological root inoculants available). These practices reduce the potential need to fumigate.
In addition, tomatoes are heavy feeders making strict attention to soil tests and tissue analysis especially important in the usually higher (than field) densities used in high tunnels. Maintaining recommended levels of Ca, Mg and K are often challenging in tunnels. Since it never rains in a tunnel, all nutrients that the roots utilize are within the drip irrigation zone. This root area can quickly be exhausted of nutrients. Both injected and foliarly-applied nutrients are necessary to maintain nutrient levels are their optimum levels during ideal growing periods.
Negative R Values
Under early and late season short and cloudy days and clear, cold nights, it is possible to have the low temperature in a high tunnel be lower longer than outdoors. Cold nights, particularly in spring, can see temperatures lower inside the tunnel than out (hard to believe but true). Using a set of heavy floating row covers under these conditions can help to keep a crop alive. Many growers will opt to use a low output heater during these conditions to keep temperatures above 45-50 F. In general, an unheated high tunnel can be reliably planted with tomatoes about four weeks prior to the normal outdoor planting season. Adding heat can speed up successful planting by eight weeks or more.
Increased Management and Labor
It is more challenging to manage tomatoes, peppers, cucumbers, strawberries and raspberries indoors. Pest populations and infestations tend to come on quickly requiring a very proactive management program. The narrow aisles require careful attention to crop canopy management, so pruning and trellising are constant chores. Is there room within your schedule as manager to accommodate the increased demands of a tunnel? We have seen many cases where the answer is No.
Playing Field Irregularities Due to Subsidized Tunnel Purchases
Recent grant programs have created two levels of tunnel purchases, those that are subsidized and those were not. Growers that purchase their tunnels without the grant subsidy may have paid 40-60 percent more for their first high tunnel. If you are not a grant recipient, your input costs will be higher, and your margin lower, than others.
In conclusion, while we remain optimistic about the role of high tunnels on vegetable farms, it is important to note that they are not ideal for all farms. Here we have tried to present some of the less glamorous aspects of tunnel production to balance the many favorable programs we’ve conducted for many years. These challenges must be met by those currently growing in tunnels and carefully weighed by those considering getting started.