After three years, on average, of growing tomatoes and other vegetables in high tunnels, salt levels accumulate and need to be reduced to promote better plant growth, according to Bruce Hoskins, a soil scientist with the University of Maine. Sheepscot Farm and General Store, in Whitefield, Maine, currently uses two high tunnels on the farm.

Using high tunnels can help farms extend their growing season, but proper management can improve the effectiveness of high tunnels as a farm tool.

The group, MidAtlantic Women in Agriculture, recently hosted Neith Little, a University of Maryland Extension agent, as she presented a webinar on the topic of “High Tunnel Best Management Practices.”

Little clarified a few differences between the high tunnel and the greenhouse.

“There (are important differences) in funding and zoning,” she said.

In general, a high tunnel is a temporary structure. In some municipalities, that means it won’t require a building permit; in others, the farmer must obtain a permit.

“A greenhouse is considered a permanent structure. so you will need a building permit,” Little said. “High tunnels are built with flexible, temporary materials like flexible rods and plastic. They can last for quite a while, but can easily be taken down.”

“If it has rigid plastic or glass walls, it’s a greenhouse,” Little said.

High tunnels use in-ground planting, whereas greenhouses use tables for planting.

One layer of plastic typically covers high tunnels, but it is not uncommon to see two layers with air pumped between to help improve temperature control. High tunnels are typically passively heated with vents and shades. High tunnels are usually not powered.

Managing the Environment Inside a High Tunnel

Temperature management is an important aspect of high tunnel use, as farmers need to prevent excessively high or low temperatures, depending up on the season and their crops.

“Monitor the temperature,” Little said. “You can do it low tech with thermometer and clipboard, or with a probe connected by Bluetooth to your phone.”

She said her temperature devices cost about $125 apiece and help her troubleshoot issues.

Small hoop covers used outside or in a high tunnel can help keep plants warm overnight but should not be kept on plants all day.

“If you don’t have the budget for a high tunnel, these little low tunnels can keep plants warmer and keep out pests,” she said.

When using an active heat source, farmers should make sure the device is well-ventilated and kept away from anything flammable.

Shade cloths can prevent tunnels from becoming too hot, which can be a problem for plants like lettuce, especially.

“Even plants like tomatoes, once it gets to 120 (degrees), it will definitely decrease your tomatoes’ vigor and yield, and cause serious damage to your plants,” Little said.

In addition to shade cloths, painting the inside of the high tunnel white can offer low maintenance shade for cool-season crops like lettuce.

“Taking the plastic on and off can cause a lot of damage, so I don’t recommend that,” Little said.

Roll-up sides, roof vents and end wall vents also help; however, Little said that vents can be prone to leaks.

Pest management is not as intense in high tunnels as in open fields; however, it can still be an issue in high tunnels. Screening vents and using shade cloth can help keep pests out.

“Once you are controlling irrigation, you start to see pests that like dry conditions, like thrips and mites,” Little said.

Soil salinity should also concern farmers using high tunnels. Little said that salt occurs naturally in the soil and rainfall typically leaches salt deeper into the soil. But the irrigation in high tunnels does not have the same effect.

“In high tunnels, when you irrigate, the soil gets wet down to 6 to 8 inches and then it gets hot and dry, and the moisture gets drawn back up to the roots,” she said. “It can draw the salt back up to form a crust on top.”

Salinity can also come from fertilizers, compost, purchased soil or growing medium.

“Compost is often formulated to be added in small amounts to raised beds or fields,” Little said. “They can be high in nitrogen and salts, which isn’t a bad thing necessarily, but you have to make sure they’re not too high (because) you don’t have that leaching action. Get an analysis from a lab so you know what proportion to mix it with other material. If (the levels are) too high, (salt) can reduce water availability.”

Salt-stressed plants can exhibit stunted root growth, purple leaves and necrotic leaf margins.

Little also said that the soil pH is important to monitor twice a year, since it can change over time. Between 7 and 8 is ideal.

“At high pH levels, you see nutrient deficiencies,” Little said. “If it’s too low, you can add lime like you would outdoors.”

Little also advised producers to be careful when working in a high tunnel, because high temperatures, poor air quality and pesticides can pose health and safety risks to workers. Those especially in danger include the young and the elderly, and those with other health issues, particularly ones involving breathing. And, while protective equipment is important, it can increase the risk of heat-related illnesses. That is why using equipment and chemicals according to their label and providing adequate ventilation is so important.

Deborah Jeanne Sergeant is a freelance writer in central New York.