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Farming Under Cover: High Tunnels, Low Tunnels and Greenhouses


IN DECEMBER, JUST BEFORE THE GROUND froze, we started building our second high tunnel. It will be an imposing structure: 96 feet long, 30 feet wide and over 13 feet high—that’s nearly 2,900 square feet of covered space. Tomatoes are destined to be grown inside this tunnel, along with lesser quantities of peppers, basil, salad greens and thyme. A high tunnel looks a lot like a greenhouse, though it serves a quite different purpose. But I’ll get to that later. Let’s start with greenhouses.

Most people know that a greenhouse is a protected environment for growing plants with lots of sunlight. When I was a boy, greenhouses were made of many panes of glass, held together in a grid-work frame. They had straight sides and a pitched roof to shed water and snow, and they were often called glasshouses. These structures were expensive to build but, if well-maintained, had a long lifespan. No doubt, some still exist today.

In our era of ubiquitous plastics, the typical contemporary greenhouse has a different look. It is shaped like a Quonset hut and constructed of steel hoops or bows, with a polyethylene covering. It may have endwalls made of plywood or some other solid material, with a door for access. It will invariably have a propane heater (or possibly an oil- or wood-fired one) that turns on when a thermostat detects the inside temperature has dropped too low for the well-being of the resident plants. And it will have a fan to exhaust hot air when temperatures climb too high. Today’s greenhouses don’t cost as much to build as the old glasshouses, but the owner of a modern Quonset-style house will have to replace the polyethylene covering every few years.

Most greenhouse growing is done in trays, flats or pots that sit on benches two or three feet above the ground. The greenhouse manager makes sure the plants have the right amount of moisture and the optimum soil and air temperatures in which to thrive. This is your basic greenhouse—any commercial vegetable grower who starts plants from seed, understands it is a very handy thing to have.

In my first two years of farming, I managed to get by without a greenhouse, mainly because I didn’t have one. Everything I planted had to be seeded directly in the field, except when I was able to purchase transplants from someone who did have a greenhouse. When you direct-seed into a tilled field you enter into serious competition with countless numbers of weed seeds, many of which will germinate and grow more rapidly than the seeds you have just planted. Space, sunlight and water are in high demand in a patch of soil, and weeds tend to be more adept at capturing all three of them. In those first two years, I spent many an hour battling the invading hordes or, to put it more prosaically, weeding. Without my ongoing efforts, the tiny rows of vegetables that were making their first appearance in the world wouldn’t have stood much of a chance.

It quickly became evident that a greenhouse would give me a distinct advantage over the weeds and free up a lot of time for other work. I hastened to build one. Now, except when dealing with vegetables that do not tolerate transplanting (like peas, beans and carrots), we always set out young seedlings in our fields. The little transplants usually have at least four weeks of greenhouse growing under their belts, which puts them a few steps ahead of the un-germinated weeds. True, the weeds will still come and claim their space, and they will have to be dealt with. By the time they arrive, though, the crop plants will at least have a foothold.

The heated and protected environment of the greenhouse also enables us to start plants a lot earlier than we could in the open field. Some crops, such as onions, shallots and certain herbs, are seeded in flats in our greenhouse as early as mid-February, and many others are seeded in March. Planting outside in the often frozen and snow-covered ground at this time of year would be impractical, if not impossible.

From the outside, a high tunnel closely resembles a greenhouse—it is constructed of the same galvanized steel hoops and polyethylene covering. It will probably have end-walls and a door for access. But it is not likely to have a fan or heater, and the plants inside will not be growing in trays—rather, they will be growing in the ground and will remain there until harvested. For ventilation, a high tunnel will have sides that roll up about 4 feet along the entire length of the structure, allowing outside air to flow through when temperatures on the inside get too high.

The great thing about high tunnels is that they don’t use any energy other than the energy required to build them. The sun shining through the plastic warms the soil and air inside the tunnel. When it gets too hot, the sides are rolled up to an appropriate height, allowing cool air to enter. At the end of the day, unless it’s going to be a very warm night, the sides of the tunnel will be rolled down so that some of the heat captured during the day stays inside. On cool, overcast mornings, the sides may be left down for an hour or two. But, when the sun comes out and the temperature begins to climb, they will be rolled up. All this raising and lowering of sides is very straightforward and easy to do.

The two most important advantages a high tunnel offers the vegetable grower are season extension and a protected environment. Crops, especially cold-sensitive ones like basil and tomatoes, can be planted earlier in a high tunnel and will mature before crops planted in the open field. This means they will provide marketable produce sooner than could otherwise be expected—often by as much as two or three weeks. The same thing happens at the other end of the season—our last basil and tomatoes in the fall are usually harvested from the tunnel, after the same crops in the field have been destroyed by frost.

A couple of weeks here and a few weeks there may not seem like much, but, for a northern vegetable grower whose season is already limited by cold weather at either end, an extra month of harvest can bring significant income. The fact that early crops usually sell for a premium at market just sweetens the pot. With a little luck and good management, a high tunnel can pay for itself in two or three years. This is why these relatively inexpensive structures are increasingly common sights on small, diversified farms. I estimate that our new tunnel, which is a very large one, will cost around $10,000. This price tag includes materials, some outside labor and a bulldozer to level the site.

Our first high tunnel was built eight years ago, right next to our greenhouse. At 21 feet wide by 96 feet long, it is about two-thirds the size of new tunnel currently under construction. It has served us well, providing many early harvests of lettuces, basil, peppers and tomatoes, as well as late-season salad greens. It also supports a veritable forest of rosemary, which I am especially proud of.

Rosemary is a perennial herb, but a tender one. In California and other places where winters are mild, rosemary can live for many years—I’ve even seen it used as a ground cover. But in our northern climate, the winter months are almost always too severe for this wonderfully aromatic, Mediterranean herb. Invariably, it dies, probably because its roots cannot endure the damp, frozen soil. Inside the tunnel, conditions are more hospitable: The ground does not freeze as deeply and stays a lot drier (all irrigation is halted in the winter months). To provide our rosemary with a little additional protection from the cold, we drape a blanket of row cover over it. This spunbonded polypropylene covering traps some of the warmth captured in the tunnel on sunny days. Even so, on a cold, clear night, I doubt the temperature the rosemary experiences is more than 4 or 5 degrees warmer than on the outside. But this seems to be enough to keep it in business. Thyme is another perennial herb that is in high demand but often cannot survive our winters. I’m planning to put at least a couple of hundred thyme plants in the new tunnel.

Protection from severe weather any time of year is another major reason for owning a tunnel. Excessive amounts of rain are seldom welcome on a farm. Flooding, poor root growth, leaching of nutrients from the topsoil, splitting of fruit like tomatoes, and proliferation of fungal and bacterial diseases are all worries for a vegetable grower when hard rain starts to fall. June 2009 was a cold, wet month in southern New York. We got over 12 inches of rain. The worst result of this extended spell of inclement weather was the spread of the fungal disease, Late Blight. Over the course of a few weeks, our field grown tomatoes (more than 2300 of them) succumbed to the disease and were lost.

Because Late Blight is airborne and travels on the wind, it inevitably found its way into our tunnel, where we had an additional 50 or so tomato plants growing. They, too, began to show signs of infection and I was resigned to losing them. But because the foliage of the tunnel tomatoes remained dry (except for some early morning condensation), the disease was not able to advance as effectively as it did in the field. Wet foliage is a major factor in the spread of many plant diseases. Warmer weather in July also slowed the spread of the disease. As it turned out, we were able to harvest between 50 and 100 pounds of tomatoes from the tunnel each week for a couple of months—not much compared with what we could sell at our markets (and certainly less than the tunnel’s full potential), but still a lot better than nothing.

Strong wind also is stressful for plants. It robs them of moisture and slows their growth. A high tunnel, even with the sides partially rolled up, offers significant protection from wind. And that sudden hailstorm that can devastate to many crops in the open field? Inside a tunnel—no problem! High tunnels are also a deterrent to most herbivorous competitors. While I once came across a nest of baby rabbits in our tunnel, I’ve yet to encounter a deer or woodchuck brazen enough to trespass inside.

All this protection—from severe weather and marauding critters—often results in a better looking and more marketable crop. Yet another reason to invest in a high tunnel.

Since we don’t use chemical pesticides, low tunnels are our best defense against this scourge of the cucurbits.

Low tunnels are similar to high tunnels but much smaller—only about 3 feet high and 4 feet wide. They are a lot simpler and less expensive to erect. Usually, wire hoops are pushed into the ground at about 6 foot intervals and then covered with row cover or plastic, which is weighted down with rocks, soil, or some other heavy material. There are no end-walls, doors, or roll-up sides. Just an enclosed tunnel. To gain access to the plants inside, you have to remove the soil or weights along one side and lift off the covering.

A farmer might plant a frost-sensitive crop like cucumbers or string beans in April or May and immediately protect it with a low tunnel. A month later, when the days and nights are warmer, the entire structure, which will very likely be used again to protect a different crop in a different place. Low tunnels can also be used to protect cold-hardy crops like kale, lettuces and salad greens in the fall. and, I’ve been told, some farmers cover spinach through the entire winter, to have it ready for spring harvest.

On our farm, low tunnels are mostly used to cover early plantings of summer squash, zucchini and cucumber. The tunnels serve two purposes: they keep the plants warmer, which makes them grow faster, and they keep insects out. This second function is vitally important. Cucumber beetles, which are attracted to all members of the cucurbit family (squash, melons, cucumbers, zucchini), dine voraciously on the young leaves and flowers. While feeding, these insects often transmit a deadly disease, called bacterial wilt, which can kill the plants within weeks. Since we don’t use chemical pesticides, low tunnels are our best defense against this scourge of the cucurbits and a few other diseases as well. However, as the plants gain size and set flowers, it is important that the row cover be removed to allow pollination to take place.

Low tunnels, high tunnels and greenhouses are important and relatively low-tech structures on our farm. Each, in its own way, provides benefits that help keep our business profitable. Without them, we would have a shorter season and a less reliable supply of food to bring to market.

Snow and freezing conditions in December brought the construction of our new high tunnel to a halt. Right now all you can see is the skeletal structure of the hoops and their many supporting corner braces, cross-ties and purlins (the long poles that run the length of the tunnel and hold the hoops in place). By late March or early April the ground should be thawed and the days warm enough to finish the job. We’ll build the end-walls, staple on a coating of polyethylene film and set the first tomato seedlings in the ground. Another season will be underway.

Hudson Valley Restaurant Week is back this April 8-21!