Tunnel farming can double the vegetable harvest and the price

Walk into a wholesale vegetable market on a July morning and the stalls look thin, with what little there is selling at a steep price. Walk into the same market in January and cauliflower, cabbage, tomatoes and beans are piled so high that many growers cannot recover what it cost them to pick the crop. Same soil, same farmers, same vegetables; the only difference is timing. One old but effective way of working that gap is tunnel farming, known in Bangladesh as the canopy or chhauni method.

Bangladesh's soil suits a remarkably wide range of vegetables, and the list of crops that can be grown in winter and summer is long. Yet farmland shrinks year after year as houses, roads, factories and markets spread over old fields. In many village households the vegetable plot is a sliver of ground behind the kitchen or the cowshed. If a family's nutrition matters, that is not enough. With less land to work with, the only realistic route to meeting the country's need for vegetables is to grow them intensively, all year round.

Most of Bangladesh's vegetables are grown in the rabi season, from October to March, when cool, dry and sunny weather suits most crops. In the kharif season, April to September, conditions are much less favourable and far fewer vegetables are grown, so prices climb. From May to July in particular, heavy rain in some places and drought in others cut production and push market prices higher still.

The picture reverses at the height of the season. Supply is so heavy that farmers cannot get a fair price. When one district is lifting its cauliflower, so are ten others around it, and with more sellers than buyers the price has nowhere to go but down. The arithmetic that follows is plain: a farmer who can produce vegetables before the season starts, and get them to market early, can earn more than double the usual price. The same holds for anyone who can keep producing after the season ends.

That is why farmers in the north and elsewhere have been turning to early winter vegetables. Almost every year the papers report growers in Rangpur, Dinajpur or Rajshahi getting good prices for early crops, with the Department of Agricultural Extension (DAE) training and advising them in the field. The great risk of growing early in the open is late-monsoon rain. One long downpour can rot seedlings and strip flowers. That is where the tunnel comes in.

Tunnel technology is a way of growing vegetables all year, or of raising winter vegetables early. The name comes from the look of the structure. Bend bamboo or steel hoops over a bed, stretch polythene over them, and the result is a long tunnel shaped much like the curved roof of a country boat. Under that roof the crop gets some shelter from rain, fog, cold wind and harsh sun.

For growing a crop outside its natural season, few methods match it. There is one limit worth knowing from the start, though. A tunnel makes it hard to grow winter vegetables in the heat of summer, because producing the cool conditions those crops need inside an artificial space is expensive. Cooling takes machinery, electricity and money that an ordinary farmer does not have.

The opposite job costs far less. Growing summer vegetables through the winter under an artificial environment is comparatively straightforward. Sunlight passes through the clear plastic, the soil and plants absorb it and warm up, and much of that heat cannot escape. Even on a cold day the air inside stays warmer than outside, much as a car with its windows shut stays warm on a winter night. That extra warmth is enough to carry summer vegetables through the cold months.

In the rainy season the same cover works like an umbrella. Heavy drops do not hit the leaves and soil directly, the soil is not washed away and tender seedlings are not battered. Less rain splash also means fewer disease organisms thrown up from the soil onto the leaves. Nursery beds and early winter crops can therefore grow far more safely under a tunnel in the last weeks of the monsoon.

Among the vegetables that do well under a tunnel with little trouble are the cucurbits, tomato, spinach, cabbage, cauliflower and beans. Cucurbits are the gourd family: cucumber, bottle gourd, pumpkin, bitter gourd, ridge gourd and their relatives, most of which love warmth. In the winter cold they struggle in open fields, but they can keep growing in a warm tunnel.

Tomato, cauliflower, cabbage and beans are mainly winter crops. In a tunnel their seedlings can be raised earlier, safe from late-monsoon rain, so the harvest reaches the market well ahead of the season. Fast-growing leaf crops such as spinach can give one quick harvest after another.

With this method a farmer can sell early vegetables at a higher price than in the true season. Farmers in many parts of the country already grow vegetables this way, but far fewer than there could be. Most either do not know the method or hold back because of the start-up cost and the work of building the frame.

By size, tunnels come in two kinds: the low tunnel and the high tunnel. The low tunnel is for small plots and farmers with little capital; the high tunnel is for larger commercial growing. The idea behind both is the same. What differs is size, material and cost.

The main material for a low tunnel is bamboo, which is close to hand in most villages and relatively cheap, making this the natural starting point for a small farmer. The tunnel should be 98.4 feet long, which is 30 metres, and 14.76 feet wide, which is 4.5 metres. Inside, it stands 7 feet high along the centre and 5 feet at the sides. The centre is kept higher than the edges so rain runs off both sides instead of pooling on the roof.

For irrigation and drainage, a channel 6 inches deep and 2 feet wide runs between two beds or two tunnels. In dry weather it carries irrigation water; in the rains it drains the surplus away. Vegetable roots cannot stand waterlogging, so a tunnel without a drain can lose all its advantages.

Once the frame is up, it is covered with clear, black or blue polythene, drawn over like the canopy of a boat. The edges must be buried firmly in the soil or tied down so the wind cannot lift the sheet. On bright, hot days, or when the air inside turns warm and muggy, lifting the side sheets a little during the day lets air move through. Too much humidity encourages fungal disease, and too much heat can make flowers drop.

The main material for a high tunnel is a steel frame, though bamboo can also be used. It is far bigger than the low tunnel: 130 feet long and 32 feet wide, 12 feet high at the centre and 10 feet at the sides. At that height people can stand and walk inside to work, and climbing crops such as tomato, cucumber and bitter gourd can be trained upwards on stakes or trellises.

For irrigation and drainage, a channel 6 inches deep and 3 feet wide runs between two beds or tunnels. Plants or seeds are spaced 1.5 feet apart. The frame is then covered with clear, black or blue polythene in the same boat-canopy shape.

A steel frame costs more at the start but lasts for many years. Bamboo is cheaper but has to be replaced after a few seasons of sun and rain. Polythene also turns brittle in sunlight over time, so taking it down and storing it at the end of the season, or buying a sun-resistant grade, makes it last longer. Which frame to choose depends on the land, the capital available, and which crops the farmer plans to grow for how long.

The land chosen for tunnel growing must be fertile, with a soil pH between 5 and 7. pH measures how acidic or alkaline soil is: 7 is neutral, anything lower is acidic. Most vegetables do best in slightly acidic to neutral soil, where their roots can take up nutrients properly. A soil test through the upazila agriculture office or the Soil Resource Development Institute takes the guesswork out of it.

After the tunnel is built, the soil is dug thoroughly with a spade or by other means until it is loose and crumbly. Each tunnel then receives plenty of organic manure along with the following:

• Oil cake: 2.5 kg
• Urea: 800 g
• TSP (triple superphosphate): 500 g
• MoP (muriate of potash): 700 g

Each has its own job. Oil cake is what remains after mustard or other oilseeds are pressed; it breaks down slowly and adds organic matter and nutrients to the soil. Urea supplies nitrogen for leaves and stems. TSP supplies phosphorus, which builds strong roots and helps flowering and fruit set. MoP supplies potassium, which helps the plant resist disease and drought and improves the quality of the produce.

Half the urea and potash goes in when the land is prepared; the other half is top-dressed two weeks after the seedlings emerge. Top-dressing means spreading fertiliser on the soil around plants that are already growing, and working it in lightly. Putting it all in at once wastes part of it in the water, and too much fertiliser can harm young seedlings. Split into two doses, the feed arrives when the plant needs it most.

Once the fertiliser is worked in, beds are formed before seeds or seedlings go in. A bed is a long, slightly raised strip of soil beside the drainage channel, which keeps water from collecting around the plants.

Spinach shows how quickly a tunnel can pay. Sow 200 grams of spinach seed per tunnel at the beginning of the Bengali month of Jyoishtha and the crop can be cut a month later, in Asharh. Jyoishtha runs roughly from mid-May to mid-June, and Asharh from mid-June to mid-July. In other words, the spinach reaches market just when heavy rain is ruining leafy crops in the open and supply is at its lowest.

Spinach is normally a winter leaf. Growing it in the open at the start of the monsoon is risky, because steady rain rots the tender leaves. The tunnel roof keeps the rain off and the drains on either side carry away the excess. After one harvest in a month, the same tunnel can be sown again, or used to raise seedlings of another crop for the coming season.

A tunnel pays best when it is planned for the whole year rather than a single season. Late in the monsoon, in the Bengali months of Bhadra and Ashwin, when farmers hesitate to sow nursery beds in the open for fear of rain, tomato, cauliflower and cabbage seedlings can be raised under cover. Planted out, they come to harvest weeks ahead of the neighbours'.

In mid-winter, in Poush and Magh, the same tunnel can take warmth-loving crops such as cucumber, bottle gourd or bitter gourd, which all but stop growing in the cold outside. By Falgun and Chaitra, when summer vegetables are only going into the open fields, the tunnel grower is already picking. Then come the leafy crops of the Jyoishtha and Asharh rains, such as the spinach described above.

Seen this way, one tunnel serves three or four different purposes a year, each time sending produce to market when supply is short. The right crop for each month varies by area, so it makes sense to set the calendar with the local agriculture office and one's own knowledge of the market.

The pull of tunnel farming is price. The same land and the same labour produce a crop that reaches the market when buyers are many and sellers few. The same quantity of vegetables can earn a farmer double the peak-season price or more. Protection from rain, fog and hail also reduces the risk of losing the crop, and because several crops can be taken from a small plot in a year, the land works harder.

Many countries, India above all, grow off-season vegetables under tunnels, meet their own needs, and export large quantities to the Middle East, Europe and other parts of Asia, earning foreign currency. If Bangladeshi farmers are made aware of the technique and the government lends a hand at different levels, Bangladesh too could meet its own demand and earn foreign exchange by exporting vegetables.

That help would mean a few concrete things. First, hands-on training, so farmers can build frames to the right size and manage the heat and humidity inside. Second, start-up money, because polythene, bamboo or steel all have to be paid for in the first year. Third, links to buyers, so that early vegetables reach city markets or exporters on time. The DAE's upazila agriculture offices and its sub-assistant agriculture officers are the advisers closest to any farmer, and a conversation with them before building a first tunnel is time well spent.

A few points are worth thinking through beforehand. Starting small in the first year, with one or two low tunnels, keeps the cost of learning down. Finding out which vegetable sells highest in the local market in which month helps set the sowing date. The plot should be on high ground that does not flood in the rains. The temperature and humidity inside the tunnel need a daily check, with the side sheets raised or lowered to control airflow.

Pests and disease need watching too. The warm, humid air under cover suits certain fungi and insects as well as it suits the crop. Removing infected leaves quickly, leaving enough space between plants and, where needed, using only approved pesticides on the advice of the agriculture office are the safest course.

Tunnel farming is no magic. It is a way of buying a little time from nature. For the price of bamboo, polythene and some extra care, a farmer can take vegetables to market when buyers are waiting and prices are high. As farmland shrinks and demand for food grows, a method that draws more crops and more income from the same patch of ground could open a real door for Bangladesh's agriculture, and leave the farmers who use it better off.

Photo: Vegetables growing under a polytunnel, Peter Barr / Wikimedia Commons (CC BY-SA 2.0)