Closed glasshouse formats have been the subject of much research and development in recent years. But while the advantages of lower energy input, higher CO2 levels and improved pest and disease control make them appealing, growers have been reluctant to take the plunge into this still-emerging technology in the current economic climate.

But one Finnish firm believes that it has developed a relatively low-cost way of regulating both temperature and heat within a closed glasshouse using nothing more than water droplets.

In the Novarbo system, developed by Finnish horticulture supplies firm Biolan, a "droplet curtain" emitted by a row of nozzles along the glasshouse roof conducts both moisture and heat away from the glasshouse. The water is then collected and channelled to an exterior reservoir, where it is filtered and allowed to cool - aided by a propeller system that sprays the water in the air like a fountain.

The temperature of the water both entering and exiting the system is monitored, while sensors in the outdoor pool also trigger the cooling "fountains". Climate control software provided by Finnish developer ITU ties the whole thing together.

Biolan researcher Jukka Huttunen explains: "The aim with Novarbo is to have more vegetables per square metre, for as little energy and CO2 as possible."

The system is being trialled at three sites in Finland, but only one of these is in commercial production - at salad grower Harkalan Puutarha, a family firm based near Biolan's headquarters in south-west Finland, which has been supplying the local market for 30 years.

Biolan sourcing director Krista Joensuu says: "It is known for producing the best tomatoes and salads in Finland, even though they are twice the price of the cheapest on the market."

It is surprising enough seeing salad crops being grown further north than the Shetland Islands, but even more remarkable is the fact that 70 per cent of tomatoes sold in Finland are home-grown - and produced year-round.

Harkalan's 10-year-old, 0.5ha glasshouse, used for conventional growing until two years ago, is split into three separate sections for cucumber, tomato and lettuce growing. Modifications to incorporate the Novarbo system were scheduled so as not to disrupt production.

As a precaution, the glasshouse can revert to conventional production and the vents can be reopened.

Even at this latitude, outdoor temperatures can exceed 30 degsC in summer. And on a cool spring day at 12 degsC outdoors, the water re-emerges from the glasshouse at temperatures around 15 degsC.

"In summer this will be more like 20 degsC," Huttunen explains. "And even in May it would be up to 40 degsC in the house with no cooling. In summer, that would go up to 50 degsC.

"But with Novarbo, if either the heat or the humidity gets too high, the sprinklers kick in," he adds.

Each nozzle can spray one litre of water per hour. This mechanism of using water to reduce humidity as well as temperature is unintuitive, to say the least, he explains.

"If a droplet is cool, moisture will condense on it. However, each droplet has 3.5m to fall, and it gains heat as it falls, so at some point it starts to evaporate again, which we don't want. It's a lot of work to get the balance right between temperature and humidity."

Huttunen points out another potential sticking point for growers. "People are afraid of having water in the glasshouse because it's thought to promote disease - it's hard to convince them otherwise," he says.

And there are marginal cases where growers will want one without the other, he adds. "Sometimes we have to limit the humidity control if the temperature gets too low - say, below 25 degsC."

He suggests that requirements in southern England would be comparable, although days are warmer for longer.

But this far north, effective heating is essential. The glasshouse uses lower and upper heating that can be fuelled by heavy oil and propane gas. "In the winter you need both," says Huttunen.

That would seem to tie in with one of the advantages of closed growing, namely the ability to keep CO2 levels high - but in practice this is applied separately, he says.

"If you use your own from burning, there's always something else in it, like nitrogen oxide, which is harmful to the plants, so you end up having to ventilate."

However, this usually pays dividends in the yield. "We get 4.5kg per square metre per week - that's one cucumber per plant per day," explains Huttunen. "You can't have such a large crop without CO2."

He adds that Finnish customers prefer smaller cucumbers. "We could grow them a day or two longer, but consumers don't like them as much," he says.

"Artificial lighting is also key to crop quality, even in summer. The lights also heat the glasshouse, so you need to burn less oil. But peppers are even more difficult to grow economically, and only very few Finnish growers attempt it."

Both tomatoes (mainly Espero) and cucumbers (Rapides) are grown in peat rather than rockwool, held 1m above the floor to allow air to circulate, which is promoted by the ground-mounted heating ducts.

Integrated pest management allows chemical use to be kept to a minimum, but being well away from other glasshouses reduces the incidence of pest and diseases. Meanwhile, the recovered cooling water is treated by copper ionisation rather than chemically to prevent growth of algae.

The Novarbo system was premiered at this year's IPM trade show in Germany. "We have had interest from a wide range of countries, including the UK," says sales manager Katja Huhti.

"The economic situation means now isn't the best time, but we have to keep doing the work so we're ready when the economy improves," she points out.

The technology has not been extended to ornamentals production, which exists only on a small scale in Finland, explains Huttunen. "You don't need so much CO2 with ornamentals, but you do want a constant temperature to control flowering. And edibles growers would have to already have very good CO2 systems to benefit."

The system could be adapted to a range of climates, including the UK, he says. "You just need the climate data. In a hot, dry climate such as California it would also work well. But we're not sure how it would cope with long periods of high humidity, such as in Florida."

There is already a lot of interest from Finnish growers, he adds. "We want to learn from it - it's something new for all of us."

BUILDING ON SUCCESS

In a sign of optimism at Biolan's headquarters in south-west Finland, company owner Pekka Kariniemi is putting into place a plan he has been nurturing for 20 years. The new three-storey thatch-roofed office building and staff canteen, dubbed the "Asterix house", is due to open in October.

With a turnover of EUR30m (£25.7m) and 200 employees worldwide, Biolan has interests ranging from water treatment and waste processing to organic fertiliser, composting, charcoal and, most recently, glasshouse technology

But its main focus remains peat. Finland has over 50,000sq km of the stuff - an area larger than the Netherlands. Horticulture accounts for only around six per cent of peat harvested in Finland, with around 90 per cent being used for fuel.

"Horticultural peat is a very competitive market, so you need high quality and a low price, and our location isn't ideal," says Biolan's Katja Huhti. "So as well as looking for alternatives, we think of how to help growers in other ways."

Biolan's 38ha factory processes 300,000cu m of raw materials every year, with 80 per cent of production leaving the plant in a three-month window between the melting of snow and midsummer.

The company is also building its own quality-control glasshouse on the site. Sourcing director Krista Joensuu says: "It will let our staff see the result of what they make."