The plastic roofs of Da Lat

From the pass above Da Lat early in the morning, the first thing you see is not the pine forest but the plastic. Whole hillsides are covered in greenhouses, their pale roofs catching the light like a frozen lake. Inside one of them, Mr Hoa is cutting chrysanthemums that will be packed into boxes by lunchtime. His father grew potatoes in the open on the same slope. "In a good year he made enough to feed us," Mr Hoa says. "I can make that in a couple of months."

The appeal is easy to understand. Under plastic, farmers can control temperature and moisture, shield delicate crops from heavy rain and harvest several times a year instead of once. Over the past two decades, greenhouses have turned a town once known mainly for its mild air and French villas into one of Vietnam's most productive farming areas. Its flowers and vegetables are sold across the country, and some go as far as Japan. For many families, the switch has been nothing short of a route out of poverty.

The trouble is that what works for one farm can go badly wrong across a whole landscape. Rain that used to soak into the soil now runs off thousands of roofs at once and pours into streets and drains that were never built for it. Parts of the city have flooded in recent years after storms that once caused little damage. Many residents also believe the trapped heat is nudging local temperatures upwards, though that is harder to measure. If they are right, the greenhouses are slowly eating into the cool climate that made Da Lat famous in the first place.

Then there is the plastic itself. Sheets usually need replacing every few years, and much of the old material is burned or dumped in ravines rather than recycled. Torn strips hang from the bushes along almost any stream on the edge of town. Local officials have tried to limit new greenhouses near the centre, but enforcement has been patchy, partly because so many livelihoods depend on them. Mr Hoa is blunt about it. "Ask me to pull this down for the view, and I'll ask you who is going to pay my children's school fees."

There is no single fix, but some farmers are trying things. A few now collect rainwater in tanks instead of sending it into the roads below, and others are testing crops that need less cover. Planners have proposed keeping the steepest slopes free of plastic. None of this is glamorous, and all of it costs money. Mr Hoa has put two large tanks behind his greenhouse, mainly, he admits, because the stored water saves him money in the dry season. Officials who want to change the hillsides may find that arguments like his carry more weight with farmers than appeals to the view.