Weatherproofing the greenhouse of the future

No construction can withstand the hail that came down two weeks ago in the south of the Netherlands. But weather conditions are becoming more and more extreme. How can horticulture arm itself against this?

"Nothing really helps against this type of extreme weather," is the assessment of Michael van Spronsen of Glascom Tuinbouw. "Even roof tiles were broken, just like solar panels and entire trees. Everything was damaged by the hail." Van Spronsen is a glass supplier of various types of material. Development of stronger materials could prove tricky, he says. "A greenhouse is a relatively inexpensive construction, but with a very strong structure. What's more, horticulture has high demands on the material properties, in terms of light transmission and diffusion. A new or alternative material would also have to comply with this, so you can imagine there being additional cost."

This, for example, affects the choice for plastic on the greenhouse. "It’s more expensive than glass and it scratches more. And other materials can react differently in this weather. About 25 years ago we had many greenhouses around Aalsmeer with channel plates. According to the brochure they were supposed to be hail resistant, but when a cold front preceded the hail most were still pierced."

What about tempered glass? Does that offer new possibilities? Glascom Tuinbouw has participated in the development of the Winterlight Greenhouse. That has been fitted with 5 mm tempered glass. "But whether that makes a difference against hail, I'm not so sure, that’s how hard it came down. Also, this glass is a lot heavier. So the structure must also be heavier and that also costs extra."

Looking at the breakage pattern, something else stands out. "Tempered glass is much stronger than unhardened glass, but hailstones hit the glass full on. When you look at the breakage pattern in some photos, you can see that in some greenhouses with unhardened glass many windows have been damaged, the remains are still stuck in the roof, while it seems that there are fewer windows left in greenhouses with tempered glass. Because tempered glass crumbles, the glass completely falls out of the roof. So we need to look into which greenhouses have the most damage - those with tempered glass or those with unhardened glass. I do believe that most of the greenhouses in the affected area still have unhardened glass."

So Van Spronsen is not thinking about new greenhouse roof materials in terms of future weather resistance, but more along the lines of construction alternatives. A curved greenhouse, for example, with curved glass. "Then hailstones can be deflected more so you can better guard against extreme weather."

So what do the greenhouse designers say about this? BOAL Systems can tell us more about the development of new greenhouses. Can roof systems be better reinforced against extreme weather? The aluminum structures supplied by the company are already tailor-made for any circumstances, says Maurice Verbakel of BOAL. "No greenhouse anywhere in the world is the same. We take what happens in the greenhouse into account and look at which forces play a role. So we also take external forces that impact the aluminum roof system into account, such as local wind and snow conditions." This also affects the development of new roof systems like the double glass roof system for garden centers and greenhouses. "The aluminum parts and the steel substructure are then completely designed with the use of this roofing material in mind."

In the past, BOAL has actively contributed to the investigation of the impact of hail on glass. "Under normal circumstances, in case of damage, the glass is the limiting factor so a heavier roof system is not required," says Verbakel. These tests have supported the introduction of tempered glass instead of float glass in the recent past. Apart from that, aluminum components have been adapted to prevent visual damage from normal hail.

Growers can of course consider to opt for film-based roof systems. These high-quality roof systems are already widely used in areas with regular earthquakes. "Then you should also consider the probability of recurrence of such an incident, restoration costs and the economic impact of growing under film,” Verbakel continues. "Hail like we saw in Someren will shatter a film roof cover as well. This was a very extreme weather incident. Hailstones of this size will always generate damage."