Polytunnel

Polytunnels first appeared in the 1960's when it became possible to manufacture wide-width polythene sheeting. Although widely used in the soft fruit industry, there have been few attempts to grow vines in polytunnels. This is in part due to a perception of grapes/wine as a 'natural' product whose image would be tarnished by the use of plastic in growing. However, during wine making, sugar is often added to boost natural levels in open field grown grapes in a process known as chaptalisation. Grapes grown under cover have a longer growing season and less likey to need such additions. This leads to the issue of which is the most 'natural', adding sugar as an extra ingredient or using plastic to modify the growing environment? 

A handful of polytunnel vineyards have existed in England and Wales growing grape varieties such as Cabernet Sauvignon, Merlot and Gewurztraminer which would be otherwise too late for the growing conditions. In Scotland grapes were grown in a polyunnel at Polycroft of the Isle of Lewis up till 2016 with some wine being made non-commericially from the variety grown, Black Hamburg, despite this usually being considerd a table grape. Wines  were also made here at Dalrossach from Golubok and DeChaunac grapes as a test in 2018, both reds. Investigations continue to find the most suitable varieties for polytunnel wine-making. Candidates for polytunnel wine grapes on this site are Golubok and Michurinets for reds, L'Acadie blanc and Madeleine Sylvaner for whites. The red varieties Chancellor and DeChaunac can be good but ripening for these is at the very limit for polytunnel growing here.

Table grapes rarely get a mention on vineyard websites but the potential for for growing these in polytunnels would appear to be huge. Factors such as size of grapes and bunches becomes important as well as maximising the cropping season, which here runs from mid August through October.

Below: The polytunnel was made from two standard 6 metre tunnels and has extra strengthening to cope with severe gales and heavy snow-fall. Currently it is 9x3 m, soon to be extended. The plan was to have 23 vines inside, pruned to single rods and grown as bushes. These were arranged in three rows, with 3 ft spacing between the vines  which were arranged in a hexagonal close-packed pattern to try to minimise shading between vines, i.e.  the vine position in one row is opposite the inter-vine position in the adjacent row. Vines in the central row therefore have six equidistant neighbours. In reality, the inter-vine spacing has become used as test spaces for new varieties which have not yet been given permanent positions.

Bottom left: Below: Kandiyohi grapes from the polytunnel, October 2020. This large blue table grape by Elmer Swenson has proved to be productive and very disease resistant. Plus, its late budding which helps avoid frosts in April. Pound coin for scale.

Bottom right: Below: Grapevines are supposedly wind-pollinated but in June 2019 these critters, mining bees I think, were found assisting pollination as they happily rolled through the flower clusters on Alioshenkin.

The green nylon mesh infused into the plastic of the covers is not ideal for light transmission, and as the cover is now 6 years old, it will shortly be replaced. One advantage was found in the blizzards of February 2021 as the mesh meant a tear made in the cover during snow clearing could be repaired by stitching.

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