Polytunnels for Welsh Seedling Leeks
9th Jun 1999
There's no doubt that the introduction of the Welsh Seedling Leek by Ivor Mace a few years ago was a definite boon for all exhibitors, as it is such a vigorous grower. Nearly every exhibitor can grow it to a large size, regardless of whether it is grown under cover or directly outside.
However it has one drawback that becomes apparent from now on as the summer temperatures rise. The foliage puckers up or forms bubbles on the upper surface. This happens as the plant matures and is because the flags or foliage resemble the foliage of the pot leek which is much thicker than the previously exhibited soft flagged blanch leek which didn"t appear to suffer from this puckering of the upper surface.
Naturally there is no problem as such with the foliage puckering up under normal circumstances when grown outside, but when the temperatures can soar into the upper nineties during the following weeks, the puckering will burn out with the direct rays of the sun and leave the foliage looking really tattered. Preventative measures are a must with this leek if you are growing it under polythene in tunnels or under glass in a greenhouse.
The majority of the top growers who grow leeks to a very high standard usually grow them in polytunnels and there's no doubt that without these structures the quality and size of the leeks that we see exhibited today could never have been achieved. Sadly however there is a drawback with these structures in that, even though they offer first class protection in order to get an early start for the plants and therefore greater size, they can be like ovens during the peak growing season and some form of protection to the plants is therefore essential if the burning problem is to be counteracted.
There are a number of things that can be done to improve matters. The first is to allow an adequate flow of air to get to the plants and this can be achieved through the correct design of a tunnel as well as by using electrical fans designed to change a vast amount of air in a relatively short period of time.
Size of Tunnel
Many growers when they buy a polytunnel for the first time get carried away and buy the biggest they can afford so that they can grow both their onions and leeks in a structure probably extending to over forty feet in length. However, with such long tunnels, even though they have large double doors at both ends, the air movement in the centre is next to zero and the plants really suffer in trying to grow to their optimum size. The first time buyer would be far better off purchasing two twenty foot long tunnels, one for the onions and one for the leeks and this way you can vastly improve the air flow within both of them. In my twenty foot tunnel at this time of year, I suspend a fan at one end of the tunnel about two feet above the leek flags and pointing slightly downwards so the air can be seen lightly moving the flags at the far end.
When I approached Northern Polytunnels with my idea for two new tunnels, the question of the build up of heat was discussed, and we agreed in the end that the added benefit gleaned from having wind up curtains along both sides of the tunnel would be an excellent way of getting rid of any excess heat. Of course we have to realise that even though an adequate air flow could ensure that the temperatures inside are no higher than those outside, you are still faced with the problem of the direct rays of the sun searing through the polythene onto the flags. As we tend to support these flags on pipes or canes in order to take the weight from the barrel of the leek, the flags are even more vulnerable to be burnt.
Last year I managed to overcome this problem by covering the leeks over with some fine lightweight fleece. This was draped over some wires positioned above the leeks and the fleece clipped to the wires by using plastic clothes pegs. This certainly did the trick but I do need to have a hotter summer than last year to really prove the point.
If you are growing the leeks in a greenhouse then you have the option of spraying the outside of the glass with Coolglass to prevent scorching. This can also be done to the polytunnel but with such a vast area it may be better to utilise the above method. If care is taken when using the fleece and rolling it up carefully at the end of the season, it can be re-used the following year.
Green netting can also be utilised as shading material but you have to be careful that you do not drop the light levels so low so that you are unable to get the plants to grow well, possibly preventing them from achieving their full potential.