This is a picture of the same parsnip bed in June of 2006 and you can see the young vigorous plants through the green netting. The netting helps to keep aphids off and also filters the wind and prevents breakages.
The glass panes over the top serve two purposes...in the early days they stop cats from digging the sand and leaving their deposits (I like cats.....but when they do that in my well prepared beds I could quite happily kill one....even ours!) and also keeps off excess rainwater. The idea is to keep the bed as dry as possible so that the tap root of the parsnip goes down in search of moisture. As the sand is a supposedly sterile medium the roots only stay in the compost mix that you trickle into the bore holes (see yesterday's post).
There should be no need to feed during the growing season as the compost mix has various fertilisers in it. I don't usually suffer from parsnip canker growing them in this method either and the roots do tend to come up quite clean with the minimum amount of cleaning required.
In 2006 I got 22 'station's in this bed but some of them turned out far too small so I'm going to decrease the amount I grow in this bed in the hope that the extra room will help them swell out a bit more.