Parsnips experts - I need you!

If "station-sowing" would 2Litre pop bottles (i.e. with bottoms cut off) do, instead of cloches - or do you think that cloches warm the whole area more, and just offer a better solution?

Thanks all muchly! The bed they will go into this year is last year's brassica bed on neutral soil which I limed to sweeten it (never done it before - always learning) and the top soil added during last year to top up the beds was alkaline. I do add calcified seaweed in spring too before planting as a boost to the B.F. and Bone on sowing. Will sow in late April (weather-dependant) and will station sow at 6" to get smaller ones.
One thing - what about the bitterness problem...?