I sowed my onions in February, and parsnips in March.
Think it is most probably the weweds as Dove said. Planted parsnips and carrots here on 5 May and they are fine if not huge. I think the roots suffer most from weeds because we sow them without fres fertilizers and there isn't enough nutrition to go round. other crops that are freshly manured cope better with competition. Does that sound resonable?