I have used vermiculite as part of my mixes for both onions and leeks for a few years now and I find that I have quicker growth, probably as a result of a stronger root system as more air is able to get into the compost. My sowing mix is three parts F1 and one part vermiculite thoroughly mixed together. Also there is no doubt at all in my mind that vermiculite is best to cover the seed over in preference to the same compost in which they are growing.
Growing Parsnips
25th July 2007 I am often asked what is the best method of growing both long carrots and parsnips for the show