I have never had saleable leaves from sowings before late July, except in a polytunnel, or under fleece/fine mesh which has no tears. So I grow wild rocket instead, sow now and it works well as clumps a foot apart in the tunnel, planted September, usually just one cut in autumn. Tidy plants of winter debris in February and new growth through spring gives repeated cuts, in fact I am still cutting now because of the lack of heat, with a few flower stems to remove.