Should the plants be thinned out and if so, when? Some folks like to thin the plants to between eight to ten inches apart when the seedlings reach a height of five to six inches. Brianna Perry2023-03-20T22:31:01+00:00March 17, 2023|Planting and Growing, Poppy Seed General Information| Share This Story, Choose Your Platform! FacebookXRedditLinkedInWhatsAppTumblrPinterestEmail