When should I fertilize my plants? When the plants are about 10 inches tall, start to fertilize with a liquid fertilizer. Fertilize and water on a regular basis until you see the seed pods start to form. Brianna Perry2023-03-20T22:30:53+00:00March 17, 2023|Planting and Growing, Poppy Seed General Information| Share This Story, Choose Your Platform! FacebookXRedditLinkedInWhatsAppTumblrPinterestEmail