Hi Master Gardener, I seeded some nicotiana seed about 8 weeks ago. They eventually (2-3 weeks) germinated. But here is the question: They are simply refusing to thrive. They have stayed the same size since they germinated. They have four leaves, but the second set of leaves is as tiny as the first leaves. I have them under a grow light. I am beginning to despair that they will ever be big enough to put outside.
Don’t despair! Your seedlings may be a little tardy but not very far off the expected timelines. Your germination time is right on with suggested germination times from the sources I checked. Nicotiana are typically started 6-8 weeks before the last spring frost. Based on climate zone information provided by the Ontario Ministry of Agriculture, Food and Rural Affairs, the average last spring frost for the GTA is April 30th. Note this is an average date specific to this climate zone. I’ve included a link below to a Climate Zone Map of Ontario in an article by the Ontario Ministry of Agriculture, Food and Rural Affairs in case you are outside this area.
Based on these numbers, the growth of your seedlings may be a little slow but not to worry. Nicotiana does like warmth so this may be a factor in their slow growth. You may need to hold them back to let them get a little sturdier before moving them outdoors. You could provide some extra nutrients at this point with the application of a water-soluble balanced fertilizer initially at ¼ strength then at 50% strength every other week. Once your seedlings have a second set of true leaves you can water with the fertilizer at full strength.
Nicotiana is planted out after all danger of frost has passed so this takes you past the average last frost date. The OMAFRA article notes that fourteen days after the average last frost date, the risk of frost decreases to 10%. Use the weather as your guide remembering that nicotiana likes warmth. Your seedlings must be hardened off which is the process of gradually acclimatizing them to the outdoor environment. Put them outdoors in a sheltered location for an hour on the first day. For the next several days, increase the length of time and the exposure to the sun. I like to spend a full week getting my plants use to being outdoors before they go into the garden.