Step 2. Fertilize the seedlings with the dilute houseplant fertilizer every two weeks. Start the first fertilization when the second set of leaves emerge, or one week after germination, whichever occurs first.tep 2. Fertilize the seedlings with the dilute houseplant fertilizer every two weeks. Start the first fertilization when the second set of leaves emerge, or one week after germination, whichever occurs first.