In this video I’m finishing up my indoor growing series with the simplest fertilizing routine I’ve found for seed starts and transplants—indoors or in a greenhouse. The key is water-soluble fertilizer only. I don’t recommend mixing dry, granular organic fertilizer (or compost) into your seed starting mix because I want that mix to be sterile. When you sterilize with boiling water to cut down fungus gnat problems, you wipe out the microbes that would break down organic granular fertilizer anyway—so it gets fuzzy, can attract insects, and it’s basically unnecessary. I walk you through exactly when to start feeding and how to scale it up as plants grow. After germination, it’s just water. About 7–10 days after the seedlings break the surface, I give the first water-soluble feeding, usually about once a week when they’re small. As they size up, I move to twice a week, and when they’re close to full size (like indoor lettuce), I’m often feeding at every watering. The big warning is fertilizer concentration—unused salts build up as cells dry—so I always say “slow and low.” Start light (for many chemical fertilizers, about a teaspoon per gallon for indoor strength), adjust if plants yellow, and avoid the mistake of overfeeding that can damage or even kill your transplants.