To fertilize or not? Well, that is always an area of research by first finding out what type of plant you have indoor. Knowing the size of the grow pot, you will know the quantity of fertilizer to apply. Spring-Summer months are the time to fertilize indoor plants. Always it's important to research the specie of your plant, to know exactly the type of nutrients it will need. Dry granules vs. liquid fertilizer, for example. Happy fertilizing!