I will answer this way. When I get a plant from Nicole Harry, and I see those leaves, I know its something that I wont see again, they are very large. They usually get a bit smaller in my garden with just regular fertilizing. From that, I draw the unscientific conclusion that yes indeed, fertilizer can change things. Cant change genetics tho,,,and those do differ from plant to plant. From what I have observed, depending on sun and water and feeding, you can buff up a daylily plant pretty good.
Im sure there are many sellers that have plants as big as Nicoles, it was just an example that I have experienced.
