There are two main questions to answer anytime you think about how to fertilize lilies: when, and how much? Neither answer is very complicated, but unfortunately it can’t be answered very specifically in a general article, since it depends on many individual factors. Follow the directions on the package for that. Still, there are some guidelines that will give your lily all it needs to grow big and strong.
As a first approximation, you can go with no fertilizer at all. If you have average soil, lilies are very good at getting all they need and producing the rest themselves. Ample sunshine and a bit of water allows their leaves to provide energy and their roots to supply the bulb underground. That feeds the rest of the plant. The bulb concentrates sugars that are sent up through the stalk to produce beautiful blooms.
If your plants don’t flower one year (yet have ample sunshine), or produce fewer blooms than you want, consider adding a small amount of fertilizer just before flowering season. That will vary by a few days to a few weeks depending on your USDA number. Colder regions (represented by a lower number) usually see spring warmth a little later than warmer areas (higher numbers).
Gardens in colder regions can also benefit from lilies that bloom at different weeks of the sunny seasons. Asiatics, for example, are hardy in zones as cold as 2 and flower early to mid-season. Martagons may not bloom at all the first few years after planting. Easter lilies, by contrast, bloom in mid-summer. Yes, that is a little odd, considering the name. Isn’t gardening fun? They bloom in spring, around Easter, only when forced through special techniques.
Consider those season/blooming characteristics when deciding when to fertilize. As a general rule, if you add fertilizer it is best to lay it down just as the shoots are emerging from the ground. That gives the plant ample time to absorb the extra nutrients and incorporate them in order to give you the best possible flowers.
How much to add depends (apart from soil type, as we saw above) on how large your plants are and how closely spaced.
Oriental lilies, for example, often reach heights of 3-6 feet. Asiatics are a bit smaller, from 2-5 feet, typically. The taller the mature plant, the more fertilizer required. Taller plants tend to have larger bulb circumferences and correspondingly larger bulb volumes. The bigger the bulb, the more fertilizer they can profit from.
Likewise, if you plant your lilies closer together, they will require a bit more fertilizer than if spaced widely apart. Common sense explains why. More earth between plants means more nutrients available to each individual lily as their roots spread and seek out what they need. If more plants are drawing from the same soil, there’s less nutrient available per plant.
In general, it’s better to use less fertilizer and risk fewer blooms or smaller plants than to overdo it. Too much fertilizer can burn your plant and recovering from that, like recovering from too much water, is much harder than compensating for too little.
As a rule of thumb, a 10-20-20 mix of slow-release fertilizer is best to keep your lilies in peak condition. For large plants with lots of blooms a second application just before flowering is fine.