Such a direct question deserves a direct a answer.
Yes, both lilies and peonies will respond favorably with added fertilizers (including Miracle Gro). Just like any fertilizer, one wouldn't want to over do it, or apply at a wrong time. As we near the end of the growing season, the question of whether to fertilize or not becomes more important. We can't advise you on this until we know what geographic area you garden in.
Miracle Gro is a fertilizer, but more importantly, it is a liquid fertilizer. Compared to granular types, liquid fertilizers are fast acting and short lasting (2-3 weeks). Depending on what you want your fertilizer to do, these characteristics can work to your advantage, or you may decide a slower acting, longer lasting granular fertilizer is what you want.
Regarding NPK ratios, lilies would prefer higher nitrogen in the spring and early summer, and more potassium in the late summer and fall when bulb building is paramount. Miracle gro, as it is formulated now, has more nitrogen than it used to, and because nitrogen is an easily leached nutrient, I think it would be fine for both seasons. A "booster shot" now and again (but not continuously) is a good plan, in my opinion, but don't fertilize
instead of adding organic matter. Do it in addition. LOL on the "optimum nutrient" comment. At the retail center where I work, if a customer asks for some obscure NPK content (11-9-6.2 for instance), we always know what they are trying to grow optimally.... marijuana.
Plants are just not that picky. But better to under fetilize than over fertilize. Someone else will need to advise regarding peonies.
