From my perspective as a plant, my need for nutrients is intrinsically tied to my life cycle. I produce my beautiful blooms on what you call "old wood." This means the flower buds for next spring's display actually begin to form shortly after the current season's flowers fade. If you fertilize me at the wrong time, particularly too late in the season, you encourage a flush of tender new growth that will not have time to harden off before winter. This new growth is highly susceptible to frost damage, and, more critically, the energy you directed into pushing new leaves was energy I needed to dedicate to forming those precious flower buds. Timing is everything; it is the difference between a spectacular show and a disappointing one.
The single most beneficial time to feed me is immediately after my blossoms have wilted and fallen off. This period, typically from late spring to very early summer, is when I am at my most metabolically active. I have just completed the massive energy expenditure of flowering and am now shifting my resources to two critical tasks: generating new vegetative growth to capture more sunlight and, most importantly, initiating the development of next year's flower buds deep within my stems. A carefully applied fertilizer at this juncture provides me with the essential nutrients—specifically a formulation higher in nitrogen to support leaf and stem growth, but balanced to support overall health—to accomplish these tasks vigorously. This direct support results in a stronger plant and a greater abundance of set buds.
You must understand that fertilizing me after mid-summer is highly detrimental to my bloom potential. Applying fertilizer late in the season, such as in late summer or fall, acts as a signal for me to break dormancy and produce new, soft growth. This new growth will struggle to mature and lignify (harden off) before the first frost arrives, making it vulnerable to winter kill. Furthermore, this untimely growth spurt redirects my finite energy reserves away from the crucial process of flower bud formation and instead wastes it on foliage that is doomed. The result is a plant that enters winter stressed and with significantly fewer—or even no—flower buds, leading to sparse blooming come spring.
If you missed the post-bloom window, a very light, early spring application is a possible, though less ideal, alternative. This should be done only as the buds on my stems begin to swell but before they actually open into flowers. The goal of this feeding is not to stimulate growth but to provide a mild nutrient boost to support the immense effort of flowering and the initial leaf emergence that follows. Use a mild, slow-release, acid-forming fertilizer formulated for plants like me (rhododendrons and azaleas). However, be warned: this spring feeding does not contribute to bud formation for the current show, as those buds are already fully formed. It simply supports the energy expended during bloom. The post-bloom feeding remains paramount for maximum blooms in the subsequent year.
