Faithful reader Lee asks, "When should I fertilize my crepe myrtles? They are old and about 25 feet tall. They never get pruned and don't bloom the way I would like them to."
Grumpy's 124% Guaranteed Correct Response: Unless you're planting them in pure sand or in very confined areas, crepe myrtles don't need much fertilizer. They're well-adapted to most soils and grow just fine without it. If you were to fertilize, the best time would be in spring just after they leaf out. Any controlled-release tree & shrub fertilizer would do.
You say you're unhappy with the way your old crepe myrtles are blooming. My guess is that most of the blooms are way high up atop the branches. This is just the way that tall-growing types like 'Natchez' and 'Muskogee' do as they get bigger. Plus, their flower clusters get smaller, even though there are more of them.
The only way to change their blooming habits is by drastically shortening them in late winter. Cutting them back to thick stubs, a practice I call "crepe murder," will give you bigger flower clusters lower down. I don't do it, because I prefer my crepes to express their natural form. I grow them not solely for their flowers, but also for their beautiful bark and wonderful architectural shapes. However, this is a matter of individual taste, so you can choose to prune or not.
You can avoid Lee's predicament by reading the label when you buy a crepe myrtle and choosing those that grow no taller than 10 to 12 feet -- ones like 'Acoma,' 'Black Diamond,' 'Delta Jazz,' 'Early Bird,' 'Pink Velour,' 'Siren Red,' 'Tonto,' 'Velma's Royal Delight,' and 'Zuni.' Check out their descriptions in The New Southern Living Garden Book. You'll never have to murder them to keep the blooms down low.