In a 2-year field study, 5-year-old field-grown ‘Natchez’ crapemyrtles were either not fertilized with nitrogen (N), or received 100 kg N/ha (89 lbs N/acre) as urea-ammonium nitrate solution applied in six biweekly applications either mid-June through early August (bloom stage) or late August through late October (post bloom). Treatments had virtually no effect on the development or degree of cold hardiness attained as measured by lowest survival temperature on twig samples collected monthly from November to March. Nitrogen application increased leaf N concentration in 1994, but not in 1995. Nitrogen fertilization reduced leaf phosphorus (P) concentrations both years, presumably due to the dilution effect, but only reduced the leaf potassium (K) concentration in August 1994. Nitrogen treatments increased stem diameter 4% compared to the control, but sidedress timing had no effect.
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Lindstrom et al. (2002) studied this question.