How wide and how tall does a spiraea hybrid bush grow?

Question:also what fertilizer do they like

Answers:
Sorry, but that's an ambiguous question. The genus Spiraea, common name Spirea, has many species, and dozens of varieties. The simple answer to how big do they grow, is how big do you want them to grow? All are deciduous, and all can be pruned at will.

On the smaller side, like 'Shirobana,' 'Anthony Waterer,' 'Little Princess,' they get around 4' x 4' if not pruned.

Compare this with Vanhouttei (result of a cross between S. trilobata and S. cantoniensis) at 6'-8' tall and a spread up to 12.'

As far as fertilizer? In the spring, I use Esoma Plant Tone, or Flower Tone. Go to www.espoma.com and put in your zip code. It should be available in your area, if not, read up on it at the site and substitute for a similar analysis.