flower gardens in Holland in July??

Flowers are the Netherlands' business, so you can count on floral displays somewhere. I am more familiar with dahlias, and these won't be blooming until later in July, but you may want to check out websites and links for the Nederlandse Bloembollen group (bulbs). Major bulb fields are in Hemsteede and Hillegom. Lisse has Keukenhof with large displays and is headquarters for the Bloembollen group. Aalsmeer is another commercial location with a popular flower market. Most likely you'll find lilies, carnations, and other summer-flowering plants in profusion.
Every 10 yrs. the Floriade is a year-long hortic. exhibition on a huge territory near Amsterdam. See what's viewable at this time by Googling 'Floriade" or 'Floriadepark'. Public parks and the grounds of Dutch castles are another focus for floral display. Also contact some flower growers directly: Van Bourgondien (kvb.nl) is one of the better known sources.