Lace-cap hydrangeas (Hydrangea serrata Blue Billow) are a delight in any garden. In nature, this is a mountain species, so it tolerates cold better that the mophead hydrangeas. The lacy blue central flowers that give lace-caps their name are surrounded by big blue florets with sparkling petals, creating an interesting effect. Most blue hydrangeas want acid soils to retain their blue color, but lace-caps aren't as fussy. And although the plants grow quickly in summer, they don't get overly large. Along with the Oak-leaf hydrangea, this is one of the few species with good fall color.