Hi Atlanta,
Not really enough info here to help you much.* Were they just planted this year?* Has it been several years?* What type of hydrangea does your mother have? Were they florist's hydrangeas?* Some hydrangeas bloom on new wood and some on old wood.* Have they been pruned?* You don't talk about growing conditions or if you see any leaf problems.* Take a look here to see if you can id the type if you don't know.* There's loads of growing and pruning info too.
http://www.hydrangeashydrangeas.com/
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.