Where you are located and the specific type of hydrangea you have will dictate what type of winter protection may be required and pruning needs, if any. Many - but not all - white flowering hydrangeas are selections of arborescens or paniculata, both of which are very hardy and bloom on new growth. They require no special protection in winter and can be pruned back hard, but that is generally a task left until early spring.

If the type is unknown yet still in flower, posting a photo here for identification can help with more specific instructions. And let us know your zone.

I am also interested to know what needs to be done. We planted hydrangeas in my mother-in-laws yard over the summer in northern Michigan (bordering zones 4 and 5). Luckily there is no snow yet, but what should be done with them? I'm not sure exactly what type they are. The flowers bloom blue/pink/purple depending on the soil pH and are of a small to medium size.

It is critical to know the type and variety because that determines what, if anything, you need to do. Hydrangeas that bloom pink/blue are Hydrangea Macrophyllas, most of which are not hardy to zone 4 and 5 so they need protection during the winter. However, Endless Summer -a hydrangea macrophylla variety that blooms on old & new wood- is hardy to zone 4 and requires no protection.

If your hydrangea is planted in the right USDA Zone then about 4" of acidic mulch should be all you need to add. If unsure on what zone your hydrangea macrophylla is good for, consider applying some of the techniques described in the link below.

Luis

Here is a link that might be useful: Overwintering Hydrangea Macrophyllas

Go to www.hydrangeashydrangeas.com
There will be pictures there that might help you identify the type of hyrdangea you have. There are also instructions on winter protection.
Good Luck!

Just purchased a "pink Sensation" from the grocery store. How hardy is it and can I possibly plant it outdoors later in the spring?