Calgary's a bit quirky in terms of weather patterns. I'd recommend starting by checking out the city's recommended shrubs and trees and then pick from that list.

You could also try the Calgary Carpet Juniper
Juniperus sabina "Calgary Carpet"

http://www.calgary.ca/UEP/Water/Pages/Water-conservation/Lawn-and-garden/Water-wise-gardening-and-plants/Water-Wise-Trees-and-Shrubs.aspx

Here is a link that might be useful: city calgary plants annuals perennials

You may want to get your sister a membership to the Calgary Horticultural Society. Lots of good info on the forum there.
Caroline

How nice of you to do this for your sister. I am in zone 2 and a total fan of peonies and my daylily does well too. The only shrub I have is a false spirea and it does very well.

I think Calgary needs its own gardening zone! Just make sure that -- whatever you plant -- it will look good after frequent batterings from hail storms.

I can recommend western mockorange and creeping oregon grape.

Calgary has what are known as Chinooks which are warm winds that can occur during winter and temperatures can go from well below freezing to well above, then back down again. This can be very harmful to plants as they break dormancy and then suffer from freezing temps.

I don't live in Calgary but if I did I would try to create a mini-environment with hedges, shrubs, trees, and perhaps even rock walls in an effort to protect plants. Also mulch and other winter protection is essential.

The local garden club was very helpful to me when we moved from a warmer zone to zone 3. Your sister might want to join now to get ideas before gardening season starts.

Here is a website which might give some ideas.

Here is a link that might be useful: Calgary gardening

I've got hansas and mordens planted on the south side of my cabin and they've done well there for decades. I'm in zone 3 Alaska. If your sister has a fence or building with a good southern exposure, that's where I'd put them. I laid Typar, a road covering, on the beds and cut X's where I planted. I pruned them well last summer, and both still doing great. They get watered from the rain coming off the roof, so are pretty much trouble-free. Good luck!

Thank you all so much! You've given me a lot to think about. I'll be visiting my sister in late May - we'll hopefully put some stuff in the ground! Destinyvp and luckygal, thank you for the very helpful links. I've been perusing both. CLBlakely, peonies?! Really? I LOVE peonies! Tree or herbaceous or either? Or ITOH?

Tree peonies are not hardy here. Itoh do fine but are still expensive.

No cedars allowed. Don't even think about it.

Amend soil. Ours is mostly clay. Do not apply any sort of lime product as our soil is plenty alkaline.