You'll probably want plants that:
- don't reseed freely
- don't need to be constantly dead-headed, like some daisies, coreopsis or campanula
- don't spread out like crazy
- are long lived and don't need frequent dividing to stay nice-looking.

With this, you can plant tidy clumps, mulch thickly in-between and not have to worry too much about adding to the weed problem.
Coneflowers do tend to reseed freely, popping up everywhere after a while. The hostas and peonies are good choices in the above categories.

Here just a few low maintenance choices for both color and texture:

Sun:
Sedum autumn joy
Pretty dwarf shrubs, such as dwarf weigela or dwarf spirea (sorry, forgot the names), hydrangea 'pee wee'
Butterfly weed
Butterfly delphinium
Hardy geranium, cranesbill
Daylilies (do need deadheading)
Phlox
Liatris (the finches love those in the fall - do not deadhead)
Instead of mulch, try a dense groundcover like creeping and wooly thyme to crowd out weeds.

Shade or partial shade
Hosta (more hosta!)
Ladies' mantle (will take sun, too)
Bleeding Heart
Ferns (some do tend to spread)
Forget-me-nots (doesn't matter if they spread - they don't tend to choke out things)
Heuchera
Monkshood
Hellebore (lenten rose)
Bergenia

Thanks for the suggestions. We can't grow delphiniums and monkshood in NC, too hot...so at least I'll be able to see them in my mom's garden. Love the way those plants look.

hi i was thinking columbine would be good and lupines also. both are easy to care for have lovely flowers and very beautiful foliage

Ooo- great suggestion. Lupines are another plant I love the look of but they hate our heat & humidity in NC.

What about pulmonaria, another shade perennial that doesn't do well in NC heat - does it do well in Michigan?

There are a variety of hydraneas and viburnums. I love them both for their fascinating foliage. If choosing hydrangeas, be aware that the colored types (which bloom on last years growth) will not bloom in Michigan since they die to the ground each winter.

Perenniel Sunflower is a fav of mine for back of border planting. Nice and tall and no work at all. :)

Though I love lupine I find they need too be babied in my area (SE MI zone 5/6).

Veronica (Speedwell) seems to do well for me.

Asiatic lilies are always a treat.

If you have moist shade I highly recommend Astilbe.

False Indigo is also great for moist shade. Love the sweetpea-like blooms and the seed heads are interesting too.

Happy gardening!

Thanks for the additional suggestions. Glad to know astilbe does well, I have a large patch that needs thinning...

Squirrelsput, yes, pulmonaria does great here. I have several different varieties, but Trevi Fountain is one of my favorites. Another great shade plant is brunnera. The 'Jack Frost' variety is awesome!

Love the foliage on the pulmonaria and brunnera!

Ohh, and coreopsis in full sun. There are several types and all are very hardy and easy to grow.
They bloom throughout the season but will need deadheading. However, this can be easily done with a pair of scissors and your mom can cut some to take in the house while doing this. Baby Sun is especially cheerful in a small vase and looks great with daisies.

I'm in the same zone as your mom..only in northern Indiana. Here's my suggestions:

Salvia--Salvia should be deadheaded to look it's best but I don't always get to mine and it does just fine.

Nepeta--cat mint. always does great and looks lush.

Perennial geraniums...Does usually need to be cut back to the ground in July so it can relief but will survive without it.

Pasque Flower...great plant for early spring blooms...blooms early April for me. Has interesting seed heads.

Cone flowers--super easy...and now come in lots of colors.

Gaura--dainty flowers that move in the wind

Oriental & asiatic lilies..plant and forget them to be rewarded with fabulous blooms. I especially love the oriental ones because they smell so fabulous.

Russian Sage...grey green folliage with purple spikey flowers. There is a miniature version that stays more contained.

Butterfly bush...great summer color. Dies back to the ground every year.

Annabelle Hydrangea...the white hydrangea is the only snowball type hydrangea that is reliable for me. The others are a pain in the @@@ to get to bloom.