I'd like to add some hardy perennials to my garden. Is it too late in the season to add daylilies and catmint (hardy to zone 3 according to the garden center tags).
Not too late to establish, but they will not bloom this year. If your area is still hot and dry, I would wait another week or two. Perennials will establish better in the cooler, wetter conditions of early fall rather than summer. Perennials planted in the fall will grow roots until the ground freezes and be in good shape to take off in the spring.
As a follow-up to Judy B above, I would like to say that any transplanted perennial needs to be in the ground six weeks before the hard frosts. So, for instance, around Ottawa, you should get your transplants in the ground no later than mid-September.
Daylilies are one of the hardiest, low maintenance perennials out there. In fact, some spread so quickly they are almost like weeds! Daylilies THRIVE when divided with relative frequency. I have actually left some clumps o daylilies utside on the bare ground over winter, and in 1 example, I found 2 clumps this past OCTOBER/early November. They both clearly still had good roots, so I planted them and expect to get blooms next year.
The only caveat to late season planting is the stronger risk of heaving. Like bulbs, though, they can be planted right up to snow in some cases. If the ground is diggable to the necessary depth, and doesn't have ice grains, you can plant them.