What does perennial dictum mean?
I don't know if there's a specific context you're looking at, but "perennial" refers to something that is long-lasting, and "dictum" is a pronouncement/declaration/etc. with some authority behind it, so a perennial dictum, in general terms, is a decisive statement with lasting strength.
It depends on a lot of things - where you live, etc etc. Why don't you go and see your local gardening centre and see what you really like. You might like peonies, etc. The choice is enormous. You might also consider bulbs. And think about plants at the back of your borders - climbers or shrubs. Concentrate on every third plant being an evergreen. But a good idea is to go to the garden centre every month, to see what is in flower. That way you have continuous interest. And read gardening magazines and visit your library. There are books on small gardens with plans. Enjoy your garden.