Unless you're a die-hard grow from seed person (no guarantee anything would be ready even by late June in zone 5) - go to any good greenhouse or garden center (even the big boxes) and buy a few flats of ready to plant, already in bloom things. And, unlike perennials, no great expense.

Alyssum is good for borders; pansies; Dianthus Chinensis - which come in every shade of red, pink, white, salmon double, ruffled; verbena hybrids; petunias; Gerbera Daisys; geraniums. The list is endless and most are good for accent and infilling without detracting from your roses. And by buying established plants, they'll be ready when you need them.