KHT - I had to laugh - here our window is just really opening!! I planted the veggie garden on the 4th of July one year, and it was a wonderful garden. We had a frost on may 30 and it was downright chilly last night. First frost of fall can come at the end of August...small window, indeed.
Just starting to sow here in WPA-had hard frost night before last. Never plant anything out before Memorial day-learned tht over lot of lost plants. In 1974 when one of the hurricanes I think it was Agnes or Hazel swiped the applachians I lost 400 tomato plants on June 14 to 14 degree weather. Replanted the next day and lost them the next night.
Pebble~ We still plant things around here till FROST. In june I can still plant anything that will make in 90-100 days or so, then in late july I plant the first of the Fall garden stuff you know greens and brassicas, carrots, biennial flowers can also be planted pretty late, they're not going to bloom this year anyway.
Rise, are you going to plant them in containers, or direct-sow?
If you are direct sowing perennials, you run a risk that they won't be large enough to withstand winter temps, but I think summer can be a good time to start some slower-growing perennials or biennials.
I've got several perennials that I thought I'd start this summer, either indoors or out, (depending on our temps) and keep them in containers in the crawlspace or a cold frame with cow manure (for bottom heat) until next spring.