Hmmm... I generally plant pea seeds directly into the garden in August and my first frost is the first week of October. Your dates sound pretty good to me. HOWEVER, the date of the first frost isn't a sure thing. Last year the first frost was the first week of Sept and other years it waited until end of October. Also, the first frosts likely won't harm the peas as they can take it. The above paragraph is my way of saying go ahead and plant a little earlier and stagger your planting. If you get an early winter your planting earlier still gets you a yield and if your winter comes late your later plantings will give you a higher yield. I wouldn't bother starting the seeds inside for a fall planting though as the soil outside will be warm and result in rapid germination with no need for transplanting. It is spring when I indoor germinate seeds.

Other late fall bloomers are celosias. Annual asters were my grandmothers' favorite fall bloomer and she planted them fairly late after spring flowers were done (in her gladiola cutting garden). The seed pods on Castor beans are extraordinary deep reds and purples and textures that would look good against the smoothness of pumpkins. Speaking of, how about ornamental gourds, the small ones? That are so neat to look at. If you go with zinnias, be ready to protect the plants late fall from powdery mildew, to make sure that they keep blooming late. If you have neighbors who have roses, the late season blooms are often the best of the year, and many rose growers like to share their blooms.

The document linked below has been presented here in this forum multiple times. I think that you'll find that it goes a very long way in answering most of your fall gardening questions. Note especially the chart on page 7. Here is a link that might be useful: UT Ag Extension Publication - Growing Vegetables in Home Gardens

Hi Lindsay. I live in Michigan USDA Zone 6. You won't be able to use above ground planters as most perennials will suffer badly from the freeze if not completely die. That said, plants will grow around broken glass, etc. as long as the soil itself is decent. I suggest you pay a couple guys from a high school football team come and dig out a large trench along the fence to bury narrow horse trough planters. They don't have to be brand new [the planters not the guys] https://www.pinterest.com/pin/154952043399694610/ Fill the planters or trench with garden soil--not top soil. I just re-landscaped an older home in Pontiac MI and aimed for low maintenance perennials like hostas, daylilies and coral bells mass planted in groups of 3. Look for reblooming daylilies which will add massive color with little work. https://www.oakesdaylilies.com/. Daylilies love Epsom salts. You might want to add height with pillar form shrubs with color like Orange Rocket Barberry or that flowers/berries. Maybe hollies? Talk to your local nursery about what they have unless you want to buy online. Roses require a great deal of time and attention and chemicals especially if the Japanese beetles find them. I had to pull out a 12 bush beds of roses due to beetles. https://en.wikipedia.org/wiki/Japanese_beetle Clematis would be very beautiful, but can make maintenance of the fence difficult and they grow slowly. If you have little ones consider whether the flowers will draw stinging insects. I really like but didn't have much luck with micro clover. Died after the 1st year.