A garden is never finished...when something come out...something goes back in.
When I cut heads of lettuce, I plant radishes around the root that is left in the ground. New lettuce grows from the root and the radishes are out before the lettuce is big enough to cut.
When a bed of onions is dug, I turn in a little compost and go immediately back in some okra. Okra transplants easily so I can sow a short row and transplant in the evening and water in well for a couple of days until they recover.
When cabbages come out, I plant a late crop of squash. I usually leave a few cabbage plant roots with a few buds for side shoots. I pinch off the weakest and can get a second smaller head from the root.
When potatoes and beets come out, I usually follow with beans. I sow turnips when the beans come out.
Plants like tomatoes, peppers, brussel sprouts will be growing from mid spring to fall three seasons
Broccoli and red cabbage will be two seasons from spring into summer. If you cut the broccoli back in mid summer to about 6 inches, side shoots will produce some salad size heads.
Early fall is a good time to plant a second crop of carrots or greens like collards, spinach. To get them to come up, I cover the rows with a 1 x 4 board to cool off the soil and check it frequently. When I see baby sprouts, I remove the board .
Isn't gardening fun! 
