Even when I start plants on time, late August often finds me with bushels of green tomatoes. On the other hand, if you are willing to provide frost protection during early fall frosts and don't mind letting an unripe crop turn colors indoors, give it a try. If the worst happens, your plot's bald spots will be filled with foliage and you'll be able to survive on fried green tomatoes until Thanksgiving. If you are one of those amazing persons who do not care for tomatoes, this is a time when you are actually more fortunate than those who do like a bit of red in their salads. Virtually all of the leaf greens can be planted from seed or transplant throughout the entire month of June and be providing you nourishment by, at the latest, mid-July.
Spinach is also a good bet.
There still is time to grow other salad essentials. An added treat is that they come in many colors other than the traditional red. That other pungent vegetable, the scallion, is another wonderful last minute addition. Bags of sets are incredibly cheap and produce edible onions in a matter of weeks. Like radishes, scallions can be pushed in anywhere you see soil and don't need any tending beyond watering.