Answer: Tomatoes are able to set fruit in San Antonio primarily during early summer and fall. Some varieties will set some fruit in summer, cherry types being one example. The problem is not just these infernally hot days, but the warm nights too. (By the way, I admire your diligence in digging up a garden spot in this heat!) Keep your tomato plants healthy and growing and by September, they should start setting a good fall crop.
Winter or storage types of squash can be planted now for fall harvest, but summer squash may have a difficult time as it will start setting fruit earlier, but I wouldn't give up on it. I'm afraid your onions will have the worst time of it. You need to plant sets (not the marble size bulbs but the pencil sized sets) in January for best results. Bunching onions may be set out in fall.
Enough of the bad news! Veggies you can plant now include okra, amaranth greens, southern peas (like blackeye, purple hull, crowder, cream, and pinkeye), sweet potatoes, Malabar "spinach", cantaloupe, winter squash (like acorn, butternut, Heart of Gold, Lakota, Buttercup and Ambercup), and pumpkins.
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