Try New Zealand spinach. It's not really spinach, it's a vining plant that will spread all over the ground. It's been a few years since I grew it. It needs to be cooked, you don't eat it raw in a salad. The bugs pretty much left it alone.
I don't remember it being that great compared to many other greens but that time if year it doesn't have much competition. If you want a cooked green in the heat of summer give it a try.
We prefer New Zealand to all other spinaches! Probably because that was what DH grew up on. If picked small and tender it can be eaten raw. We like it steamed with a drizzle of vinegar.
Folks might say "chard" but I haven't grown it in many years. Instead, when I can find the seed, I've grown perpetual spinach.
It isn't a spinach either but rather, a type of chard. How is it different? It doesn't have that heavy stem. It tastes more like a beet to me, which is another possibility for you but it doesn't do too well as a green over very many weeks of summer for me.
Escarole that I had for the first time last year, did fairly well into the first part of summer. Yeah, it's supposed to join all those other greens during springtime but I'd like to push that season envelope, too !
Supposedly, a large leaf kailaan was gonna help me do it this year ... I don't know what I did wrong - it's already bolted ! Several years ago, that variety (South Seas) did quite well into summer.
Portuguese kale - that might be my best suggestion. After several years of growing it, I'm just about to declare it a winner! However, I've got it's sister, Scotch kale, right through the summer, too. What about their cousin, collards? Are you not a Georgia collards fan, 'Dorie?
For that matter, what about the other cousins, cabbage? I've got quite a bit of Savoy cabbage, DW's favorite, well into summer.
Steve
Keep in mind, it's hot and dry here during the summer - not hot and humid. Not perfect "greens country" but a bit of contrast from much of the country.
There is also Malabar spinach, though by that point you're really in the realm of "tropical" greens, and that too is a vine (and unlike New Zealand, one that actually does need a trellis)
Steve mentions collard greens, one of my favorites..it's good to eat all summer long, no bitterness. I have also had pretty good luck with gai lan as well, although the leaves/heads get progressively smaller as the summer wears on. Both are a steaming green, I don't eat them raw, like you can the smaller rhubarb chard leaves.
Gai lan
I see Steve mentions kailaan, which I think is the different spelling for gai lan..It's also known as Chinese Broccoli. I must have just gotten lucky with the variety I first chose, as it's been slower to bolt. Course I keep cutting the heads off to eat, and just leave a few for next years seeds.
Personally, BillS, I like the spelling "Guy Lon" since I've known and liked a Guy and a Lon (Leon).
The University of Melbourne has page after page as they try to work out Asian vegetable names and English translations . I think it's an unaspirated "k" and English just wants to aspirate all those consonants.
Whatever the spelling, it grows okay for me only some years. It's probably like the Chinese cabbage. I've fairly well given up on that. It can only grow well under plastic here - probably doesn't like the temperature swings and low humidity ...
Steve who nibbled some bok choy flower stalks while weeding this morning
I tend to prefer the "Gai" spelling, if only because it seems to be the one used in more of the names of other ones, and using the same pronunciation reminds me roughly what kind of a veggie I am dealing with, the same way I know that if a name includes "gwa", it's some sort of curubit or "choi" some sort of cabbage/mustard family member (except for ong choi [water spinach] of course)