I would really like to grow brocolli, potatoes, blueberries, and/or spinach in Oklahoma (zone 6). But all my previous attempts have failed miserably. Does anyone have pointers on how to do this, or should I just give up? Even tomatoes are difficult to grow down here, because the summer heat can be brutal.
I grew up in Eastern Kansas(hardiness zone 5b). I had the most success with tomatoes when I bought them as seedlings and selected heat loving varieties. Late frosts were also a problem. I also found they needed to be in the sun but did poorly if the area did not also receive shade(especially during the hottest part of the day. I also found that spinach did not do well after June. Early Spring or late winter was a good time to plant it.
I have not personally grown blueberries in Kansas but I have a friend that successfully grew them after many years of trying out different plants and soil combinations. She built a netted shelter around them that keeps creatures away from them. As I am not familiar with the type of blueberries that grow well in Oklahoma I suggest this article. I used a similar site when I started growing blueberries where I currently live on the West coast.
A couple things I have learned is that you need to plant more than one bush. I am trying several varieties that are supposed to do well in my region. I tried to choose a well drained protected location. I mulched around them during the winter to try and protect the roots from the cold. The birds always seem to get my blueberries so netting or some other type of protection is necessary.
Haven't tried spinach or blueberries, but I've grown potatoes and tried broccoli here in N.Texas (similar climate to OK but our frosts are not as bad).
Your growing season for these plants are limited to spring (or possibly autumn), so there's a race from when growth starts (or last frost) until it gets too hot. As Ed says, watering is important. For the potatoes, I've found they do best on a north facing wall and with good watering later in spring when the rains are less frequent.
In order to lengthen the growing season my first attempt to grow broccoli was with grow lamps but became a failure due to aphids on the seedlings. Will try again in autumn or next year.
For tomatoes, you'll need to get the soil to hold enough moisture between waterings, and to not dry out too deep in the heat of the day. So you need lots of humus, and lots of mulch.
If your garden's small and dollars aren't tight (or you can get it cheap), my favorite quick-and-easy source of humus is composted cow manure. But there are many other options.
For mulch, I prefer one that's a grass of some kind - but be sure not to use too much that's too green (which will burn). Some folks also worry about introducing weed seeds, but it hasn't been a problem for me.
Of course, you must keep an eye on it. You'll probably still have to water heavily daily in the heat of the summer. You're best off watering in the morning, to avoid molds. For a small garden, consider use of a simple sprinkler that just has holes for the water to squirt out of. These put out big droplets, so you lose less to evaporation. Just don't spray hard directly at the leaves - it should "falleth as the gentle rain, upon the place beneath".
You also should try to use seed varieties that are heat-tolerant.
I live in New Hampshire, so take this with a grain of salt. You may need to do more than this - but it's a good start. For the other plants besides tomatoes, I think I'd better defer to folks who have experience with your climate.