Shade in the garden questions.
So... we have a pretty small yard (less than an acre, maybe 1/4 acre available for our urban garden). We currently have four raised beds. We're in zone 5, but very arid and at altitude the sun gets very hot. Our tomatoes get too hot to grow well, and we have a bed we're using for root veggies we'd also like to cool down a bit. I've read here and elsewhere about lightly shading the tomato plants, and I'm thinking I need to provide some shade to that root veggie bed. My SO suggested taking one of our fruit trees and planting it between beds to provide some shade and use water and space more efficiently. Even if it doesn't shade the tomatoes (which definitely need less shade than the root plants), I figure shading an adjoining bed will cool down the air around the tomato bed. The beds will be almost due north of any tree planted.
The trees we have to pick from that need to be moved around our yard are: crab apple, peach and pear. None are full-sized so it's hard to judge how much shade/how long a shadow they'll provide.
Here's the brunt of my question, I guess: How tall a tree will provide too much shade? I think the crab apple might get to be 10 feet tall, the peach and the pear up to 20 feet each.
Anyone have any ideas or similar experience to share? Or is using trees to partially shade veggie gardens just out-right nutty? 
= new reply since forum marked as read
Actually woke myself up thinking about it (she admits while typing at 4am). Not only is there the problem of a tree needing to grow/growing too big eventually, but a tree is permanent shade. Fine for certain crops, certainly, but it doesn't lend itself to crop rotation very well. I'd been thinking crop rotation was only applicable to gardens planted directly in the ground, but I after I posted this I found a web site that talked about crop rotation in raised beds, which of course makes sense. So... I'm now thinking a moveable "wall", such as you describe, or even a clothes line I could hang old lace curtains from would be smarter options over the long run.
Thanks! 
And had everything in raised beds, which have so many advantages.
After I moved South I realized how different gardening here was, where the main problem is sun and keeping the jungle at bay.And the millions of garden and people eating insects.
The best option turned out to be a lightweight "floating" row cover, that can put on, pegged in and left loose enough for all the crops to grow as tall as they needed to,
get plenty of sunlight, not too hot, and keep the bugs out.
( obviously we are not talking about corn or tomatoes)
Actually, I have used those white polyester sheer curtains from the Dollar Store with success, they are very re-useable.
Tomatoes I actually can grow in light filtered shade under existing tall leafy trees, they do fine down here.
Problem is summers are getting hotter, so I have to think shade more often than usual.
I love that you were gardening in your sleep! I know the feeling well.