green/shade houses for the U.S. South?

So I bought a cheap 5’x5’ish greenhouse online so I would have somewhere to start seeds.  The problem is it gets warm so early here, I end up frying my starts by forgetting to open the greenhouse.  And then I’d like it to double as a cooler place to grow lettuce and such in the summer.  We have very little natural shade, and anything I plant in has to be fenced or otherwise inaccessible because of our dogs.

Any brilliant solutions for me that doesn’t involve machinery?  I live in a subdivision and have limited income.  Do I need to modify my little greenhouse more, buy/build a better one, or is it as simple as row covers?

Nevermind, lol.  We have a trampoline, zipties, hardware cloth, and a few yards of mulch.  

I'm presently growing Goji in an attempt to shade a raised bed. The leaves come out before the last frost but the shade it casts isn't dense.

What about just covering the greenhouse with some shade cloth ?

Kiko Constantinescu wrote:What about just covering the greenhouse with some shade cloth ?

I've tried that, but the greenhouse faces west, so the shade cloth covers its south and north sides.  It still gets too hot in there.  If the door opened to the east, it might work a bit better, though I'd need more shade cloth.  It's a light frame... maybe I could relocate it to spot where it can face east...

I relocated it.  There's a spot on the north side of my yard that gets morning sun, but afternoon sun is blocked, and I can orient the door to the south, so the shade cloth is now covering the east & west sides.  I'm going to plant some lettuce seeds soon and see how it goes.