I'm guessing it depends on the quality of the sail you purchase. We installed one for a client this spring that we purchased via the actual ShadeSail website (middle price range, stock item) and it looks and functions great. I'd recommend talking to the tech support folks where you purchase the sail (which probably rules out Amazon) about the tree attachment. Pine's a softwood and with the amount of tension the sails are under, I'd worry about a screw eye popping out spectacularly. If you read the install instructions they specifically say a 6x6 post will not withstand the forces, so you know they're pretty intense.

Thanks for your responses. After posting my question, I found sites with information for DIY installation, and I think maybe we are not ready to do that without a bit more prep and information. We're going to just hang some patio shades for the moment and take more time to think about the options. I think the sails are a very good idea, but I guess before make a permanent installation, I should have a better plan. Thanks everyone. Valerie

They need sturdy posts per mfg. specs. Rectangular give better cover.

@Yardvaark That is surely a concern to think about. Thanks for the tip. I do like the idea of the roll out shades but we are looking for something low cost. So may land up with a good old patio umbrella if nothing else.