I have an orchard. I put in posts and wire to suspend the main line around 2 feet off the ground. There is some buried main line as well, but I don't like having too many buried lines because if they should leak it might take quite some time before you notice, and quite a long time to find the leak location. Burying one relatively short line shouldn't' be a problem, though.

You could use garden hose as your main line and move it when you mow.

You can connect your system to use the rain barrel at all times. Use your garden hose to manually fill up the rain barrel when needed. Or you could even use a float valve to automatically top off the rain barrel from the hose. Then everything would be zero pressure.

I think a hose timer will work OK with zero pressure. So you could have branch A be main line, back-flow preventer, pressure reducer, hose bib A. Branch B would be water barrel, hose bib B. Connect the hose timer to hose bib A when you are home, and hose bib B when you are out. Don't forget to turn on the hose bib before you go on vacation!

If you can get your rain barrel 10 feet above your garden, everything will work much better. If you only have one or two feet of pressure, even drip irrigation might not work very well.

I believe most drip irrigation is designed to work at 5 psi (hopefully someone will correct me if I am wrong). That is like 11 feet of water pressure, I believe.

--McKenzie