We need more information.
What specifically did you plant?
did you till the soil?
did you add compost?
re wild plants...
yes they can take care of themselves... if everything is to their specifications.
if you scattered seed from wildflowers over the lawn... nothing would happen.
those vegetables that you planted... originally came from other places... they didn't evolve in your area.
plastic? no.
mulch? not over freshly planted seed.
mulch the soil after the plants are large enough not to be harmed by it.
re water...
touchy subject...
if the rains are enough, watering shouldn't be required. And... there are things we can do to conserve moisture, the mulch... you've already touched on.
adding compost and/or animal manure, and "green manure", also helps the soil to hold moisture.
to address the why can't the plants take care of themselves question... many can.
at my house, I have climbing beans, crowder peas, tomatoes, calico peppers, maxixe, poppies, iris, and many other things all growing from dropped seed from last year's garden... thing is... I did a lot to improve the sand.
