Plum trees are great trees. They will give you a wind break to your summer garden. Over time they will provide enough leaf drop to greatly improve your soll. In the first few years, you can give your soll a helping hand and cop & drop under your trees anything that you can get a hold of so that they will help keep the weeds down as a mulch and transform into
compost.
I tend to plant my trees about 2 inches above the rest of the ground and then mulch two inches and taper it to the surrounding ground. 9We get enough moisture by winter snows and rains the rest of the year.) Give it lots of water first few weeks and then weekly (if need be) for the first couple of years. You do not want your plants to be in a drought and missing water. They will become drought tolerant in a few years. I think that plum trees are still quite forgiving and will usually do fine with some neglect.
If you heavy mulch around your new trees, then you might not need to water if you get sufficient natural moisture as the mulch will trap the rainwater. Just keep an eye on them.