To be successful you have to use nursery grown plants. During the winters you can dig up dormant aspens and conifers, plant in potting soil in pots, keep protected from the cold because they are now in pots and their roots are the most fragile part of a plant. And you might have a 30 percent chance doing this to use a 'free' wild plant and you'll have a larger chance of getting fined if caught. Grins. Especially if caught digging up little Alpine firs. I've been sorely tempted.
I lived in Moscow, Idaho for 20 years. Zone 3. The best trees are: Amelanchier alnifolia (a rare 4 season beauty tree with few to no insect or disease problems) also called Serviceberry, Aspens of course, Choke cherry, Sambucus or Elderberry. The best shrubs are Salix purpurea 'nana'...or Blue Arctic Willow, Paxistima myrsinites or Oregon Boxwood, Acer ginnala, super multistemmed little maple a large shrub or small tree, the Camperdown Elm, Ulmus something Camperdownii, if your zone is 4 then Yews or Taxus are superior conifers that actually grow in shade OR sun, I like Taxus baccata repandens Taxus baccata repandens You must be at least zone 3. Micro climates make a big difference. This yew grew well in Zone 3 with a bit of shelter. Handles minus 28 degrees F. Mulched roots.
Do you own a Western Gardens Sunset book? Gotta have one. There are tons of other species of plants that thrive in S. Idaho.
Update: really need a more precise location for you now. Your expectations of watering them once or twice per week might depend on your soil. When plants are first planted they need regular watering for at least the first season. You might be able to erect shade cloth to protect against the hottest months and prevent too much evaporation. You should also insert 3' of 3" pvc pipe at least 3 of them, drilled with holes down the length leaving a foot above ground with no holes. Insert around the outside of the rootball and one right in the rootball when watering put the water down into these pipes. A number of times before you leave. Soak the soil as much as possible using these pipes. This should last a week in the heat but I would water at least twice per week until your plants become established.
Remove all burlap completely. All twine and labels. Prune branches that are heading toward the center of the tree at the trunk. Use alcohol on your BYPASS pruners. Do not dig the hole any deeper than the depth of that root ball. Do not amend the soil in any way. Do not bury any deeper than the root ball, the trunk has to be clear of soil, mulch, weeds...
Get a soil test. I'd call WSU Cooperative Extension Service. Call them and ask for a master gardener, they might know the pH where you are planting and/or do soil tests for you very very cheaply. WSU has a powerful Cooperative Extension service. Utah might as well...I've used WSU's. I am dying to know why Acer t. 'ginnala' should not be planted in S. Idaho, N. Utah. A soil test needs to be done at least twice for your landscape. One now and one in a few years. Valuable information for success. And not wasting money.