Yes you can plant in summer! Many are fearful of it and believe summer planting is a bad idea, let's go over how it can actually be a good thing. The first thing to understand is that we still have many hours of sunlight during each day. This means plants are actively growing and will be able to send large amounts of energy into their root system which is critical to ensure the plants survive the winter. The more root-mass a plant has, the more moisture it can take in when it begins storing it in the fall. Roots will even continue to grow well into fall even though the above-ground portion of the plant is going dormant. Getting a head start by planting in summer is especially beneficial to broad-leaf evergreens and conifers since they lose a great deal of moisture through their exposed foliage during the winter months. Deciduous plants are less susceptible to moisture loss since they lack foliage in winter. Perennials also benefit from summer planting since they have time to root in. Planting perennials in late fall can cause them to heave up, popping them out of the ground when the ground freezes and thaws. Another benefit of summer planting is in plant selection. Gone are the days of a big up-tick in fall planting which this industry enjoyed for many decades. The "fall is for planting" theme has all but disappeared for whatever reason. Garden centers just don't carry the levels of product that they used to in fall. Summer inventories are still relatively bursting in summer with large selections of flowering plants, trees, and evergreens.
A word of caution though; remember to water! You will not have success if you just plant and forget. Proper watering will be necessary to prevent plant failure and must be consistently maintained. Depending on the plant, watering frequency may be daily, weekly, or a few days per week. If going on vacation it is a good idea to have someone take over watering duties for the time you are away. Saving gray water from your kitchen sink for your plants is a great conservation technique as well as setting up rain barrels on your property to collect rainwater. For trees you can purchase a 'water bag' watering system which releases water over the course of a week or two depending on the product. Continue to keep the soil moist all the way until the ground freezes, an unusual fall drought can be detrimental to plant health.