Answer: It is very difficult to grow trees successfully in containers under home conditions and in a cold-winter climate such as yours. The limited root space is stressful on the plants, as is the lack of insulation for the roots against cold as well as being subject to oscillating oscillating temperatures. In many cases they are replaced each spring.
Be sure you are using a large container such as a half whiskey barrel and are using a good quality potting mix formulated for container plants. There must be a drainage hole in the bottom of the container. Feel the soil with your finger to see if you need to water; if it is still damp, do not water yet. If it is dry then water slowly and very thoroughly. The goal is to keep the soil evenly moist but not sopping wet or saturated, and never bone dry.
You might experiment with a smaller-growing cultivar of the crabapple tree (spring bloomer and fall color, but deciduous so it will drop its leaves in the fall) and possibly junipers (needled evergreen) and see how they do for you.
Another option might be to try a fast growing flowering annual vine such as the morning glory and grow it on trellis in each barrel. This would provide both height and blooms, and you can pull them out and discard them in the fall before they lose their leaves -- they will be killed by a hard freeze.
If fall leaf drop is a problem, wait until the leaves on the deciduous tree begin to fall and then pick off the rest by hand. Or, consider placing screening over the pavers and then use a leaf vaccuum to gather them up. Keep in mind that evergreens also shed some of their oldest needles every year, depending on your drain they may or may not cause a problem. Certainly it is critical to prevent the drain from becoming blocked.
Good luck with your project!
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