Answer: Crepe myrtle needs full sun and a well drained location, so if possible plant it on a slight slope. Do not plant it in a low spot or where a gutter drains, for example. This is especially important with a clay based soil. You also want to avoid a spot that receives strong winter winds. Allow ample space for the plant to mature. Crepe myrtles come in a range of sizes so without knowing the cultivar I can't tell you how large yours can potentially grow. (Dwarf can still mean rather large, depending.)
At planting, loosen the soil over a wide area, several feet across, and as deep as the container. Leave the sides of the hole rough, not smooth and polished as clay can do. If the roots are growing in a circle inside the container loosen them or cut them and direct them outward. Plant it at the same depth as it grew in the container or a half inch higher, and water thoroughly to settle the soil. Then mulch with several inches of organic mulch over the disturbed area, do not allow it to touch the trunk, however. Notice I have not suggested fertilizer at all. New plants do not need it and a crepe myrtle should not be fertilized late in the season as this can encourage late growth that will not be hardened enough to survive the winter.
The most important thing you can do is maintain soil moisture. Keep the soil evenly moist like a wrung out sponge, not saturated and not dry. To know if you need to water, dig into the soil with your finger. IF it is still damp do not water yet. When you water, water slowly and thoroughly so it soaks down deep and encourages deep rooting. This is better than sprinkling lightly every day. After watering, wait five or six hours and then dig down to see how far the water went, it can be surprising. Clay soil can hold moisture for a long time, but once it dries it is difficult to re-wet, so check it often.
This fall after freezing weather, add a deeper mulch. Next spring be very patient waiting for it to come back into growth. These leaf out very late. You may also find that it has died back to the ground; if so, trim it back and hope for new growth from the roots. Be patient with it, especially since it is being planted in the fall rather than spring. Good luck!
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