Answer: It will take time for them to develop caliper and stature, just as it would for any tree. According to your zip code, you are gardening in zone 6. This is toward the northern end of the range for crepe myrtles and this is of course a plant that prefers warm weather -- lots of heat and sun. So they will grow somewhat slower due to the shorter growing season than they would enjoy if they were planted in a warmer climate. This is a relatively small crepe myrtle and its growth rate is not quite as rampant as, say, the species. How fast yours actually will grow is going to depend on the site where they are planted and the weather each year, but I would expect at least a decade or more before they begin to look at all imposing; you might see a foot or so of height added each year. As far as helping them grow faster, do not be tempted to overfertilize as this can cause soft growth that does not harden off before winter cold sets in. The best you can do is make sure they are planted in a sheltered spot out of winter wind with a warm microclimate, (if not maybe you can provide a windbreak each winter) fertilize each spring with a top dressing of compost and a slow release, complete, granular fertilizer such as 10-10-10 per the label, water as needed to supplement rain to keep the soil slightly moist, and mulch year round. Use an organic mulch several inches deep spring through fall, then in late fall once the ground has frozen you can deepen it a bit. In early spring as it begins to thaw, pull some of the mulch back again so the soil can warm up sooner in the sun. Mulch should always be in a flat layer over the root area, never allow it to touch the trunk. Also make sure you are training each one to just a few stems, three or so. If there are too many more than that they will not size up as well. I hope this helps.
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