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If you would like a multi-trunk tree with 3 or more trunks you would want to select the 3 or 5 best, most upright growing ones 🔥 After selecting the best trunks remove all the rest, cutting them off as low as possible to the ground 🔥 You should do this in the late winter to prevent new growth from appearing in spring 😉 You can then remove any lower branches that are not needed to reach the top of the trunks. Be careful not to remove branches that could damage the form of your canopy. The top can be trimmed if the branches are too long or spindly. From the area where you made cuts, two or more branches can grow. You should not trim too many of the lower branches as this could cause too much damage to the canopy. The canopy’s lower limbs are usually located between 5 and 6 feet above the ground in most crape myrtle trees. This cultivar is smaller in size and may have side branches that are too long. It might not be enough to form a full canopy if this was the case. The smaller bush crape myrtles may still be a good choice. Pruned to be smaller trees These plants look amazing in any garden. You may not need to walk under their canopies and therefore a smaller tree It works well and looks great. You can find a great article on Gardenality here. Prune the branching to develop a nice canopy each year after your shrub is pruned into a small tree form. 
I am a novice gardener and need some assistance. In early 2006 my backyard was landscaped and three crepe (crape), myrtle trees were planted. Two were already mature and had “tree (upright form) when they were planted. The third tree, the crepe myrtle Acoma, was unavailable as a mature, so they were planted. Planted a little tiny seedling and told me it would grow The tree is now a lovely specimen. After leaving it alone for 3 years, I am now seeing that it has become a bush instead of a tree. It stands at 4 feet tall by 7 feet wide. It is currently 4′ tall and 7′ wide. 
Crape Myrtles are TREES and that means they grow TALL from 20 – 40’ in height. The Crape Myrtles have been affectionately called the Lilac of South. They are scented and bloom for a long time during the summer. The exfoliating bark of Crape Myrtles is so wonderful in the late spring/early season that I remember as a child, peeling off the trunk and discovering a velvety, cinnamon-coloured trunk. It is very useful. Leaves also provide great fall colourr from yellow If it is sunny, the plant can go from yellow to red. We have here a plant with an exceptional summer bloom time, wonderful fall foliage (not many trees can do that in this area), and a beautifully sculpted trunk if it is allowed to naturally grow. We thank Silas Ryder who highlighted this. 
To create a 3 to 6 inch tall dike of soil, use your hands. It will trap water and prevent it from sinking into the soil. Soak the roots in water. To retain soil moisture, spread 3-6 inches organic mulch on top of the root ball. Use straw, leaves or pine needles. To determine what nutrients your soil is lacking and which fertilizer to use, you might consider a soil test. It will help you to prevent diseases like leaf spot.