How to stop saplings growing from exposed or cut roots of a crepe myrtle?
Undiluted Round Up will help kill the roots. You will need to wound the roots (normally, people cut the suckers down and use those wounds) and then paint the undiluted Round Up on the wounds. The bigger or more wounds you paint, the better. The roots will suck the Round Up in and it will kill them. You may have to repeat the process a few times to fully kill the roots if the root system is still strong.
Live in city. Crepe myrtle replaced another tree in sidewalk opening.
Newly planted trees need significant amounts of care. Make sure the soil is tamped down well around the root ball to make sure there are no air pockets and make sure that you are following the watering directions in this article: https://www.gardeningknowhow.com/ornamental/trees/tgen/watering-newly-planted-tree.htm
I have 3 crepe myrtles on my property, around 15' - 20' tall. When I moved in 22 years ago, they were already close to this size. The past few years they are looking thin, and flowering less and less. Are they coming to the end of their lifespan, or do I just need to prune and fertilize - I haven't done much of either. I live in Fla, Tampa Bay area.
It is hard to say for certain. With good care, the trees have a fairly long lifespan. If they have not been pruned, this could be an issue. These articles should be of some help: https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/pruning-crepe-myrtle-trees.htm
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/best-crepe-myrtle-pruning-time-when-to-prune-crepe-myrtle.htm
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-not-blooming.htm
There were no leaves on the at that time. The trees look nicely shaped with lots of leaves, but no buds visible. Should we have blossoms by now?
April is a bit too late for pruning and this may have resulted in the loss of blooms this year. That being said, late pruning will not harm the tree, it only sacrifices the blossoms. The following articles should be of some help to you in the future: https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/best-crepe-myrtle-pruning-time-when-to-prune-crepe-myrtle.htm
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/pruning-crepe-myrtle-trees.htm
I planted a young burgundy with white flowering crape according to directions. I watered well. I noticed overnight the leaves are a little drier and curling. When I planted it, it looked much more moist and healthier. It is in sun and I firmly believe I planted it well. Can it be in shock? I do not believe it has fungus or disease. What can I do. . . just sit and wait?
It's probably just suffering from stress-related shock from the transplanting. Newly planted trees need lots of water for the roots to establish. These articles should help: https://www.gardeningknowhow.com/ornamental/trees/tgen/watering-newly-planted-tree.htm
https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm
This tree is young, planted a couple of years ago. It bloomed but the leaves started getting white on one side only. There is no visible sign of insects and the leaves aren't dying, just white looking on one side. Can you help me?
It sounds like it may be powdery mildew. It won't harm the tree but does look unsightly. If you treat the tree with a fungicide, it will clear it up. This is normally caused by a lack of circulation among the leaves. You may want to look at doing a little pruning to open up the tree a bit and allow better air circulation. This will help prevent this and other fungus in the future.
My crepe myrtle is not blooming. Does it need a certain soil and/or fertilizer?
If your soil is lacking phosphorus, this would inhibit blooming and a soil test would confirm it. A phosporus-rich fertilizer or bone meal will help. For additional suggestions, this article will help you:
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-not-blooming.htm