Answer: It is not always necessary to stake trees when you first plant them. If the crepe myrtle is subjected to wind you might want to stake it (use 3 stakes, equal distance from one another). If your garden is not in a windy area, staking probably won't be necessary.
As for the crabgrass type weeds, you can use a selective herbicide (one that specifically targets grasses but not broadleaf plants). However, if the bed is completely overrun with the weeds, it might be simpler to dig everything out, roots and all (save the desireable plants) and then start all over again by replanting your phormium and liriope in a clean bed.
Enjoy your garden!
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