I hope you planted a cr@p ton of spring bulbs in that bed ;)

I don't think it's missing anything, the plants just need time to grow and fill in.

I wouldn't hedge the back, I like the informal view of the trees. There isn't an ugly view to screen out so leave it natural with maybe a couple of shrubs.

I picture a garden bench on the lawn and a pedestal birdbath near the tree.

The hedge is not necessarily a bad idea but we can't see your whole yard so we don't know. Are you trying to block the view of some of the trees in an open yard, and want a sense of enclosure? Or block a view of the neighboring house? Maybe add a touch of formality?

I agree the open feeling around the tree is nice and the beautiful tree trunk. Don't hide the base of the tree.

For flowers, buy a few sixpacks of seasonal flowers, and plant them in the bed along the border of the lawn.. Do this every three months. Pull out any sad plants. New colors will be in the stores like pink in Spring etc.

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The hedge could be of any height.