Picket fences aren't usually meant to be edged. The soil underneath can be neatly levelled though underneath but it is important the soil does not touch the wood to prevent any possible rotting. A photo would be useful though to clarify your post.
As Lizzie said really. Except I have seen one locally that are panels set between concrete posts like you would have with more traditional panels. They have put a narrow (height) concrete gravel board below. Sounds naff but looks quite neat. Though it is a more chunky wide not so traditional "picket" it is still about a metre high.
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Though it is a more chunky wide not so traditional "picket" it is still about a metre high.