The single word is better than the hyphenated one for numerous reasons, mostly the traditional assumption by most people.
But I would also purchase and redirect the hyphenated version to the single word one. I think this greatly helps seo as well as prevents someone else from picking it up and maybe diverting your viewers.
When I first heard about the seo potential I added the hyphen version of one site and immediately appeared on google page one for the key words used.
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Good Success! Douglas County Master Gardeners
"We don't always get what we want, but we always get what we expect."