Do you know how much it costs to buy one of these things? A big boulder, engraved or just painted, especially with a design like those flowers, can set you back hundreds of dollars. And somebody paid hundreds of dollars in this case, for an incredibly heavy typo!
Either a family named Gardner bought it with no idea how to pluralize, which is admittedly the most likely scenario, or a gardener with a lot of pride in his job title bought it (see how the flowers fit in, now?) and then misspelled the job title. Either way, it's proof that the personalized yard boulder industry needs an editor.