A few suggestions: platycodon(balloon flowers),the shorter variety,,,,,,geranium cinereum;ballerina......coreopsis....sweet williams,dianthus....blue asters.......lavender.....sedums........geums.....yarrow.....silver mound for leaf texture.....liriope,landscape greenery but they do flower........

carol

I never knew they had a short variety of agastache.
Here are some pics of sun worshipping front row plants that stay under 18".

Coreopsis 'zagreb'
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Scabious 'blue butterfly'
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phuopsis stylosa
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penstemon fruticosa
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nepeta
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Also try 'red fox' veronica, 'first blush' euphorbia, ajuga, dianthus, geraniums, creeping phlox......the list is long for possiblities.
flora

Dianthus 'Bath's Pink', 'Firewitch', or the new one 'Wicked Witch'. Campanula carpatica, camp. 'Blue Waterfall'. Brunnera macrophylla 'Jack Frost'. Phlox subulata. Anemone pulsatilla. Dwarf everblooming daylilies like 'Going Bananas', 'Purple D'Oro', 'Stella Supreme','Rosy Returns', 'Red Hot Returns'. Pulmonaria. Dwarf asiatic lilies. Hardy chrysanthemums (pinched back to make compact.) Dwarf asters e.g. 'Snowflurry', 'Wood's Pink', 'Wood's Blue'.