Answer: I'd use bamboo skewers to stake up the stems that are falling over. Standard plants can reach 4' in height so your miniature mixture will probably grow at least 12" high. If the plants are healthy, and if they are getting adequate sunshine, the height is normal. If they are not getting enough sun, the stems will elongate and may not be able to support the leaves and flowers. Since you've already gotten them established in your planter boxes, I'd just stake them up.
Best wishes with your plants!
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