Answer: Based on your description, I think this is most likely a soil related problem. The plant may not have rooted into the surrounding soil, or the soil is too dry.
If the soil was not well prepared prior to planting, or if the plants were rootbound and the encirclign roots were not untwined or cut at planting time, the roots may not be growing outward, thus stunting the plant.
If you are growing it in a container, one geranium needs a large pot, say about 12 inches across. It should be filled with good quality potting mix formulated for container growing. Fertilize with a slow release granular fertilizer or with a water soluble fertilizer per the label directions. Overfertilizing can cause "fertilizer burn" and stunt the plant.
Although quite tolerant of heat and needing a well drained soil, your geraniums also still need to be watered so the soil stays evenly moist like a wrung out sponge. It should not dry out entirely. Overly dry will also cause stunting at the roots.
You might also check your soil pH. For geraniums, it should be about 6.5; if the soil is more acidic then that can stunting.
Another possible cause of stunting would be accidental chemical exposure or herbicide exposure.
I should also mention that some of the zonal geraniums do have colored foliage patterns that can change slightly due to increased or decreased light levels.
I hope this helps you trouble shoot. If none of the above seem plausible, you might want to consult with your local county extension for a more specific diagnosis.
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