Answer: Since your Hershey reds are blooming well, I would not expect your fertilizers or iron additive would need to be different for the Gumpos unless they are in a different location and the soil is rather different there. If this is the case, you might want to run some basic soil tests; however the fact that they have lived for so long and reached that nice size would seem to indicate that they are healthy.
Another location-dependent factor on blooming can be winter low temperatures. If the Gumpos are in an exposed location out in the wind, or away from the protection of a building, perhaps winter cold has been affecting the blooms. I doubt this would be the case, however, because we have had several very mild winters recently, but it is possible because they are actually less hardy than the Hershey Reds -- some authorities rate the Gumpo only to the zero to 10 degree range or zone 7.
Yet another factor can be shade, if the location is too shaded they may not bloom as heavily as they would in more light. Again, though, if they are in the same location as the others, I would not expect the light to be a factor.
This leads me to wonder about pruning. If you tend to tidy up the plants each spring or summer, you would want to take into consideration that the Gumpos bloom quite a bit later -- by weeks -- than the Hershey Reds. Is it possible that you have been trimming off flower buds by accident? Any pruning would need to be done right after blooming and yet before July when the new buds begin to set for the following season. Pruning in August would remove blooming wood for the following June bloom season.
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