Answer: When plants in a group perform differently like this there is usually some sort of explanation. One might be that they are in a colder microclimate due to say exposure to more wind in the winter, or the soil is wetter or drier where they are planted, or some other aspect of the growing condition is different and it has slowed them down. Another possibility is that they did not root as well and become as well established as the healthier plant did. If the branches are still alive then you would not want to trim them off because the buds on those branches from last year will be the buds that bloom in the early part of the summer. If however they are not alive, or have died back somewhat, you would trim off the dead portion. (Cut just above live wood.) Buds from lower down on the plant will continue to develop as the season progresses and the plants should be able to bloom on new growth of this year -- that is how they have an extended blooming season well into the fall. You might want to check with your retailer and/or county extension to try to troubleshoot what has happened with the two slow plants. In the meantime you may find the following web site about Endless Summer interesting.
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