Answer: There are two common reasons for this to happen. The first is water stress, and you may find that there are minor differences in your watering pattern or soil or mulch layer amount of shade they receive to account for this. Also, some plants are possibly just less well rooted than the others which could explain it. The second is early frost damage which happened just as the leaves were unfolding or emerging. Often this damage does not show up until much later in the season when the plants are fully leafed and the vascular system is attempting to work at full capacity.
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