Answer: This is usually a trouble free plant and based on your description I am not certain why the tips would be turning brown. Since the other ones are fine I would expect your planting technique and care are good, so I would look for an environmental cause. If these are new plants, it may due to overly dry soil. Sometimes plants in the same area will somehow recieve more or less water due to say drainage patterns, location near a building that blocks rain from hitting the ground, or some other factor. You can check this by observing and digging down into the soil a bit to see. There might also be something happening such as de-icing salts washing into the soil (this would cause salt to build up there and could cause root damage) or accidental herbicide contact such as from a rotary lawn spreader. Overfertilizing can also cause root damage which is signaled by foliage symptoms.
On the other hand it might be due to poor rooting, if the plants were rootbound at planting and those encircling roots were not cut or untangled and directed outward. If this is the case they can be lifted, the roots cut and then re-set, then be certain to keep the soil evenly moist until it freezes this fall. Your goal in watering is to keep it damp like a wrung out sponge, not sopping wet and never bone dry. Water less often but deeply so it soaks in and encourages deep roots, this is better than a daily light sprinkling. If these are new plants and the problem continues, you might check back with your retailer as well.
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