Answer: Based on your description I am not certain what is happening. It might be due to incorrect watering or poor rooting, or perhaps there is a problem with the soil such as overfertilization, winter de-icing salts, excessively high pH, or even watering with water that has been through a softener system. It might be that the plants were potbound or root bound at planting time and were planted with encircling roots, these would then continue growing in a circle and strangle the plant. It might be also be some sort of disease problem. It is very difficult to diagnose this type of symptom long distance so I would suggest you consult with a certified professional nurseryman and/or your county extension to see if they can help you troubleshoot.
In the meantime, make sure you are not overfertilizing (a springtime top dressing with good quality compost and/or a general purpose granular or slow release fertilizer with an analysis of 10-10-10 should be adequate. Be sure to read and follow the label instructions. It is better to underfertilize than overfertilize and risk "burning" the roots. If you are using a water soluble fertilizer, also be sure to read and follow the directions -- this can damage the foliage if over applied or misused.
Over watering and under watering can both cause problems. Watering is done if needed to supplement rain and keep the soil evenly moist. It should not be bone dry or sopping wet. Your goal is damp like a wrung out sponge. To know if you need to water, dig into the soil with your finger. If it is still moist, do not water yet. When you do water, water slowly and deeply. Apply it at ground level, not to the foliage. Then make sure it soaks down deep to encourage deep roots. It is better to water less often but deeply than to sprinkle lightly every day. Watering in the early morning or evening are both fine, but avoid wetting the foliage to reduce the chance of fungal or bacterial infections.
Good luck with your plants!
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