Answer: Based on your description I think they are suffering from too much sun and inadequate watering. (Overfertilizing can also cause foliage problems, as can early season frost damage.) This is a plant that needs shade. It does well in bright dappled light all day, or some gentle morning sun. Full sun all day is too much.
Also, they do best in soil that is organic and humusy and evenly moist like a wrung out sponge, not saturated/sopping wet and not dried out. It is better to water deeply but less often, than lightly every other day. To know if you need to water, dig into the soil with your finger. If it is still moist, do not water yet. A slow, thorough, deep watering will reach the deepest roots and hydrate the plant thoroughly. After watering, wait a few hours and then dig down to see how far it went; it can be surprising.
Using an organic mulch helps keep the soil evenly moist and also helps feed the soil slowly over time. Apply it in a flat layer two to three inches deep over the root area but do not allow it to touch the stems.
I hope this helps you trouble shoot.
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