Answer: Unfortunately based on your description I am unable to tell you why they are browning. Browning (particularly at the base of the plant) can sometimes indicate planting too deeply or overwatering, while browning in other areas can indicate drought stress, or poor rooting, or a pest or disease problem. Sometimes browning of just the oldest, interior foliage is normal.
Grass clippings can be a good mulch if spread in a very light layer (too deep and they can pack together and exclude water) or mixed with other mulch materials and as long as they are hercide free. Generally a good organic mulch for a shrub planting would be old rotted leaves and/or shredded bark or wood chips. It should be in a flat layer over the root area, about three inches thick, and should not touch the bark or stems of the shrubs.
It is not a good idea to fertilize new plants that seem to be stressed. Instead the most important thing you can do is water correctly. To know if you need to water, dig into the soil with your finger. If it is still damp you do not need to water yet. When you do water, water slowly and thoroughly so the water sinks down deep and encourages deep rooting. Less frequent deep watering is better than a daily sprinkling. After watering, wait a few hours and then dig down to see how far the water went; sometimes it can be surprising.
Since these are new plants I would strongly suggest you consult with a professionally trained and certified nurseryman to try to troubleshoot the problem. I hope it is nothing too seriouos.
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