Last year, a rose in my garden had almost white leaves with green veins. That rose did not make it through the winter. A group of seven emerald cedars are characterized by lighter coloured foliage with some small areas dying. Could chlorosis be the most likely problem? If not, what are some other reason causes?
It sounds like the rose which died over the winter was suffering from iron chlorosis, and this document from the Marin Rose Society describes the issue and offers diagnostic possibilities and solutions. We hope that you will find it helpful.
With regard to your emerald cedars, Thuja occidentalis ‘Emerald Green”, the yellowing of foliage and small areas dying could indicate the presence of Spruce spider mites, Oligonychus ununguis, or red spider mites whose piercing-sucking mouth parts enable them to suck the juice from the foliage which will turn yellow then brown. The infestation of mites can be detected by the webs that they spin over the branches so search inside the foliage for the fine webbing, the mites themselves are very tiny.
Light infestations can be treated by spraying plants with a forceful stream of water taking care to include the underside of the foliage and the woody material. Do this every day for three days. If mites continue to be a problem, spray with insecticidal soap every three to five days for two weeks in order to interrupt the cycle of eggs hatching. It is advisable to spray the soil as well as the plant.
Good cultural practices include planting in full sun or at least partial shade, and providing plenty of moisture during the growing season. Apply a mulch of compost or other organic material to help retain moisture as well as to feed the plant, but keep the mulch several inches away from the trunk.
We wish you success in returning your cedars to glowing good health.