I am not sure why your shrubs are yellowing. Such symptoms are usually due to either over saturated soils which drown roots by excluding oxygen, or from drought symptoms which also damaged roots. The fertilizer spikes could burn roots if you put too many of them ride around the plant but this is very unlikely. I suggest you give the plants some time while you try to keep the soil moist but not soggy wet. If the initial damage was not too severe they should survive.
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