Some yellowing of spikes is natural. As the spikes age, they turn from green to yellow, then tan before they die. If just a few spikes are turning, you can trim them off. If you see a great deal of yellowing, it may be an indication of disease or insects or an environmental problem such as overwatering. In these cases, the yellowing is more likely to be generalized throughout the plant. Check to plant closely for additional signs of insects or disease, and control those pests, if present.