It is normal for new growth to sometimes get a tad yellowish but the older leaves suggest iron chlorosis. That's usually triggered either by alkaline soil (this prevents the uptake of iron thru the roots) or by too much rain (the wet soil also ends up preventing the uptake of iron thru the roots).

To treat alkaline soil, you can add garden Sulfur, aluminum sulfate, iron sulfate, greensand or liquid iron-chelated compounds to acidify the soil. The liquids are faster acting and acidify the soil and provide iron that the roots can more easily absorb. But they have to be applied more often. Follow label directions (amounts to apply and the recommened frequency) in order not to burn the small roots. A/S should not be used around azaleas and rhodies.

To trest cases of too much water, water only when the soil is almost dry or dry. If water is accumulating in this area then consider how to change the flow of water elsewhere.

I would also add 2-4" of mulch up to the drip line and get rid of the grasses/weeds under the hydrangea. The mulch should minimize the chances of weeds coming back.