Yellowing of old leaves -with no dark green veins- could signal heat stress, too much sun or a nitrogen deficiency.

Too much sun affects only the leaves in direct contact with the sun so if shaded leaves are affected too then this may not be the cause of the problem. Morning sun and afternoon shade (or dappled sun) will prevent the leaves from getting scorched by the summer sun in warm areas.

Soil kits are available on some plant nurseries and can help you tell if the soil needs (or does not need) nitrogen; you can add the fertilizer that you never added or use blood meal (high in N). While at it, make sure that the soil has not water issues (too little or too much). A ph kit can also tell if the ph is too low and the plant has difficulty absorbing some minerals at low levels.

Wilting is common during the summer months so water if the soil feels dry to a depth of 4". About 1g of water per watering should be fine. Do not over-water as this causes wilting too due to root rot. 'Normal' wilting is something that the shrub can recover on its own if the soil is moist. Root rot wilting episodes develop and 'stay' 24/7.

Unfortunately. home soil testing kits are notoriously inaccurate. I wouldn't waste my money :-) And even professional soil tests often don't measure N, as it is the most mobile of the plant nutrients and is almost always required. Mulching with compost will supply adequate levels.

The leaves don't look all that bad to me. Often the paler color is due to insufficiently acidic soils which makes it difficult for the plant to access iron, resulting in pale or yellow colored foliage. And that is a common situation in our area as our soils are typically only slightly acidic (usually around 6.3-6.5). Fertilizing with a water soluble acid lovers fert - like Miracle Gro for Acid Lovers - acts almost like a tonic, supplying iron and greening up the foliage very rapidly. Since soil acidifers take some time to become effective in the soil, this may be all you need for now.

I agree that the leaves do not look too bad but they do not look iron deficient to me though. The leaf veins look yellow, opposite to what one would expect with iron deficiency (ie, probably deficient on another mineral due to high pH, etc). Adding a fertilizer, manure/etc will add whatever may be missing. But I would still check the soil. Yeah, the kits are not accurate but you do not need accuracy, just an idea if they are high or low.

Thanks everyone.

I think it's got enough water (I live in Seattle, so hard not to have enough water this time of year) and it doesn't have too much sun. So sounds like I need to fertilize/mulch with compost and maybe more acidifying. I was avoiding it at first as I've read that in some cases if a plant looks sick, fertilizing may make things worst.

I've tried the home soil tests, and didn't find they were great. I've sent soil samples to UMass soil testing, and they do pretty elaborate testing - not sure how accurate it was though.

You have received good comments, but I just wanted to add that I don't think you have a problem at all. If the plants are blooming, strong, and appea healthy, I would guess this change will be minimal and short-lived. Happens to many of my hydrangeas as they set buds and begin to flower, then the yellowing stops. Nice Nikkos!

If I lived in zone 8 my garden would be sponsored. LOL

Looks fine to me. If there were, say over 30% if total leaves looking bad, I'd be worried. The blooms look healthy on their own with no droop, so that's a good thing as well.

I live in Shoreline - just north of Seattle. The two Nikko Blues that I just planted out - 2 year new plants looked very much like yours a couple of days ago. It got really hot - at least for us - and so I watered very deeply and they came right back up. The yellowing of the leaves are very much the same for the three we have in the new location. I will be adding a bit of fertilizer in the fall because there was a lot of peat moss - previous owners - where they are now planted. The leaves on the mother plant - in another location are just fine but they are naturally a lighter green than most of my other hydrangeas. Several others varieties are doing just fine in the new garden - only the Nikkos seems to be stressing a bit. Today it was considerable cooler and they look just fine.

Thanks much.. it was very hot last week (high 70's/low 80's).. I'll make sure to water more when it's that hot and also do some fertilizing...