Can I use ph down mixed with water to lower soil ph when soil is still wet?

I hope you have a soil pH tester. You can do two things. Either mix the pH down to a level just below where you want to be or you can get a soil sample and see how much it takes to get the pH down to the desired amount.

I am not sure if you are wondering about absorbing the water or if the issue is further dilution. In either case the answer is yes you can.

tree is under stress from wrong soil PH, lack of water missing correct fertilizer, excessive water, wrong soil ( clay or other non water absorbing material
talk with a nursery professional and get advice

I love these plants for their ease of growing and spectacular colours!
The answer to your question partially depends on what colours you have or want. Other than that they pretty much grow anywhere in a good sunny spot, and unless your soil is terrible, they dont need fertilising, just plenty of water. If you do fertilise then everyday fertiliser is fine but depending on its composition it may change the colour of the flowers.

This excerpt is from a web page listed below:
"Hydrangeas are amazingly versatile in that you can actually alter the flower colours to suit your needs. The flower colour in most forms relates to the acidity or alkalinity of the soil. In acid soil (pH 5 or less) hydrangeas are usually always blue. As the soil pH climbs towards the neutral and alkaline end of the scale (pH 7 or more) hydrangeas turn mauve, pink and red. A blueing tonic (containing aluminium and iron) will turn pink or mauve hydrangeas blue. It should be applied once a month in March and April and again in August, September and October, following the directions on the pack. A cup of lime (calcium carbonate) added to the soil in spring will cause blue or mauve hydrangeas to turn pink. White flowering hydrangeas will remain white regardless of soil pH."

http://www.burkesbackyard.com.au/fact-sheets/in-the-garden/flowering-plants-shrubs/hydrangeas/#.WAWfKzJg3cM

I know this is an old post and I didn't see it until now. I missed helping last year, but if you have the same question....

They grow best in rich, well-drained soil with a basic pH (they like more alkaline than acidic). If your soil is naturally acidic, adding lime will help to provide these conditions. They are also heavy feeders. The addition of compost or manure to the soil as well the use of granular fertilizer will result in healthier, stronger plants.

check you soil ph sometimes the soil Ph can change. and can cause this. also try some plant food in water. out side plants sometimes need some food over the cold months. also if it gets to cold for to long it can kill the plant off.

Every 2 weeks , in a frost free enviroment .

pH of water is important. Buffer pH at PH 5 . Then add 2% Roundup. Homesprayer is 1 lt therefore 2 % is 20 ml.
in a 20 lt Rucksack sprayer a 2% is 400 ml.
If weeds are above kneeheight and thick use a 3% solution.

Very important to buffer the pH of the water at pH 5. Then add Roundup /Glyphosate 360 at a 2% for grass weeds and 3% for brush weeds. Ensure that the grass is dry when treating and no rain is expected within 6 hours. Please take care with roundup as it is poison .