A few points to ponder about restoration of your decks
Whether you resort to power washing or use of any other deck cleaners for the purpose, you must see to it that the contents are biodegradable and safe for the environment. You should only use such cleaners that would not kill the vegetation around. Yet some of them might cause a discoloration of the vegetations and tree leaves around. You need not pay much attention to this discoloration as new growth will replace these old disfigured particles in the trees soon.
Your deck could be sealed with the use of good quality sealers after a couple of days of cleaning a deck so that the deck dries up properly. Sealing should not be done unless and until the wood reaches the right moisture level for accomplishment of the task. Ordinarily, after a cleaning even if the sealing is delayed due to rain or some other reasons, no re-cleaning would be necessary even after a week.
If you are constructing a new deck or remodeling an old one, you will have to use a finish to the surface of the decks soon after the construction is completed so that surface degradation is prevented.
Ordinary wood stains may not be decent looking and may require quick removal but some semi transparent stains can be helpful in designing the show of the decks. You will have to use some water repellants too since clogging of water may cause serious damage to the desk and its beauty.