Answer: Drought is plain awful. First of all, do not expect plants to actively grow -- simply hope they survive in good enough shape to make it through winter and into next spring. Plants that are in general good health with have the best chance.
The lawn will go dormant and will probably recover when it finally rains. Most long planted trees and established shrubs with deep roots will manage somehow. The newly planted and the naturally shallow rooted plants such as rhododendrons and azaleas (and those that prefer ample moisture) may suffer terribly and may even die outright.
The best you can do for stressed shrubs and newly planted trees is to water slowly and deeply when you can (once a week) and maintain a layer of several inches of mulch to try to keep the soil cool and moist. A drip system or hand watering slowly with a bucket or even barely leaking milkjugs set in a circle over the root zone next to the plant can all be effective. You may need to dig down to see how moist the soil is.
The most water-saving way to water is to avoid evaporation as much as possible. Water in the evening or early morning, apply it directly to the roots. Apply slowly so it can sink in. Avoid sprinklers if possible and definitely avoid the daily light sprinkling.
Perennials may surprise you by looking dead but coming back from the roots when it eventually rains or perhaps even next spring. The best defense for them is to prepare the soil well prior to planting -- add lots of organic material to help the soil structure hold water and then water only deeply if and when you do water. Again, mulch is very important.
When you truly need to be selective about what to water, it is terribly difficult. My personal method is to select certain plants I need to keep -- the trees and more valuable or strategically placed shrubs and water them because perennials are more easily replaced. I will also make an effort to water plants that are severely wilted or are wilting early in the day before others that seem to be lackluster but maintaining some level of turgidity. Remember that wilting is worst in the late afternoon, too.
Probably the last thing you should do is fertilize; plants will actually die back to try to balance their tops with their roots/water capacity, so forcing new top growth is harmful during a dry season. You may see trees and shrubs defoliate or shed branches, but do not assume they are totally dead at that point. Sometimes even just one good watering can make the difference. Water them if you can and wait and see -- they may rejuvenate just fine next spring.
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