Answer: Most flowers do better when watered deeper but less often, so that the roots are encouraged to grow deep where the soil naturally stays moist longer. The rule of thumb is an inch a week from the sky or the hose, applied in one long soaking rather than by daily sprinklings. The best way to know when to water is to dig down into the soil and see if it is dry or not. Teh best way to judge the effectiveness of your watering is to water, wait about twelve hours, and then dig down and see how far the water penetrated.
Strategies to boost the soil's natural ability to hold moisture longer include adding ample amounts of organic matter such as compost, rotted leaves or aged stable manure and bedding during the preparation phase (a twelve inch deep layer is not too much), maintaining an organic mulch layer several inches deep year round and repelnishing it as it breaks down, and taking advantage of naturally moister (low spot) areas when planting flowers that require extra water such as astilbe. Conversely, drier areas such as south facing or western slopes would be planted with flowers that are adapted to handle a hotter drier situation.
Artificial watering systems all have their pluses and minuses, but proper installation and regular routine maintenance and checking for smooth operation are imperative to get good performance. They should also be set to run by hand rather than on a seasonal timer so that you can take into account precipitation patterns as well as heat or windy conditions.
I realize this is not a direct answer to your question, but it is important to start with the soil when talking about watering needs. With luck, your flowers will be able to handle all but the hottest and driest weeks of August without extra watering, and with a little planning your watering chores will be greatly reduced.
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