Hand watering, though very gratifying in the sense that one is directly nurturing the plants, is most likely the least efficient and reliable way to water plants. People love to water in full sun during the day, which leads to high evaporation rates. Also, in splashing what appears to be a large volume on top of the soil, folks often think they have really watered the plant, when in fact, soil can only absorb limited amounts of water at a time. In fact, if you come back to a hand watered area after the water has been absorbed, generally you will find there is water in the top inch or two of soil only.
Irrigation is much more efficient than hand watering for a number of reasons. First, irrigation systems sprinkle or drip water over the soil emulating rain, and allowing the soil to absorb water a little bit at a time. Irrigation systems can be programmed to run at dawn or dusk, when there is far less evaporation. As a general rule, lawn irrigation spray heads for example, are eighty percent more efficient than hand watering. The problem with spray heads of course, is they were developed for lawns, and not for perennial and flower beds.
As mentioned earlier, many of the perennials, shrubs, and ornamentals commonly available, evolved in different environments, so why would you deliver the same amount of water to them? With the exception of water plants, all plants need varied levels of moisture for stronger roots. Trees for example, generally appreciate deep heavy watering with a week r so to absorb moisture allowing roots to grow and reach for more. Shrubs similarly will want water twice a week, and smaller perennials perhaps three times a week. These generalities regarding timing will not apply to drought loving plants of course, or soils that are more saturated with moisture because they are located near a body of water, but they illustrate that watering a bed of mixed trees, shrubs, and perennials with a broadcast spray head is unlikely to deliver the needs of all the plants in the bed. Instead, look for ways to deliver plant specific watering.
Drip irrigation is the simplest means to deliver plant specific watering. Drip irrigation is also sixty to eighty percent more efficient than spray head watering. Drip systems were invented by the Israelies for growing fruits in the desert with minimal water. Israely farmers found that if they slowly dripped water directly to the roots zone of the plants, the plants absorbed a higher ratio of the water, reducing evaporation and creating the most efficient watering system on the planet.
Drip systems come in several forms; the original form is a thin spaghetti like tube with an emitter on the end that literally drips water into the root zone of the plant. Emitters come at different drip ratios so that when watering trees, you can deliver much more water per an hour than lets say a flower pot whose roots are closer to the surface and requires less volume of water to saturate the roots. The next step up is drip pipe, plastic pipe with the emitters built in. These come with different spacing and can be used when watering a bed of plants that have similar water needs. Drip pipe can also be used to circle larger trees, and for subsoil irrigation of lawns.
Finally there are micro spray heads, and low volume spray heads. These are spray heads that need less pressure and volume than lawn sprinklers and can be calibrated to be area and/or plant specific. We like to use these on greenroofs where water is only needed for the first year, or in other beds of groundcover.