I believe that automatic sprinkler systems kill more plants than all pests and diseases combined. That's not really fair
to borrow from the NRA: sprinkler systems don't kill plants, people kill plants. How? By using sprinklers that water too fast, and by setting their timers so the sprinklers run too often and not long enough.
.so the water can soak in!
Water running faster than the soil can absorb it runs off--into the gutter, the neighbor's, or into unintended parts of the landscape. This doesn't just waste water. If it happens too often, plants in affected areas can get crown rot and die. Clay loam soils, typical of this area, absorb water slowly.
Your hose running at full throttle puts out 400 - 500 gallons per hour (7 - 8 gallons per minute). If you just set it on the ground water runs all over the place, which isn't an effective way to water anything. The only way to get it to penetrate is to spread that water out by running it through a sprinkler or system, or by slowing it down.
The low-tech solution: a hose left running at about 1/4 turn of the faucet can be a very effective way to water larger shrubs and trees--just leave it there trickling for 4 - 6 hours or more (overnight is fine!).
A drip system puts the water right at the base of the plant, dribbling 1 - 2 gallons per hour. That is a fine way to water -- just keep in mind that a young tree needs about 10 - 20 gallons per week. So if there's one emitter per plant, you need to run the system 5 - 10 hours! You can add more emitters, or use 'microsprayers'. But these systems need to run at least a couple of hours--several hours are usually best.
Sprinklers that go on the end of the hose cover areas ranging from 100 to 5000 square feet. The number of gallons coming out is all the same, so sprinklers that cover large areas need to run fa-a-ar longer than small 'spot' sprinklers.
Typical spot sprinklers start to run off or puddle after 30 - 60 minutes.
Oscillating or 'wave' sprinklers should run 1 - 3 hours.
Impulse sprinklers cover very large areas, so they may need to run for several hours.
Sprinklers on an underground system vary in how many gallons per minute they put out. While the water coming into the system is still the same, the sprinkler heads reduce the flow in various ways. Which is great! Older sprinkler heads put water out too fast for our soils to absorb it. Replacing the sprinklers on older systems with 'low-gallonage' heads makes it possible to water effectively and efficiently.
so the soil can store water and roots can grow deeper and broader.
Roots grow where the soil is moist, and don't grow where the soil is dry. For roots to grow out from the planting hole, the surrounding soil needs to get moist with each watering.
'Overwatering' means watering too often, not too much at one time. You won't hurt a plant by giving it lots of water all at once. If you forget and leave the hose trickling for 24 hours, it won't hurt the plant. Just don't water that plant again for awhile.
A side note: does deep watering a tree in a lawn discourage surface roots? Trick question
No! Deep watering encourages deep roots, but as long as you're keeping the surface moist roots will grow there, too. Whether that will be a problem depends on the type of tree. Riparian species (those that grow in nature along streams) will have annoying roots in your lawn no matter what. Many other types of trees won't. Choose the right species to avoid surface roots in your lawn.
.so that the surface can dry out between watering.
Soils that contain clay 'store' water -- a big advantage of our clay loam soils over sandier soils. That means our soil stays moist for several days after a long soaking, even though the surface appears cracked or dry.
Daily watering is not necessary!
In fact, it can be harmful. Fungus spreads rapidly on a moist surface. Moisture trapped by soil or mulch around the trunk or crown leads to rot, killing plants. Keep mulch a few inches away, and remember that it helps to keep the soil moist--reducing the need for frequent watering.
Even newly planted plants rarely need daily water. Check them each day by poking your finger into the soil. Under normal summer conditions a new transplant probably needs water every 2 - 3 days for the first couple of weeks. If it's 100F or over, or if it 's windy, an extra drink may be needed.
How often should you water?
Your lawn needs water twice a week. Measure with tuna cans to see how long it takes to get one inch each time.
Roses and annual flowers like water every 5 - 7 days, as do many shade plants such as ferns.
Most perennial flowers, shrubs, and young trees can easily go a week between watering, and established trees can go a couple of weeks.
Larger trees, orchards, native or drought tolerant plants can have very long soakings every few weeks.
Many plants will tolerate more frequent watering--lawns may look fine with daily watering, for example. But remember that your water goes out as much as, or more than, it goes down--plants within a few feet of your lawn may be adversely affected. <
If you're designing a yard, put your plants into zones based on their water preferences. Native plants near a lawn will be killed, but roses, bamboo, or canna lilies will thrive on the extra watering. If excess water is going to some plants, a simple drain system or some light grading may save them by sending the water to plants that can tolerate it.
Shallow light watering with drip or soaker systems can have another drawback. The salts in our water 'wick' out to the edge of the wet zone and accumulate. You can sometimes see the white deposits of calcium and boron salts along the edge of these systems. These salts can be taken up in excess by some plants, can damage roots, and can make the soil more alkaline. They reduce the plant's ability to take up certain important minerals such as iron and zinc. Symptoms include burnt areas on the edges of leaves, chronic yellowing of the new growth, smaller leaves and stunted growth. Since these salts dissolve easily, just set a hose or sprinkler to flood the area, or turn your system on manually for several hours, every few weeks. Soil sulfur to make the soil more acidic can also help correct the symptoms.
Amending the soil with compost helps in many ways. It breaks the compacted surface, so water penetrates more readily. It adds to the soil's ability to retain moisture, so amending the soil for roses, flowers, and shade-lovers can allow you to go even longer between watering. Compost encourages earthworms, and allows roots to grow more quickly. And compost holds and releases important nutrients to plants.
Try a watering vacation!
Run your system for a long soaking, then turn it off.
Go out and check the soil with a trowel in several places of your yard each day or so. You may be surprised how long it takes to start drying out. Your plants might appreciate the break. And a drooping plant will recover
but a rotted plant won't!