Watering is one of those contentious subjects, with vociferous champions for different watering times and methods. My own approach is to look at what other functions of watering besides just keeping plants alive.
One major thing water does is to cool the soil by evaporation. So, I water at times when cooling is a good thing.
In spring, when nights are still cold, I water early in the day. That gives the soil all day to warm in the sun, and start the night with as much stored heat as possible. Later, when the days get hot, we switch to late afternoon, giving the plants the water they need to grow during the night when they are not heat-stressed.
If the weather is brutally hot, I water a bit during the hottest part of the day. This is just what “experts” tell us not to do, because the water will evaporate before the roots can use it. What they don't say is that by evaporating, it forms as little pocket of cooler, moister air to refresh the plants. When temperatures exceed 90 degrees, plants often shut down and cease growing; this little bit of evaporation can make the difference between growing and just surviving. It also cools and moistens the feeder roots, bacteria, fungi, and other organisms in the top inch of soil. Normal deeper irrigation can proceed at the normal time.
In early August, we switch again. The danger of mildew during the long warm nights of August and September is great. Mildew grows best in warm, humid air. So we water in the morning again, and let the plants go into evening with the soil surface dry.
Unintended functions of watering can be soil compaction and foliar disease. So we mulch well. Soil that splashes up from the ground from raindrops or watering is a major cause of foliar diseases like rust and wilt. Straw mulch breaks the impact, preventing soil compaction as well as the splashback that causes disease.
Another function of watering is to enhance the soil food web. When the interface between soil and mulch is slightly damp, fertility-producing microorganisms thrive. Microorganisms need oxygen, water, and food. Mulch and water put food and moisture within the critical top inch of soil, where oxygen is abundant.
The entire root zone needs water, so be sure that whatever method you use, it reaches the whole bed.
Soil Texture Affects Watering Time and Style
Clay soils that are heavy and sticky drain very slowly. They take a long time to get fully wet, and a long time to dry afterward. They need long, slow watering at wide intervals. Water spreads more than it sinks. Drip emitters can be far apart, with slow flow rates and a long watering period.
Sandy or silty soils wet or dry quickly. They need short, frequent irrigation. The "deep, infrequent watering" that books recommend won't work here--the water just drains away below the root zone, and plants dry out. Water in sandy soil goes almost straight down, so emitters in drip systems need to be close, with faster flow rates and short, frequent, watering periods.
Organic matter can hold water within the particles, not just between them. This means it can drain well while retaining water, spongelike. Wise gardeners add it to their soil in the form of compost or mulch. Loamy soils naturally have a good proportion of organic matter. Water spreads about as much as it sinks, in an upside-down cone shape underground. Drip emitters can be at medium spacing, with medium flow rates and watering periods. Because of the ability to retain water, the watering periods can be less frequent.
Choosing a Watering Method
If you are a new gardener or have a small garden, the daily interaction from hand-watering your plants is very helpful in keeping tabs on them, so problems are noticed early. However, a larger garden and busy schedules make some form of mechanical irrigation attractive. What kind you need depends on your soil, what you are growing, your climate, and your preferences.
Hand watering is best done with a wand that breaks the flow of water into fine droplets that don't pound or wash away soil. Water the soil, not the plant itself. And remember that the root zone is probably about twice the width of the plant. The photo at top shows the best way to hand water: Do you notice the wand is turned up so that the water loses velocity going upward? Then it falls gently. You should plan on hand-watering sometimes, no mater what other set-up you have. Newly planted seeds and new transplants will need extra watering because they don't have big root systems yet. Your mechanical systems should be set up for average summer weather. Plan on adding an extra watering cycle in very hot or windy weather, or spot watering by hand as needed. (Photo by Cynthia Raiser Jeavons.)
Overhead sprinklers use the most water and wet the foliage. Rinsing the leaves can be a good thing, and in high heat, overhead water may rescue plants by reducing water loss from leaves, and cooling the area. However, having to wet the leaves every time you water the roots can be problematic. There is no point watering the paths between permanent wide beds, so sprinklers are wasteful there. If you plow or till your whole area and then plant in rows, the area between rows may be part of the root zone, so overhead watering makes more sense. However, it also leads to lusty weed growth, and sometimes foliar disease, even if you can afford the amount of water involved. I suggest wider spacings if you use overhead watering, to make sure of good air circulation, which helps prevent disease.
Drip irrigation takes many forms and can be tailored to many situations; it has advanced considerably in the last 20 years. It's strength is that it is focused just where you want. Be careful, however, that you don't limit your plants root growth with too narrow an area of moist soil. A simple system for vegetable beds could use tubing with built-in emitters that deliver water at a consistent and reliable rate every foot or so. You have only to lay the tubing in the bed and plant around it. Emitters that are mini-sprinklers are also available, for a wider reach.
Landscaping is probably the easiest to put on drip, because you don't have to disturb it for yearly planting. Use permanent tubing to carry the water and add emitters or leaky hose to supply trees, shrubs, and perennials. It is worthwhile buying from a specialist, and following their recommendations. Dripworks has excellent resources for figuring out what you need. (I have no financial or other interest in them; they were pioneers in the field.)
You are watering too much if the soil is squishy, sticky, or very soft. You may also see plants turn up the leaf edges to try and get rid of some water out of their stomata (leaf pores). You may see leaves get a hunched appearance: it means water is taking up all the air spaces and depriving roots of oxygen. This leads to root rot. In the worst case, crown rot will kill the plant. The rot kills the water-carrying tubes just under the bark. The plant is girdled and dies.
How do you know whether you have watered enough? At first, when you are planting seeds, there is no substitute for digging to see how far the water penetrated. It is also good to dig a hole and see how fast it drains. Later, watch your plants, and even weeds, to see if growth is normal. Often, people think they need to fertilize when really they just need to water a bit more, or to hold the water in the soil longer with mulch.
Observation, as always, leads to familiarity with the signs and symptoms. A "green thumb" is really a watchful and thoughtful interest in the day-to-day life of the plants in your charge.