How much is too much water? How little is too little? While water is essential for any garden plant, factors such as soil, humidity and sunlight can make a big difference. Clay soils hold water best, but take a long time to absorb it. Sandy soil absorbs water very quickly, but can't hold much. Your soil should contain enough organic material to retain moisture and still allow spaces for air to reach plant roots. Always try to water in the morning, allowing plants' leaves to dry all day and cutting back on disease.
What you grow will also determine how much water you need. Root depths vary greatly. Tomato roots can reach 2 feet deep after a good season, while lettuce roots may be only be 1 foot deep. Other plants grow even shorter roots. If possible, try to take note of how deep the roots are of your different plants. Plants with deeper roots are usually more capable of finding water in the soil.
You also have to remember the rate of evaporation. Hot, dry, windy days suck up moisture out the soil much faster than still, humid days.
Either with a hose or a watering can, hand-watering is the simplest and possibly the most common method of watering in gardens. The only drawback is that people get impatient and often water less than their plants need. If you hand-water, be sure to check the rate of absorption. If your soil has a lot of clay, don't be fooled by runoff. You may think that the ground is saturated when actually the water can't be absorbed fast enough. Water your plant until water pools on the surface, then come back after an hour or so and water again.
Either attached to your hose or built-in, sprinklers are great for densely planted vegetables. If you need water placed evenly over a large area, this is the technique for you. Keep in mind, though, that sprinklers spray a lot of water on foliage, which can lead to disease. Also, weather factors such as wind and sun can decrease your watering efficiency. Don't use sprinklers on sparsely planted gardens, as this will waste water.
This technique allows a slow, but steady amount of moisture to seep into the soil around your plants. You can use a soaker hose or a trickle irrigation system and leave the water on for several hours, ensuring a good soaking. Use this method around thirsty plants, and in gardens planted on slopes, to offset the effects of runoff.
Whether the soil surface appears wet or dry, you can't always tell what's going on at the root level. To see if you've watered enough, take a long screwdriver and stick it into your garden soil. If it goes in easily, you've probably watered enough. You can also purchase a moisture meter at your local garden center to measure how dry your soil is. When watering, it's always best to opt for periodic deep soakings - the period varying according to weather and how well your soil retains moisture - instead of frequent, light sprinklings.