The garden's planted: Now how do I water?
Watering a vegetable garden depends on a number of variables, especially the weather. Here are some guidelines to follow to water for best results.
1. Methods and time of watering
There are several ways to water: by hand using a watering can or hand-held hose; by sprinklers; or by some form of drip irrigation. Hand watering works well if your garden is fairly small. Be sure to water at the soil line so the water goes directly to the roots. Try to avoid wetting foliage, as this may contribute to certain plant diseases. Sprinklers can cover a larger garden area, but tend to lose a lot of moisture to evaporation, and water may not always reach the roots, especially as plants grow bigger. Also, sprinklers usually wet the foliage and can contribute to disease, especially if used too often. Drip irrigation takes some time and expense to set up, but usually gives the best results in the long run. An easy form of drip irrigation is a soaker hose with multiple predrilled holes that, as the name implies, drip water directly onto the soil. These hoses should be placed close to the root zones of the plants, for example, along the row of planting, so you may need more than one hose.
The best time to water is in the morning. Less moisture is lost to evaporation than later in the day, and plants will have water throughout the heat of the day when they need it. Watering early in the day also gives plants a chance to dry out if foliage has gotten wet, and thus avoids humid conditions during our cool nights that can promote disease.
2. Frequency of watering
If you've just planted seeds, keep the seedbed evenly moist as seeds to promote germination and early plant development. Natural rainfall may be enough to do this, since spring and early summer tend to be rainy in western Oregon. Summers here tend to be quite dry, so if there isn't enough rain, add water gently to avoid disrupting the seeds and developing roots.
Newly planted starts need water as roots begin to grow into the garden soil. Check new plantings every day for the first few days to make the sure the soil around the plants is moist enough that the plants don't wilt.
As plants start to grow, either from seed or transplant, the goal is to develop deep roots, so that plants get water and nutrients from deeper in the soil, and can survive dry periods better. Deep infrequent watering, to the depth of a plant's roots, is preferable to shallow, frequent watering. If watering is deep, it may only need to be done every few days. Hot, dry, and windy weather can all cause plants to lose moisture more rapidly, so check your vegetable garden often to see if plants are showing signs of wilting and extra water is needed.
WSU Publication EB0422 (pdf)
OSU Extension Publication