Water is essential for all life on Earth. Water, and carbon dioxide, are the raw materials in photosynthesis to manufacture carbohydrates. The essential minerals taken up by plant roots, and the carbohydrates manufactured in green leaves, must be dissolved in water before they can be circulated to other parts of the plant. Plant cells maintain their shape because they are swollen with water. In short, plants need lots of watering.
It may surprise you to learn that only one to three percent of the water is actually consumed by the plant. The rest evaporates as water vapor from tiny pores (called stomata) on the leaves and stems. Water travels into a plant’s roots and enters its circulatory system. Traveling upward in vessels (called xylem) to leaves, it evaporates into the air. This process (called transpiration) cools the plant like sweat does for people. More importantly however, transpiration creates a vacuum in the xylem vessels which sucks more water up from the roots like an above-ground pump on a water well. (There are physical limits to how high a vacuum pump can pull, and other forces in addition to negative pressure tension help to raise water to the tops of even the tallest trees.)
Plants that are not getting enough water wilt. Plants that are getting too much water also wilt since saturated soil contains too little oxygen and roots can drown and rot, causing the whole plant to wilt and eventually die. The vast majority of plants do best when the soil surrounding their roots is constantly moist but drains freely and never gets soggy. That said, maintaining a constantly moist soil that doesn’t get too wet requires lots of water applied frequently, and near constant attention to guard against over- or under-watering.
A more practical way to water our plants is to drench them thoroughly and deeply, then allow the soil to dry out until the plants just begin to show signs of mild stress before watering deeply again. If we were to water frequently and just enough to wet the soil near the surface, the roots would tend to spread laterally. By watering deeply and less frequently, we encourage the roots to grow deeper where the soil is less prone to drying out. Plus, the roots will be protected from environmental extremes.
Lawn grasses and annuals have most of their roots within the top six inches of soil. The roots of perennials, shrubs and trees are mostly in the top twelve inches. Push a spade or shovel into the soil and pull it back to check the amount and depth of soil moisture. The soil should feel moist, not saturated, to a depth of 6-12 inches. If not, it’s time to water!
Our goal is to get the water into the root zone. If you pour the water on too quickly, it can run right on past the roots, especially in sandy soils. It’s better to water slowly so the water has time to spread laterally as well as deeply. Check again with that shovel until you feel comfortable relying on your experience.
Most plants get all their water through their roots, so there is no need to wet the foliage. (Plants that do not have roots, such as some aquatic plants and epiphytes like orchids and bromeliads, are the exceptions.) For most plants, if the foliage stays wet for too long, diseases, molds and fungi can attack. It’s best to water in the morning. They will have time to dry out before the cooler evening encourages bad things.
Use lots of mulch around your plants, even the ones in containers. Mulch reduces evaporation from the soil and holds moisture in. Mulch keeps the soil warmer in winter and cooler in summer. It also reduces erosion and runoff from extreme rain and overzealous watering.
The best way to water your outdoor plants, whether in the ground or in containers, is with a soaker hose or a drip irrigation system. With these you can water deep and slow. Plants in outdoor containers need to have water every day (maybe twice a day) in the summer. Check the moisture of the potting medium with your finger to figure out how much watering is enough. Plants in the ground will probably need watering around once a week. Depending on the kind and size of plant, the soil type, the air temperature, the relative humidity and natural rainfall. You will need to keep track of the soil moisture. Use the shovel test until you learn when to start the irrigation system and how long to let it run.