Plants that have suffered cold injury are more susceptible to drought stress, and vice versa. So this year, the Puget Sound region is really getting a double whammy, which could result in additional plant problems if our watering methods are out of sync with our garden's needs.
But... how to tell how much water is enough but not too much? The biggest part of the answer is in knowing your native soil.
Most watering guides say to water deeply and not very often, in order to promote long plant roots that can thrive despite the dry summers that are typical in the Pacific Northwest. This is sage advice, but.... you definitely need to know your native soil in order to understand how this watering advice relates to your own garden conditions.
Also, don't assume that every part of your garden has the same type of soil. Because of the construction techniques used during most home-building in the 20th century, your native soil may be intact in some areas, and either replaced by fill dirt or flipped topsy-turvy in others. This is why I check the soil in several areas of your garden during my first time consultation, and sometimes take 2-3 samples home to test for its components. If you're unsure about your soil's components and have received a garden report from me over the past few years, you can check the Soil and Drainage section for my findings and customized advice. You can also follow the instructions at http://toxipedia.org/wiki/display/ipmopedia/Soil+Test
Amending your native soil with compost is a vital component to watering wisely, because compost helps sandy soils retain more water for longer periods of time, and helps heavy clay soils drain better while still retaining water well.
Another big factor in your watering decisions is what you are growing in your garden. Your goal is to have adequate water available to plants in the level of soil where their primary roots are growing. The roots of well-established trees and shrubs can grow 12-30 inches or more into the soil, and typically need much less water than relatively new plantings. Some need no supplemental water at all. New plantings of trees, shrubs, or perennials will only have roots in the top 6-12 inches of soil, gradually growing deeper as the years go by, especially when you water wisely. Vegetable garden plants will always be in the top 4-12 inches of soil, so typically need more water than any other part of a garden.
No matter what your garden contains, you also need to pay attention to how high the temperatures are and how low the humidity is, because that has a huge effect on how quickly the plants in your garden use up the water that you put in the soil.
Here are some soil and watering examples from the ornamental beds in my home garden:
- My back garden is on a terraced slope, with planting beds on each of the 4 levels. The soil throughout the back garden contains a large percentage of clay, which is the smallest of all soil particles. This means that it holds water well and disperses it horizontally when watered. But its fine texture also means that it accepts water slowly, so if I have the sprinkler or irrigation system set too high, the water will pool on the surface or create runoff, which doesn't do my plants much good. Also, the trees, shrubs, and perennials in my back garden have well established root systems. Therefore I water my back garden at very low pressure, for a longer period of time, and less often than my front garden, which has a high sand content. In a typical summer, I water the upper ornamental beds (which drain fastest) approx. once every 10-14 days, depending on the weather. The ornamental beds in the middle and lower terraces usually only need water every 14-21 days, and some areas are only watered once or twice during the whole summer. I never water the mature Conifers, Smoke Tree, or Wax Myrtle.
- My front garden is also terraced, with only two levels. Though all the soil has been amended with a lot of organic matter (and some of my back garden soil), it is still primarily sand. Because sand is the largest soil particle, this soil accepts water at a fairly fast rate, but holds it for a much shorter time than the back garden. Also, the water tends to travel straight down through the soil without dispersing much. If I watered the front garden as long as I watered the back, I would be wasting most of the water well below the reach of all my plants. For these reasons, my front garden watering needs to be done with more thorough coverage, in shorter bursts, and more often. Most of the plants in the front garden are also well established, so in those areas, depending on the weather, I water approx. every 10-14 days in the upper level and every 21-30 days in the lower level. I never water the mature Barberry, Camellia, or Lilac. However.... during this past spring, I added all-new ornamental plantings in 3 different areas of the front garden, including 2 hot and sunny parking strips, thinking that Mother Nature would supply most all the water until July. (Boy, was I wrong about that) So in those areas, my watering has to be much more often; approx. every 4-7 days, depending on the weather, and still in short busts with thorough coverage.
Container gardens are also typically in need of more frequent watering, due to their well-drained potting soil mix, and the use of more water by plant roots exposed to higher soil temperatures. Containers placed in the hot sun may need watering every day.
No matter what you are growing in your garden, nothing beats the built-in "watering sensor" that each of us carries with us- your finger! It is your best tool for watering wisely. So before turning on the hose or the irrigation system, get that digit (or a narrow hand trowel) down in the soil at least 2-6 inches and then look and feel what's underneath. Many times, the surface layer of the soil surface is bone dry but underneath it is still dark and wet. If so, delay watering until the soil is dry underneath, and instead spend some time in your hammock!