Like most gardeners, I have inadvertently killed a lot of plants. Mostly, it is not a big deal but I loathe to lose a young tree or shrub. However, for years, I missed one of the key reasons my trees and shrubs would struggle in the summer: my soil stores a lot less water than I thought. I live in a climate where it rains from fall through early summer. We can get up to 55 inches of rain per year, most of it during the winter months. On my ten acres, that is roughly 15 million gallons of water dumped onto the soil every year. If contained all at once, that would cover 10 acres with 4.6 feet of water. It is a lot of water. And yet, when summer starts, my pasture, trees, and shrubs only have access to about 4.5 inches of it. That is a measly 8% of all that rainfall. Making things worse, this year, when summer started, they blew through 4.5 inches of water in 18 days. (gulp)
Imagine your soil like a giant water cistern. Then imagine it rains and rains and rains. All of the soil pores will become saturated with water. Once it quits raining, water drains out of the macropores downward into the groundwater system. These subsurface flows are incredibly important for refilling aquifers and for maintaining flows into our freshwater waterways. After water drains away, we are left with soil, some air space, and water in smaller pores. The amount of water present is the field capacity of a soil.

Every type of soil has its own capacity to hold water. Sand holds water about as well as a colander – it doesn’t. In comparison, clay holds water like a Stanley water bottle. I have a sandy loam. According to the University of California, my soil probably holds between 1.25 and 1.75 inches of water per foot. We’ll call it 1.5 inches per foot. So, at field capacity, the top three feet of soil holds about 4.5 inches of plant available water.
We had a warm spring followed by a rainy, cool June that culminated in a 0.33 inches of rain right at the beginning of July. We entered July very close to field capacity. Yet, by mid-July, even though it had been so wet, my young bare root trees were already showing signs of drought stress. Why?
Water exits the soil through evapotranspiration (ET), a combination of plant transpiration and evaporation. Every day, ET reduces the amount of plant available water. On July 9, the evapotranspiration rate was 0.39 inches. That was 8.6% of my plant available water, gone in a single day! Looking at effective rainfall (precipitation minus evapotranspiration), by July 12, we had lost 3.0 inches of water to ET. That is the equivalent of the plant available water in the top two feet of my soil. By July 19, we had lost 4.5 inches: the soil is dry. We aren't even to August when it really heats up! (If you want to track your precipitation and ET levels, you should be able to gather data from a public weather station near you. I use data provided by Washington State University.)


Now, imagine you are a baby bare root tree that came with a wee little root system that had been rudely ripped out of the ground. Within 12 days of the last rainfall, the top two feet of soil have dried out and you are sitting there without water. Meanwhile, your owner has been celebrating less than two weeks of not getting rained on and is completely not thinking about how you already need water. It is easy to see how things go south fast.
Even if the plant lives through the summer without irrigation, it will not thrive. It will not grow. It will not sequester carbon. It will not play a role in the ecosystem. See this “drought tolerant” seaberry whose owner (me) did not sufficiently water and allowed to get outcompeted by grass? It is not fixing nitrogen. It is not providing me with nutritious berries. It is not providing habitat or food for birds or insets. It has sat there getting rained on for three full years but does not have enough water in the growing season to get a foothold.
In contrast, here is a shore pine that I have watered regularly for the same period of time. It has gone from a foot high to towering over my head. It is well on its way to having a sufficient root system and storage organs to be able to survive summer without irrigation. However, I will still water it lightly throughout the summer because I want it to grow so that it can provide ecosystem services. Eventually, it will help slow rainfall. It will provide shade, slow the wind, and cool the ground. It will take care of itself. It isn’t there yet.

Why write all this out when it seems so obvious? For many years, I unnecessarily lost plants out of a belief that watering was wasteful. I believed that if I mulched, dug swales, chose drought-tolerant plants, etc., I should not need to water. This was wrong. It resulted in a lot of dead plants and wasted money. More importantly, it wasted time. You can only plant so many trees in your lifetime. Do not deprive yourself of the great joy of getting a tree on its way to maturity by being stingy with water. If you are in a dry summer climate, water early, water often, and watch your tree or shrub come to life. In summer, your soil may be drier than you think!
Comments