Watering during the dry summer season is crucial for plant survival and health.

“Plants can’t survive without water.” I bet you won’t find anyone who will disagree with that statement.

But many homeowners act as if it wasn’t true. They let their plants go for months during the dry summer season without giving them any water at all. When fall arrives, they wonder why their plants are unhealthy, stressed, infested with pests or disease, or even dead. Plants need water to survive and thrive.

In 2017 Seattle rainfall was over 10 inches above normal. Even so, we had a record-breaking 55 days without any rain at all.

It may be raining outside today, but it is not too early in the season to think about the water needs of your plants. Get ready for the dry season that is certain to come.

This year we plan to publish regular blog posts about the importance of water and what you can do to help your plants stay hydrated and healthy over time.

The rule of three

Humans can live for three minutes without air, three days without water and maybe three weeks without food. I would not recommend trying to see if you can exceed any of these; it can get ugly. Living three days without water would only be possible if the person was in otherwise optimum conditions: not too hot, shelter from sun, etc. Yet we expect plants to go without water for days, weeks and even months.

Some plants, such as cacti, are adapted to harsh, dry conditions. Over thousands of years they have modified their ability to survive with very little water. Their extensive shallow root systems allow them to soak up any rain that may come their way. And they can store vast amounts of water. For example, a fully-grown saguaro cactus can absorb and store up to 200 gallons of water during a good downpour!

However, cacti are highly unusual in the plant world. The majority of plants need water during dry periods. They may be able to survive for some extended periods with little or no water, but they won’t thrive. After some time, their reduced ability to pull water from the soil can have adverse reactions that will damage and eventually kill the plant.

Why plants need water

Plants use water for many important functions:

• Photosynthesis
• Transporting minerals for plant growth
• Cooling plant tissues during hot periods
• Disease and insect resistance
• Turgidity in leaves and bark. Turgidity is pressure against a plant’s cell walls, essential to keep the plant standing upright. Plant cells that lose a lot of water have less turgor pressure, and tend to become loose or limp. Further water loss eventually results in the wilting of the plant.
• And many other functions.

Without water, all functions begin to decline. This can lead to many problems. For example, a plant that is stressed and unhealthy is more susceptible to insects and diseases. A lawn that has dried out and become thin is prone to invasion by weeds and moss. A vegetable plant that has not received enough water will produce a smaller harvest, and individual vegetables will be smaller and less tasty.

Summer can be brutal

The hot, dry summer months are the most severe weather conditions that plants have to deal with. Little or no rainfall, long days and intense heat can have very adverse effects on plants.

Deciduous plants, the ones that shed their leaves every year, begin to droop first. Then they will begin to lose leaves. A plant drops its leaves to reduce the amount of leaf material it needs to support.

If caught in time, most plants will rehydrate and return to normal functions in a short while. But there is a point of no return, called past wilting point (PWP), in which the plant will die because it can no longer pull water with its roots.

It is more difficult to tell with evergreen plants. Some large trees will take up to two or three years to show adverse signs that lack of water was a problem for it in the past.

It seems like every year in the Pacific Northwest, we are saturated with rain and water for months on end. The soils are sloppy and cold for what seems like forever.

But once the rains disappear, it only takes a few weeks for the soils to become bone dry and dusty. Most of this has to do with the heavy clay soils that are prevalent throughout the PNW. The clay holds onto water a long time with its microscopic pores and releases the water slowly,. However, when it gets dry it becomes almost hydrophobic, meaning that it will almost repel water.

Soils are like sponges

If a sponge is allowed to completely dry out, you can pour water on top of it and all the water will shoot off the top, leaving the sponge dry. But if a small amount of water is applied to the top part of the sponge, it softens up the surface. In a short of amount of time, more water can be added to the sponge and it will begin to soak in and soak down. After a while water flows freely through the sponge and comes out the bottom.

Our soils are the same. If we allow them to get so dry that water runs off the top immediately, we will need to water them slowly so they can rehydrate. It will take small amounts of water and some time before the water will soak into the soil and move through the soil freely.

If we do not water at all during the summer, it may take months of rainy weather to rehydrate soils because of the need to soften the soil over time. Until that happens, plant roots will not get enough water.