Climate change is leading to hotter, drier summers that cause drought stress in trees and other plants.
[Ed. note: In honor of our 25th anniversary, this is the third of several blog posts about changes we have seen in landscape practices over time.]
When we started In Harmony in 1994, no one was talking about climate change. In recent years, it is hard not to talk about it.
Climate change is having big impacts on our landscapes. We need to learn to deal with the changing reality.
Climate chaos
Perhaps they should call climate change “climate chaos.” In the last few years we have seen extremes of weather from hot to cold, wet to dry. While the Seattle area has always seen wet winters and dry summers, what is “normal” is changing.
Hotter summers, colder winters
As reported in the Seattle Times, in 2018 we broke the record for the amount of hot days, with 32 days reaching at least 85 degrees. That was well above the average of about 11.
Average temperature lows have also risen over the years. In 2018 there were 22 days that never got below 60 degrees. All of the top five years were from this decade.
At the other end of the spectrum, in 2019 Seattle had its coldest February in 30 years, with an average temperature of 36.6 degrees.
These temperature extremes have an impact on the ability of our plants to thrive and survive. Plants that were appropriate for our climate in the past may not do as well in a changing climate.
Less rainfall
Climate change also affects the amount of rainfall. In 2018 we had a below-normal annual rainfall, with less than 36 inches of rain. The average is about 39 inches.
That was largely because of a long summer drought. From May through August, Seattle logged just 1 inch of rain. The next driest May-August period on record was 2003, with 2.05 inches.
In 2019 Seattle had the second driest March on record, with only 1.37 inches of rain. Currently, year-to-date rainfall is about 3 inches below normal.
The lack of rain has major impacts on landscapes. This year we saw plants with drought stress in our clients’ landscapes as early as April.
In recent years we have become much more concerned about the long-term impact of dry, hot summers on trees and other plants. Last year we wrote several blog posts about why it is important to water and how to water to promote plant health.
More intense weather
Seattle residents are used to drizzle, not thunderstorms and downpours. However, a recent study found a significant rise in the number of heavy rains in recent decades.
Climate models predict that we will see more extreme weather events, with heavy downpour that can dump large amounts of rain over relatively short periods of time. This can cause flooding and overwhelm our drainage infrastructure, sending raw sewage in nearby waterways.
Over the past 14 years “the stats have changed such that the definition of an extreme event has changed,” according to James Rufo-Hill, a meteorologist with Seattle Public Utilities. A storm that was previously expected only once in 100 years is now likely in 25.
Our landscapes may suffer in intense weather. Heavy rain may lead to soil compaction and erosion, especially if the soil isn’t protected by a thick layer of mulch. Overly wet weather leads to more fungal diseases. High winds that often accompany rainstorms may break or tear branches and stems.
Trees and drought stress
A recent story on KUOW said people are seeing more dead trees this year. Ken Zobrist, a forestry professor at Washington State University, said he sees dead and dying hemlocks all over western Washington.
And it’s not just hemlocks. Western red cedars and big-leaf maples are struggling as well. All three species are native to western Washington.
His “top suspect is drought—drought stress from climate change. We’ve seen records being set for heat and drought in a number of years in a row now, starting with 2012.”
While 2019’s summer was a little bit better, “that cumulative drought stress is really taking its toll,” Zobrist said.
Lack of water can weak trees’ immune systems. Then something else, perhaps an insect pest or a fungal disease, can kill them.
In recent years we have seen much more evidence of drought stress in our clients’ trees. Sometimes a tree may not show damage for two or three years.
More pests and diseases
The changing climate is causing many species to move toward the poles. We are likely to see more pests move into our gardens from warmer climates.
A study published in Nature found that “crop pests and diseases are moving toward the poles at about the same speed as warmer temperatures. The findings suggest that climate change is driving their relocation.”
We keep up to date with information from WSU Extension and others doing research and monitoring of pest populations, and our service technicians monitor our clients’ landscapes. We will let our clients know if new pests arrive.
Pollinators suffer
Bumblebees tend to overheat in hot weather due to their large size, dark color and hairy bodies. They are unable to quickly migrate to cooler climes because they depend on specific plant species to survive.
Honeybees and the flowers they depend on for nectar are getting out of sync. Flowers are blooming earlier due to rising temperatures. When bees are ready to gather nectar, they may find limited food because the flowers have already died back.
What can you do?
Trees: top priority for your landscape and the planet
Plant more trees. Trees are critical for reducing heat effects in our cities and cooling your house. They provide shade on hot days, provide shelter and food for wildlife, reduce stormwater runoff, help keep the water clean, clean the air of toxins and more.
Trees form the “bones” or structure of the landscape. They may offer beauty and interest in every season, whether it is spring flowers, colorful fall leaves or sculptural bare branches in winter. Conifers offer a greenscape throughout the year.
Help out your trees. Look at what else might be competing with the tree for water. “Grass is the worst,” said Ken Zobrist. If your tree is planted in a lawn, the grass robs all the water. Remove grass from around the tree and replace it with three to four inches of mulch.
Mature trees, not just young trees, need water. Don’t wait until your tree begins to show stress to water it.
Replace dead trees with a drought-tolerant native species. Ask your local nursery for recommendations, or check out this list of drought-tolerant trees on Great Plant Picks.
Water deeply, and add mulch
Summer watering is now more crucial than ever for plant health and survival. It will become even more important in future years as the climate continues to change.
Watering is also important for managing pests. When plants begin to have water stress issues, they often emit destress signals that many damaging insects pick up on.
Slow, deep, infrequent watering is the best method. Find out the right ways to water and tools to help. Here is a link to our blog post on watering resources.
Mulch, mulch, mulch: not just trees but all of your landscape beds. A deep layer of mulch will help your plants survive hot, dry summers and insulate roots from winter’s extreme cold.
Right plant, right place
Choose plants that will do well in our hotter, drier summers. And make sure you are not putting shade-loving plants in the sun. Read our blog post on drought-tolerant plants, including plant lists and other resources.
Plant flowering plants for pollinators. Choose a variety of plants that flower at different times of year, so pollinators can find food throughout the season.
Autumn is a great time to plant new plants. Our fall and winter rains will help them get started, and they will have a full winter to develop roots before next year’s dry season. The plants will be healthier next year, and you will have to water less.
New plants need regular watering for the first two to three years. This includes newly planted trees. Then you can water less often. But even drought-tolerant plants will need some water in our dry summers.
Help our streams and lakes
Plant a rain garden to slow down intense winter rain and help keep the water clean in our streams and lakes. Here are resources on rain gardens.
