Yellow Sky Days

If a global pandemic and a worrisome election weren’t problems enough, Colorado and many of the western states have been on fire for weeks, creating ominous yellow skies and a sense of foreboding.

Smoke-filled sky (Kate Russell)

While there isn’t much we can do about any of it, besides wearing masks, maintaining social distance, voting intelligently, and practicing fire safe gardening, knowing more about how our plants are being affected can help us take better care of them.


Atmospheric adages

Many of us grew up with the old adage: Red sky at night, sailors delight. Red sky at morning, sailors take warning. Truth is, that quote comes to us from the Bible in a slightly different form, but for the same reasons: Red sky at night, shepherd’s delight.  Red sky in morning, shepherd’s warning.


In both cases, the redness of the sky is caused by particles in the atmosphere. Yellow, orange, red, and brown are longer wavelengths and they are able to pass through particles in the air. Blues and greens have a more difficult time. This is called Rayleigh scattering and it explains why sunsets are so colorful - the light has to pass through more of the Earth’s atmosphere, hitting more particles, so we get pinks and purples. But I digress.


According to the old saying, if there are a lot of particles floating around in the evening, clear weather is likely the next day. If the sky is red in the morning, it may mean a storm is on its way. But we don’t need old sayings or Spare the Air alerts to tell us something is wrong. Spending time outside this summer causes itchy, burning eyes, a raspy voice, and general ill ease, both physically and emotionally.


But what do these wildfire conditions do to your garden and landscape plants? Is it harmful or helpful? You might be surprised to learn that it can be both.


Not too long ago, forestry experts learned that controlled fires are an important part of forest ecosystem health. Some trees need fire to release their seeds. Fires kill pests and pathogens. They clear out dead and dying materials. Surprisingly, there are other benefits.


Smokey shade and photosynthesis

We all know that shade protects plants from sunscald. The smoke and other particles currently flying around in the air is providing similar protection for your plants. Of course, too much shade interferes with photosynthesis. Unless the smoke is really thick, rather than blocking sunlight, the particles diffuse sunlight, bouncing it around in more directions. This provides sunlight for photosynthesis to plant parts that don’t usually get enough sunlight to do much good.


Carbon and carbon dioxide

Many of the particles found in wildfire smoke are carbon-based. There’s more carbon dioxide in the air. Plants use carbon for food and carbon dioxide in photosynthesis. To that degree, the fires and smoke are good for plants, but that’s probably where the benefits end.


Since more than trees and grasses are burning in all these fires, we have to assume that pollutants are also coating our plants (and us). These pollutants come from burning furniture, vehicles, plastics, and other man-made substances.


Also, research has shown that many of the normal substances found in wildfire smoke can cause several problems for plants:

• destroy chlorophyll
• reduce photosynthesis
• deactivate the Calvin Cycle
• disable antioxidant enzymes
• interfere with proteins (K+ channels) responsible for sending electrical signals between cells

Fires and smoke can also result in atmospheric pressure changes and other conditions that cause plants to close their stomata. Stomata are, in essence, breathing holes. Fires also increase ground level ozone. Ozone is very bad for plants. Basically, once ozone enters a plant through the stomata, it is transformed into a reactive oxygen that breaks down cell membranes and damages the contents of cells. Symptoms of exposure to high concentrations of ozone include:

• bronzing
• flecking
• reddening
• stippling

Further exposure can lead to chlorosis, early senescence, and necrosis. Put another way, many plants are holding their breath, suffering ulcers, and their cell phones have died. It’s tough being a plant right now.

New growth on lemon tree infested with leaf miners (Kate Russell)

If all that weren’t bad enough, stressed plants are more susceptible to pest infestations and infections. My perfectly healthy young lemon tree was inundated with leaf miners in just two days. I can’t help thinking it must have something to do with all the particulates in the air. I could be wrong, but it makes sense to me.


After the fires are gone


What about after everything gets back to normal? It’s all too easy to think that everything is okay once blue skies return, but that may be premature. Most of the pollutants and other particles that were floating around end up on the ground and on your plants. Rain and irrigation water washes those chemicals into the soil.


It’s a good idea to have your soil tested after a major fire episode to check for contaminants. It’s also a good idea to wash your produce even more thoroughly than normal. Just in case.


As for us mammals, just like our plants, we need to stay clean, hydrated, and protected.