Are Plants Stressed By Wildfire Smoke?

Given the spate of fires in North America and Europe this summer, those growing bonsai may wonder if wildfire smoke affects plants. And it does—in both positive and negative ways.

Wildfire smoke has a lot in it. There are gases, like carbon dioxide, sulfur dioxide, nitrogen oxides, ozone, and ammonia. Then there are the particulates, like carbon, ash, metals, and silica. And then there are bioaerosols, like fungi and bacteria.

Due to the particulates in smoke, sunlight is diffused. This is a benefit, as a leaf gets light from many different directions rather than in one direction, which gives interior foliage a photosynthetic advantage. Similar to shadecloth. And yet with dense smoke light intensity may fall to where this is no longer a benefit.

When smoke is dense enough, stomatal closure may result. Stomata are the small openings in leaves which allow for transpiration and gas exchange. A plant closes these during periods of high winds and high heat, so this is a normal process, but if those conditions last long, including extended heavy smoke, it is akin to plants holding their breath. Gas exchange stops. Photosynthesis drops. Energy production is cut. And the plant suffers by loss of productive time—which in the north, with its short growing seasons, could be significant.

A few of the gases present in wildfire smoke may spark plant growth by being signalers, like carbon monoxide and methane. Carbon dioxide is needed for photosynthesis. On the other hand particulates can be damaging to plant tissue, as are many of the gasses in stronger concentrations.

The upshot is that small amounts of smoke are likely good for plants while higher amounts are detrimental. Whether a plant is actively growing or not when smoke is present is one variable. But those who have studied crop yields of corn and wheat in high smoke years have found minimal impact.

You may notice that when above 150 AQI (air quality index), your bonsai use less water. Given that wildfire smoke tends to reduce daytime highs, this is likely the result of closed leaf stomata. This may be the only thing we notice about our bonsai in periods of wildfire smoke, they are less thirsty. Though washing off the leaves of soot and other particulates with hamisu, leaf watering, would be a good practice.

Other than that—which isn’t much—it’s not clear if smoke should change our bonsai work timing, like bending branches. It’s a guess that we shouldn’t give a plant added stress during stressful times. But keeping an eye out for reduced water needs, and giving a periodic flushing of the leaves with water, are perhaps our main tools during smoky days.