How Wildfire Smoke Affects Garden Crops

Updated 9/8/2021 – How Wildfire Smoke Affects Garden Crops

I’ve been seeing so many images out on social media of your gardens with hazy, smoky skies in the background. How does all that wildfire smoke affect garden crops?

A few years ago, we were wine tasting in the Russian River Valley. We did a tasting of two consecutive vintages from the same Syrah vineyard, the second of which was harvested after a huge wildfire ravaged nearby the vineyard. The wine had a unique smoky flavor which gave the wine an interesting complexity. But it tasted very different from the previous year’s vintage from the same vines, which were not exposed to smoke. The vintner told us that the grapes were salvageable due to their more remote location, but many other grapes in the vineyard were not, due to over-exposure to smoke.

This got me thinking. With this year’s Dixie fire and last year’s lightning complex fires raging across Northern California, I wanted to know how Wildfire Smoke Affects Garden Crops. In particular, what would be the affect on my beloved garden heirloom tomatoes and tomato sauce? What about peppers, squash or the fruit out in the orchard? And what about the ash falling in the soil affecting next year’s crops?

Smoke Taint

Research into so-called ‘smoke-taint’ is relatively new. Scientists in Australia, at Washington State University, at the University of British Columbia, and the University of Exeter all have programs to look into this issue. Much of the study done so far has been with wine grapes. Makes sense, right? Not only are grapes one of the more lucrative crops, but they also have quite thin skin membranes.

However, researchers are procuring additional funding to test orchard fruit, among other crops. Study on wildfire smoke affect on crops seems to fall into three categories: reduced photosynthesis, reduced pollination, and flavor degradation.

Reduced Photosynthesis

Wildfires release large amounts of carbon dioxide (source: NOAA) We all learned in grade school how plants need carbon dioxide to grow. They turn that into sugar and oxygen which we humans breathe. So in one sense, a small amount of increased carbon dioxide is good for plants.

However, visible smoke particulates such as are released from raging wildfires can coat the plant leaves and noticeably reduce both air quality and photosynthesis. Reduced photosynthesis leads to smaller, less productive plants. It can even lead to disease and a slow death in many crop plants.

Clearly, the type of plant matters. Trees are better able to withstand such an onslaught thanks to their woody bark and branches. Younger plants and fast growing crops will be more affected since photosynthesis happens at a more rapid rate.

Additionally, research indicates that these particulates and falling ash can also clog stomatal pores in plant leaves, reducing gas exchange. Smoke from a wildfire often contains inorganic matter and chemicals, such as when building materials, metals or plastics burn, and that too, contributes to clogging of the stomatal pores.

The University of Exeter reports that these detrimental effects on photosynthesis are detected for hundreds of miles outside the burn zone, and that plant productivity is significantly decreased. Reduced health in crops can be detected after as little as two hours exposure to smoke.

There are many factors which contribute such as the type of plant, its maturity, duration of exposure, and location. The result could be plants that are oxygen -deprived, stressed, wilted and weakened. They may even look washed out. They will stop producing new fruit and existing fruit will stop growing. Ripening will slow. Flavor could suffer (more on that below). They will put more energy into generating seeds for the next generation.

The good news is that some sources report that when the smoke clears, your plants should rebound, assuming they have not gone to seed. Spritzing their leaves and applying water soluble high phosphorus fertilizer could help them rebound more quickly.

Reduced Pollination

If smoke persists for several days, you will start to see the very young baby squash, gourds, cucumbers, melons and pumpkins wither on the vine. These are all in the cucurbit family. The baby gourd or squash might get about an inch or several inches big, but will then start to turn yellow and die. This is because the female flowers aren’t getting pollinated.

We know that wildfire smoke particulates in the air are bad for us mammals. They are bad for our pets and bad for wildlife too. Turns out they are quite bad for bees, as well. When the air becomes smoky, bees retreat to the safety of their hives. They are not out working the squash blossoms like they normally would be.

Have you ever noticed how eerily quiet it is outside when smoke is in the air and the whole sky seems pink in the middle of the day? No bees. No dragonflies and likely very few if any birds.

Extreme temperatures, such as those which fuel fires in the Midwest and Western US states, exacerbate bee productivity. When it blazes above about 95 F degrees, bees need to gather water for the hive to survive. They are not tending to our gardens, because they can’t afford the extra energy expenditure.

Even self-fertile plants in the nightshade family such as tomatoes and peppers will have decreased pollination. This is because even though a self-pollinating plant has both male and female parts for self-pollination, the bees greatly increase pollination rates on tomatoes by shaking the flower and releasing the pollen.

Manual Pollination Techniques

You can help your cucurbits and nightshades by manually pollinating them, assuming they are still flowering. For the squash, pumpkins and gourds, you need a small paintbrush. You also need both a male and a female flower. But they are easy to tell apart. The female will have a baby squash. It might just look like a swollen area at the base of the flower. It should look like the shape of the eventual fruit. The swelling on cucumbers will be elongated, pumpkins will be rounded. The male flower will not have this baby fruit.

Take your paintbrush and rub it around inside the open male flower. Check the tip of the paintbrush – you should see some bright yellow pollen. Take that and transfer it to the female flower.

Tomatoes and peppers are even easier – just shake the plant a little to get the pollen flowing. It just needs to fall inside the flower to self-pollinate.

Once the heat decreases and/or the smoke clears, the bees should return.

Flavor Degradation

Ok, here’s where the really interesting research comes in. There are several studies into how wildfire smoke affects garden crops, and specifically the flavors of fruit. Turns out, the fires produce free volatile phenols. The smoke carries these phenols to the vineyard where they are absorbed by thin-skinned grapes.

The phenol in the smoke chemically bonds to the sugars in the grape. The chemical bond changes the sugar. This chemical bonding is what causes the smoke taint. Once absorbed, there’s very little that can reverse the chemical bond.

However – here’s the good news – the taint does not show up as a flavor in the fruit until after the fermentation process. So the sugars taste ok until you ferment the fruit.

The researchers are studying what sugar-producing fruits can be tainted by this chemical bonding, and at what stage. Experiments are being conducted on pears to see if taint can be detected, and at what maturity.

In almost every scholarly article I looked at, the researchers acknowledged that there is still so much to learn about how wildfire smoke can affect garden crops. While much was made of the wine country fires a few years ago, I suspect we will be learning much more about other crops after the fires in the Salinas Valley are studied.

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