What Wildfire Smoke and Heat Do to Trees and Gardens

Does wildfire smoke hurt trees?

It can slow them down, at least for a while. In a piece for The Conversation, republished by Scientific American in August 2024, atmospheric chemists Delphine Farmer and Mj Riches described measuring ponderosa pines at a Colorado Rocky Mountain field site during the heavy smoke of fall 2020. On the first heavy-smoke morning, the needles' pores were fully closed and photosynthesis was close to zero. When the researchers changed leaf temperature and humidity, the pores reopened and photosynthesis recovered.

They could not say for certain why the trees shut down. Smoke particles may coat the needles, clog the pores, or prompt the tree to close them early, and they think a combination is likely. They also say it is not yet known how long the effects last or what repeated smoke seasons do to trees over the long term. Ponderosa pine is a common yard and acreage tree here, so the finding is worth knowing.

Can a smoky summer affect next year's fruit?

There is evidence that it can. A University of California, Davis study published in Nature Plants in October 2024 followed almond, pistachio and walnut trees at 467 orchard sites in California's Central Valley from 2018 to 2022. After the 2020 megafires, smoke reduced the carbohydrates the trees store for winter dormancy and spring growth, and nut yields fell by 15 percent to as much as 50 percent in some orchards. The effects lasted months after the fires, depressing the next season's bloom and harvest.

The researchers do not know which part of the smoke caused it. Smoke cut direct sunlight and raised ozone and particulate levels, and one or more of those may be responsible. Those were California nut orchards under months of heavy smoke, not Bitterroot apple trees, so treat it as a reason to pay attention to fruit trees after a smoky year rather than a prediction. Good care the following spring, covered in fruit tree pruning, matters more after a stressful season.

What does ash do to plants?

A light dusting is mostly a cleaning problem. Oregon State University Extension's guidance on wildfire ash says ash and smoke are unlikely to penetrate intact produce, and suggests hosing ash off plants, using mulch and drip irrigation or soaker hoses, and avoiding disturbing the soil. It says never to use a leaf blower to clean up ash, which puts fine particles back in the air.

Where ash is thick, the same guidance notes that compost or clean bagged soil may help restore soil biology, though amendments are not necessary for most gardens. Ash also hides what is underneath it, so wait until it is washed off before deciding a plant is damaged.

Is garden produce safe to eat after smoke and ash?

Usually, with cleaning, according to Oregon State Extension. Its steps: rinse produce outdoors, then again in the kitchen sink; peel tomatoes, apples and root crops; strip the outer leaves from lettuce and other greens; and for a thorough cleaning, soak produce in a 10 percent white vinegar solution, one cup of vinegar to nine cups of water, which helps with crinkly greens like kale, chard and savoy cabbage and with fuzzy fruit like peaches and apricots.

The risk is higher if the garden has a heavy ash layer or is near a structure that burned, because burning buildings release different toxins than burning forest. In that case Oregon State recommends a soil test for heavy metals before replanting. It also advises not harvesting while smoke lingers, wearing an N95 mask when you do, and checking air quality at AirNow. This is general guidance, not a health ruling; if a structure burned near your garden, ask your county health department.

What does radiant heat from a nearby fire do?

Heat is different from smoke. A fire burning nearby can scorch foliage without flames ever touching the plant. Montana State University Extension's MontGuide on fire-resistant plants explains that losing foliage reduces a plant's ability to make carbohydrates, which weakens it and leaves it more vulnerable to winter damage and pests, and that young trees are hit harder than older ones. It notes that succulent plants and plants full of water survive fire better than those with low moisture.

For conifers, scorched brown needles do not always mean a dead tree, because buds can survive inside them. Our guide to which burned trees will live covers the thresholds researchers use. The USDA Forest Service's 2025 pine engraver guide adds a warning: pines scorched by fire are highly attractive to pine engraver beetles for about two years, so watch scorched pines closely.

Does heat and drought stress make smoke worse?

They stack. Smoke seasons in western Montana come in the hottest, driest weeks of the year, and MSU Extension's drought stress guidance says drought-stressed trees are less able to resist pests and more open to secondary attacks. It lists the symptoms: yellowing, scorched leaf edges, wilting, early leaf or needle drop, and slower growth, showing most in the sunny outer crown.

MSU singles out trees planted in the last three years as the most likely to be permanently injured. A tree already short of water when the smoke arrives has less in reserve. Our guide to watering trees through drought covers how to keep them supplied.

What about fire retardant on plants and lawns?

If retardant was dropped on or near your property, expect a growth response, including from weeds. MSU Extension's post on managing weeds after wildfire reports a Montana study in which retardant high in ammonium and phosphorus acted as a fertilizer and increased cheatgrass. Watch retardant-dusted areas for a flush of cheatgrass and other weeds the following spring and treat them early.

Our cheatgrass guide explains why that matters for the next fire, and our licensed weed control team can handle treatment.

How do you protect yourself while working in the garden?

Wait for the air to clear if you can. Oregon State Extension advises against harvesting while smoke lingers and says to check air quality at AirNow first. If the job cannot wait, it recommends an N95 mask, with a cloth mask with a filter only as a backup. Afterward, take your shoes off outside, change and launder your clothes right away, and wash your hands thoroughly.

The same care applies to cleanup. Hose ash down rather than sweeping it dry or blowing it, and work gently so you disturb the soil as little as possible. Stored food that sat out in smoke is a separate question, and the Oregon State guidance lists what to replace, including room-temperature potatoes, fresh produce and dried fruit in open containers.

How do you help trees and gardens recover?

Water, then patience. MSU Extension advises watering trees slowly and deeply rather than quickly and shallowly, watering weekly in the fall until the ground freezes, spreading three to four inches of mulch out to the edge of the canopy, and clearing competing grass from under young trees. The fire-resistant plants MontGuide adds that after a fire, owners should prune out dead branches and water the remaining plants well, and that there may be no reason to add large amounts of fertilizer.

Wait for spring before judging conifers and shrubs, since many resprout or push new growth. Remove ash from vegetables and lawns with water, not a blower. Keep the lawn and the green zone near the house irrigated through the smoky weeks; the summer watering guide has schedules.

When should an arborist look at a tree after a smoky or fiery summer?

Call when the color is wrong in a way smoke does not explain: a whole crown fading at once, tops dying back, boring dust or pitch on the trunk, or scorched foliage on trees close to the house. Call too if a tree near the house was within reach of flames or heat, since the trunk and roots matter more than the needles.

Our tree health and risk assessment is done by an ISA Certified Arborist, and our tree services crews handle any pruning or removal that follows.

Where this fits

Smoke and heat are the side of fire season every property in the valley sees, even when no fire comes close. They are part of our Montana Wildfire Guide, and the best protection for the trees near your house is the same fire mitigation work that protects the house: thinned, healthy, well-watered trees with room between them. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

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