Ladder Fuels: What They Are and How to Break Them

What are ladder fuels?

The Colorado State Forest Service defines ladder fuels as burnable materials such as smaller trees and brush that give fire a way to climb vertically into aerial fuels, and notes that lower branches on large trees can also act as ladder fuels. The extension guide Reducing Fire Risk on Your Forest Property, published by Oregon State, Idaho and Washington State universities, lists them as large shrubs, small and medium-size trees, and low-growing branches on medium to large trees that allow a surface fire to move up into the overstory crowns.

CSFS adds two points worth remembering: ladder fuels are potentially very hazardous, and they are generally easy to mitigate. Of all the fuels on a property, they are the ones where a modest amount of work changes fire behavior the most.

How does a surface fire climb into the trees?

The PNW guide names four factors that control whether a surface fire becomes a crown fire: foliage moisture, surface flame length, height to the base of tree crowns, and the density of tree crowns. Ladder fuels act on the middle two. A shrub or sapling burning under a tree makes the flames taller, and low branches bring the crown down to meet them. When crowns ignite, which the guide calls torching, the stage is set for a crown fire.

Removing ladder fuels works by widening the gap. The PNW guide explains that increasing the distance from the surface fuels up to the base of the crowns means a longer flame is needed to ignite the crowns, and that removing ladder fuels and pruning larger trees raises the base of the canopy. Our guide to surface fire, crown fire and spotting explains what happens once fire is in the crowns.

Why are there so many ladder fuels in western Montana forests?

Because fire used to remove them. Montana DNRC's west-central Montana landowner guide says ponderosa pine and western larch forests in the region historically experienced mixed severity fires every 10 to 30 years, and that frequent surface fires kept the forest open and favored thick-barked, fire-resistant trees. It explains that without those fires, forests have grown denser, and dead fuels have accumulated.

Ravalli County's 2024 wildfire protection plan describes the result in ponderosa pine woodland: fire suppression has let Douglas-fir and other shade-tolerant conifers grow in the understory, adding fuel and ladder fuels that can carry fire into the crowns. On many Bitterroot and Missoula properties, the thicket of young fir under old pines is exactly that ladder. Our guide to Douglas-fir encroachment covers it in depth.

What do ladder fuels look like on a typical lot?

Look under and beside each tree near the house. Common examples are young Douglas-fir or grand fir growing under a ponderosa; dead lower branches still attached to a pine or fir; spruce and fir whose live branches sweep the ground; shrubs such as ninebark or juniper growing under the canopy; tall dry grass and weeds around the trunk; and slash or woodpiles stacked under trees.

CSFS notes that spruce and fir tend to have crowns that extend to the ground and recommends removing the lower branches that act as ladder fuels. It also recommends that shrubs in the 5 to 30 foot zone not grow under trees, and that they be kept at least 10 feet from the edge of tree branches. Ornamental junipers and arborvitae planted under or against trees are among the most effective ladders there are; see junipers, arborvitae and other high-risk shrubs.

How high should you prune to remove ladder fuels?

The guidance converges on about 10 feet, limited by the tree's size. DNRC's west-central guide says to prune tree branches 10 feet high in the area 5 to 30 feet around a home so they cannot act as ladder fuels. CSFS says 6 to 10 feet or a third of the tree's total height, whichever is less. The PNW guide calls 10 feet a good target and says 12 to 15 feet is beneficial, but also says pruning should leave at least a 50 percent live crown ratio to maintain tree vigor.

Those limits matter. A 15-foot tree stripped to 10 feet has lost most of its crown and will be stressed, and stressed pines are what bark beetles find. Pruning young trees in stages as they grow is better than one heavy cut. Our guide on how high to limb up trees for fire covers the standards, cut placement and timing in detail.

When should a ladder fuel be removed instead of pruned?

When it is a small tree or shrub under a bigger tree, removal is usually the answer. CSFS's zone 5 to 30 foot steps include removing ladder fuels under remaining trees and removing stressed, diseased, dead or dying trees and shrubs. The PNW guide's principles for fire-resistant forests are to reduce surface fuels, raise the base of tree crowns, increase spacing between crowns and keep the larger trees of fire-resistant species.

A young fir growing up through an old pine competes with it for water and gives fire a path into its crown, so cutting the fir helps the pine twice. Where there are many small trees, the job becomes thinning; our guide to thinning conifers for defensible space covers spacing.

Did ladder fuel work help in the Roaring Lion Fire?

The Roaring Lion Fire burned from the Bitterroot National Forest into private land west of Hamilton in 2016. Bitter Root RC&D's post-fire assessment looked at 21 private fuel reduction projects inside the perimeter. The stated objectives of those projects were to reduce the probability of crown fire and to keep surface fire from becoming crown fire by reducing ladder fuels, with stands thinned to 10-foot crown spacing and remaining trees pruned up to 8 to 10 feet.

The assessment's notes describe thinned stands where the fire was an intense surface fire with minimal torching, and an intact mature ponderosa stand that survived with a moderately intense understory burn. It also documents homes lost on treated properties, mostly to ember showers and surface fire. Ladder fuel work kept the fire lower in many stands; it did not, by itself, protect the buildings. That is why it belongs inside a whole home ignition zone plan.

When is the best time to remove ladder fuels?

Outside the main fire season, and with an eye on bark beetles in pine. The PNW guide warns that green pine slash larger than three inches in diameter created from winter through mid-July can provide breeding material for Ips bark beetles, which may then attack healthy trees, and advises avoiding thinning pine during that period or cleaning up the slash quickly. Late summer and fall work in pine avoids most of that risk, and our guide to pine engraver beetles and slash has the Montana detail.

During a dry summer, check whether fire restrictions are in effect before running saws or other equipment; our guide to Montana fire restrictions explains the stages. Our fire season calendar sets out when each kind of work fits best in western Montana.

Do ladder fuels grow back?

Yes. The PNW guide warns that thinning which lets significant light reach the forest floor can bring back small trees and brush, which over time become new ladder fuels, so understory maintenance is needed from time to time. Opening a stand also dries surface fuels, so slash and grass have to be dealt with or the surface fire may burn hotter.

Plan for a light follow-up every few years: cut new fir seedlings, re-prune trees that have grown, and mow or trim grass and brush under the canopy. Our guide to how often defensible space needs maintenance sets out a schedule.

Who should do ladder fuel work?

Small pruning and seedling removal are reasonable homeowner jobs with a pole saw and loppers, as long as you stay off ladders with a chainsaw and keep clear of power lines. Larger trees, work at height, anything near a roof or a power line, and properties with a lot of material to chip are better left to a crew.

HJ Property Care's fire mitigation service includes ladder fuel removal, limbing to ANSI A300 standards, selective thinning and chipping or hauling the slash, planned by ISA Certified Arborist Justin Johnson, who is also line-clearance qualified for work near power lines. This guide is part of our Montana Wildfire Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

More resources