Tree & Plant Spacing
Break the path fire uses to spread and to climb.
Spacing is how you separate tree crowns so fire cannot jump from one to the next, clear the low growth that lets a ground fire reach the canopy, and break continuous vegetation into islands. How far apart depends on your slope, and for the exact rules, on your jurisdiction.
Most spacing work is one idea in three directions
You are breaking the continuity that lets fire move. Done well, this is mostly arrangement and pruning, not stripping your property bare.
Horizontal
Separate tree crowns so a fire cannot run from one canopy to the next.
Vertical
Clear the ladder fuels so a fire on the ground cannot climb into the trees.
To the house
Keep vegetation back from the walls so nothing carries fire to the structure.
Fire moves through connected fuel
Break the fuel up and the fire loses its path. Two mechanisms do most of the work.
Fire climbs from the ground into treetops using ladder fuels, the low branches and brush in between. Break that vertical ladder and a surface fire has a much harder time reaching the canopy.
Van Wagner 1977, via the NWCG crown fire guidesA crown fire needs a continuous canopy to jump from tree to tree. Space the crowns apart and the fire tends to drop back to the ground, where it is easier to stop.
NWCG, on canopy densitySpace the crowns, then clear the ladder
Measure crown spacing from the edge of one canopy to the edge of the next, not trunk to trunk. On flat ground aim for about 10 feet. The steeper the slope, the wider the gap, because fire moves faster and burns hotter uphill.
| Slope | Between tree crowns | Between shrubs |
|---|---|---|
| 0 to 20 percent | 10 ft | 2x the shrub’s height |
| 20 to 40 percent | 20 ft | 4x the shrub’s height |
| Over 40 percent | 30 ft | 6x the shrub’s height |
Clear the ladder fuels
Leave about three times the shrub’s height of clear space between the top of a shrub and the lowest branch above it. Limb trees to roughly six feet, or a third of their height on younger trees. Do not take more than about a third of a tree’s live crown.
Islands, not carpets
A continuous bed of shrubs is a continuous path for fire. Break planting into separated islands, group plants, and space the groups.
Thin with judgment
Keep the well spaced, healthier, more fire resistant trees. Take out what is crowded, suppressed, dead, or laddering. We favor spacing and pruning over removing everything. Taking a tree out is a one way decision.
Work the downhill side harder
Fire preheats the fuel above it and speeds up as it climbs, so the downslope side of your home usually needs a wider, more careful treatment.
The physics holds everywhere. The required numbers vary.
That fire moves faster and hotter uphill is universal. The specific distances in the table are a widely used defensible-space guideline, but not every jurisdiction requires exactly these, and the band they apply over and the deadline depend on where you are.
What we look for on your lot
On a walk we read your property for exactly this, and you get back a prioritized read, in order of what matters most.
- Crowns that overlap and need separating
- Ladder fuels under your trees
- Beds that run unbroken to the house
- The exposed downhill side
- Specific trees worth thinning or limbing
- What comes first, and what can wait
In a study of about a thousand San Diego homes that burned and a thousand that survived, the near-home zone, the first 30 feet or so, was where clearing made the most difference. Going beyond about 100 feet added little, even on steep slopes. It is where your effort pays off, not a guarantee.
Syphard and colleagues, 2014See what your property actually needs
A Risk Walk turns spacing from a guess into a prioritized read of your parcel. What matters most, and what to do first. What you do with the findings, with us, with your own contractor, or yourself, is your call.