Reference
Firewise Landscaping: Defensible Space and Plant Choice
The short answer
Think in defensible-space zones measured from the house: Colorado State Extension describes Zone 1 (0-5 feet), Zone 2 (5-30 feet), and Zone 3 (30-100 feet), with the closest zones kept leanest and the most flammable plants at least 30 feet from any structure.
Near the house, favor plants with high moisture content, low oil or resin, compact growth, and green stems, and avoid tall, twiggy, resinous, high-water-need plants. Conifers are flammable regardless of how well watered they are because of their oil and pitch, so keep them out of the near zones - and remember maintenance (removing dead material, pruning, watering) is essential because fire-resistant plants lose that quality if neglected.
Defensible space in zones
The core of firewise landscaping is defensible space - managing vegetation in zones based on distance from the structure. Colorado State Extension defines three: Zone 1 at 0-5 feet, Zone 2 at 5-30 feet, and Zone 3 at 30-100 feet. Plant choice matters most in Zones 1 and 2, closest to the house.
The general principle is that the nearer to the structure, the leaner and less flammable the planting should be. The document recommends keeping flammable plants at least 30 feet from any structure or other plants, so a stray ember or a burning shrub cannot bridge to the building.
What makes a plant fire-resistant
No plant is fireproof, but some are far less flammable. Colorado State lists the attributes that decrease flammability: low oil or resin content, high moisture content, soap/latex/pectin content, compact growth form, green stems, and drought tolerance.
The opposite traits increase flammability: high oil or resin content, low moisture content, tall growth, open form, fine twiggy stems, and high water need. Conifers are called out specifically - they tend to be flammable due to their oil and pitch content regardless of their water content - which is why they belong in the outer zones rather than against the house.
Maintenance keeps it working
A fire-resistant landscape is not a one-time installation. Colorado State stresses that plants considered fire resistant can lose these characteristics over time if not maintained.
That means removing dead material regularly, keeping plants pruned and thinned so they do not accumulate dry fuel, and maintaining plant moisture through the dry season. A well-chosen but neglected planting - full of dead stems and drought-stressed foliage - can become fuel rather than protection.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Colorado State University Extension - Firewise Plant Materials — Defines defensible-space Zone 1 (0-5 ft), Zone 2 (5-30 ft), and Zone 3 (30-100 ft); lists flammability-decreasing traits (low oil/resin, high moisture, compact form, green stems, drought tolerance) versus increasing traits (high oil/resin, tall/open/twiggy, high water need); notes conifers are flammable regardless of water content and flammable plants should be at least 30 ft from structures; and stresses ongoing maintenance because fire-resistant plants lose that quality if neglected.