Defensible space: the three zones around your house

Why it works

CSU's Fire-Adapted Landscaping Practices says wildfire behavior comes down largely to two things: fuel continuity, meaning how burnable things sit relative to each other, and fuel loading, meaning how much burnable material there is. Defensible space breaks the chain and shrinks the pile.

Embers are the other half. Structures usually catch from firebrands that can travel long distances, which is why the 5 feet against the walls gets the strictest treatment. CSU's order of work: harden the house first (roofing, siding, decks, fences), then Zone 1, then outward. Together the three zones are what CSU calls the home ignition zone, the area within 100 feet of a structure.

Zone 1: 0-5 ft

The goal, in CSU's words: keep fire or firebrands from igniting materials that can spread fire directly to the house.

• Ideally nothing that burns. A concrete sidewalk, flagstone or pavers, brick, gravel or bare ground. Gravel has a catch: fine debris is harder to rake out of it than off a smooth surface.
• No wood chips, bark, pine needles or shredded rubber mulch.
• No fuel parked against the wall. No firewood or lumber stored here, and no wicker furniture, furniture cushions or door mats.
• Weekly upkeep. Keep the zone free of dry leaves and needles. Clear gutters and roofs after every big storm and at least each spring and fall.
• Branches. Keep 5 feet between the house and the nearest branch, prune overhanging branches from bigger trees to 10 feet above the roof, and keep branches at least 10 feet from chimneys and stovepipes.
• Fences. A combustible fence can act as a fuse running to the house. At a minimum, the 5 feet of fence closest to the house should be non-combustible.
• If you plant here, use plants scoring 8, 9 or 10, the bar CSU Extension's Fact Sheet 6.305 sets for the whole first 30 feet, Zones 1 and 2 alike. Our Colorado list has 49 of them.

California numbers its zones from 0: its Zone 0 is the same first 5 feet, called the ember-resistant zone, with its own state rule set, so California readers should follow CAL FIRE's defensible space page instead of this page.

Zone 2: 5-30 ft

The goal: lower the fire's intensity as it gets near the house by limiting the fuel it can reach.

• Islands, not masses. Plant in small, irregular clusters and break them up with decorative rock, gravel and stepping-stone paths.
• Green, ignition-resistant plants. CSU's Fire-Adapted Landscaping Practices lists the traits to look for: low growth, high water content, high mineral content and no volatile oils. The score bar is the same as Zone 1: 8, 9 or 10 (or, for plants CSU does not score, a place on Plant Select's fire-resistant list).
• Short grass. Lawn and other grass at 4 inches or less.
• Gravel mulch over wood, and no landscape fabric under any mulch.
• Do not go all rock. CSU says an all-rock landscape is not necessarily the most fire-resistant: green plants store water and slow the spread of fire.
• Trees and shrubs. No new evergreens; thin existing ones to 10 to 15 feet between canopies. Remove shrubs growing beneath trees, since they act as ladder fuels that let a ground fire climb into the crown. Space shrubs at least 2 1/2 times their mature height apart, so 15 feet or more for a 6 foot shrub. Limb trees up 6 to 10 feet, but never remove more than a third of a tree's height.
• Water. Keep plants at their best moisture. If water rights permit, CSU suggests deeply watering trees and shrubs every 20 to 30 days in the growing season; if you cannot irrigate, choose plants that stay green without it.

Zone 3: 30-100 ft

The goal: cut the amount and connectedness of fuel so a fire stays on the ground.

• Fuel breaks. Gravel trails or pavers break up the vegetation. Taller plants, and especially ones that throw embers, need a wider break, and CSU warns that breaks do less on steep slopes.
• Grass timing. Grass here can stay unmowed, but dry grass carries fire fast. On the Front Range the grass fire season runs September to April, so CSU says the most important cut is in the fall. On the lower Western Slope the season runs April to September and the cut belongs in spring, before green-up. Frequent mowing can thin the grass and invite erosion and weeds.
• Shrubs in groups. A mosaic of 2 to 5 shrub groups with open space between them, with weeds and grass kept out of the shrubs.
• Trees. Deciduous trees are more ignition resistant than most evergreens. Space evergreens 10 to 15 feet between crowns (more on steep ground), plant new trees about 20 to 25 feet apart, limb them up 6 to 10 feet without taking more than a third of the height, and remove dead or dying trees and shrubs.

How much defensible space do I need?

CSU's zones run 100 feet out from the house, so that is the full target, with the effort weighted toward the first 30 feet. Two things change the picture. Slope: CSU says fuel breaks do less on steep ground and evergreens need more room between crowns there. Lot size: on smaller lots the zones overlap the neighbors' yards, Zone 3 especially, which makes it a shared project. In CSU's words, a group of structures is only as protected as its least prepared member.

Slope and local code can change this; ask your fire district or the nearest Colorado State Forest Service field office or county Extension office, which CSU names as the places to get a defensible space plan for a specific property.

Plants for each zone

No plant is fireproof. CSU Extension's Fact Sheet 6.305 says all vegetation, naturally occurring or planted, is potential fuel. A high score means a plant is less ignitable, not that it will not burn.

• Zones 1 and 2 (0-5 ft and 5-30 ft): the 49 plants on our Colorado list that CSU scores 8 or higher. The finder's zone 1 filter, which also shows plants outside our picks: groundcovers, perennials.
• Zone 2 only (5-30 ft): 4 more plants that Plant Select lists as fire resistant but CSU does not score. With no score to check, we keep them out of the first 5 feet. The finder's zone 2 filter shows both groups: groundcovers, perennials.
• Zone 3 (30-100 ft): CSU's plant ratings stop at zone 2. For trees, CSU favors healthy deciduous trees in zones 2 and 3; browse trees in the finder.

The full ranked list, with save buttons, is on the Colorado fire-resistant plants page. For mulch choices, spacing and seasonal upkeep, see the fire-resistant landscaping guide.