Termites can eat redwood, but they don’t thrive on it. Redwood’s heartwood, the darker reddish-brown core of the tree, contains natural chemicals that are toxic to many termite species. In USDA research that fed Formosan subterranean termites a diet of redwood over six weeks, more than 75 percent of the termites died. That makes redwood one of the more termite-resistant woods available, but it’s not termite-proof.
Why Redwood Resists Termites
The inner core of a redwood tree produces chemical compounds called extractives as it matures. These substances give heartwood its distinctive reddish-brown color and make it unpleasant or outright lethal for termites to consume. In the USDA study, redwood performed comparably to teak, Honduran mahogany, and Alaskan yellow cedar, all of which caused high termite mortality rates over the testing period.
Not all parts of a redwood tree offer the same protection. The cream-colored sapwood that grows in the outer layer of the trunk does not contain these protective compounds. Sapwood from a redwood tree is just as vulnerable to termites as most other softwoods. This distinction matters a lot when you’re buying lumber.
Heartwood vs. Sapwood: The Grade Matters
If you’re choosing redwood specifically for its termite resistance, you need heartwood grades. Commercial redwood lumber is graded by durability, and the grading system makes this easy to spot: any grade with “Heart” in the name is cut from the resistant inner wood. The main heartwood grades include Heart Clear (the highest architectural quality), Heart B, Deck Heart, Construction Heart, and Merchantable Heart.
For any application where wood contacts or sits near soil, heartwood grades are essential. Deck Heart and Construction Heart are the most common choices for outdoor projects like decks, fences, retaining walls, and garden structures. These grades contain knots and minor imperfections that bring the cost down while still providing the natural resistance you’re paying for. Heart Clear and Heart B are reserved for visible, architectural uses where appearance matters most.
Redwood sold without “Heart” in the grade name will contain sapwood, which offers no meaningful termite resistance. If termite protection is your goal, the lighter-colored sapwood grades are essentially the same as buying pine or spruce.
How Different Termites Attack Redwood
Subterranean termites, the most destructive type in North America, build colonies underground and travel through mud tubes to reach wood. Even with redwood heartwood, these termites can find ways in if conditions favor them. A piece of heartwood sitting directly on damp soil gives termites constant access and moisture, two things that help them overcome wood they’d otherwise avoid. Over enough time, a large subterranean colony can damage even naturally resistant species.
Drywood termites, common in coastal and southern states, infest wood from the air rather than the soil. They fly to exposed wood, bore in, and establish small colonies inside the timber itself. Redwood heartwood discourages them, but no natural wood resistance is absolute. A drywood colony that establishes itself in a crack or end grain cut can slowly consume even heartwood, particularly as the wood ages and its protective compounds break down.
Protecting Redwood Structures
Choosing heartwood is a strong first step, but it works best as part of a broader strategy. The single most important thing you can do is eliminate direct contact between wood and soil. Wood siding, door frames, and window frames should sit at least six inches above ground level. Posts or steps embedded in concrete are still vulnerable because termites can travel through cracks in the concrete to reach the wood.
Moisture is the other major factor. Termites are drawn to consistently damp soil, so keeping the area around your foundation dry makes a real difference. That means functional gutters and downspouts, fixing leaky outdoor faucets, and grading the ground so water flows away from the structure rather than pooling against it. Lawn sprinklers aimed at the foundation are a common and easily fixable problem.
Crawl spaces deserve attention too. Proper ventilation, typically one square foot of vent opening per 150 square feet of crawl space, reduces the humidity that attracts termites. Laying polyethylene sheeting over about 75 percent of the exposed soil acts as a vapor barrier, cutting down on evaporation that would otherwise condense on floor joists and subfloors. Vents should stay clear of leaves, dirt, and vegetation that block airflow.
How Long Does Redwood’s Resistance Last?
Redwood heartwood’s natural resistance is not permanent. The protective compounds gradually leach out or break down over years of exposure to moisture and UV light. A freshly milled piece of heartwood is significantly more resistant than one that has weathered outdoors for a decade or two without maintenance. Applying a UV-blocking finish or sealant helps slow this process, preserving both the wood’s appearance and its chemical defenses.
In ground-contact applications like fence posts or raised bed frames, even heartwood will eventually soften and become more hospitable to termites and decay fungi. Periodic inspection is the simplest way to catch problems early. Probe the wood near soil level with a screwdriver or similar tool. If it feels spongy or crumbles easily, the protective compounds have likely degraded past the point of usefulness, and the wood should be replaced or treated.