Do Ashes Help Grass to Grow?

While ashes and grass may seem unrelated, the remnants from a cozy fire can ultimately benefit your lawn. Certain ashes improve soil in many parts of the United States, creating healthier growing environments for plants and delivering valuable nutrients to grass. Whether ashes will help your grass grow depends on the ashes and your soil.

All plants require the same essential nutrients to fuel healthy growth. Like grasses and other plants, trees contain these necessary nutrients — even when harvested for firewood. When wood burns, nitrogen and sulfur are lost in the process, but other plant nutrients remain. The amounts depend on the type of wood.

A cord of logs from a dense hardwood such as oak produces more ash and more nutrients than a cord from less-dense softwood such as pine. Wood ash from natural, untreated wood is the only type you should use to fertilize your grass. Ash from cardboard, painted wood, pressure-treated lumber, and similar trash or debris contains toxins harmful to grass and plants.

Wood ashes offer significant potassium, but their greatest potential benefit to lawns comes from being able to raise the soil pH. The ashes have properties similar to lime products used to raise soil pH. Lawn grasses generally prefer slightly acidic soil in the 6.0 to 7.0 pH range, but high-nitrogen fertilizers tend to lower soil pH over time.

Many parts of the country also have naturally acidic soil. When soil pH drops below 6.0, it limits the ability of plants to take up essential nutrients, including nitrogen, phosphorus and iron. Lawn beauty, health and resilience suffer. If your soil is acidic or potassium-deficient, wood ashes can help your grass to grow.

Apply wood ashes with the same precision used for other lawn fertilizers. Be sure to moisten your grass first and then spread the ashes evenly over the area. Next, rake them in lightly and water thoroughly. The ash will affect pH more quickly than if you apply lime. Wait at least one month before using any nitrogen fertilizers.

In general, 10 to 15 pounds of wood ashes per 1,000 square feet is safe for most soils, but a soil test confirms your lawn's needs. Ash your lawn only once every five years or when a new soil test reconfirms a need. Add any leftover ash to your compost pile.

Even if your ashes come straight from the fireplace, be sure to treat them with the respect high-alkaline products deserve. Wear gloves, protective clothing, protective eyewear and a dust mask when spreading ashes. Work when there are no winds.

Too much soil alkalinity causes the same problems as low pH, so apply ashes at recommended rates only. Overdoing it can cause long-term harm to your soil. Wood ash also has high levels of soluble salts, so never leave piles on your grass, and don't use ashes when planting or near seedlings. Keep wood ashes away from acid-loving plants, such as those in the ​Rhododendron​ genus, and from potato plants (​Solanum tuberosum​). The ashes encourage potato scab.