A dark, slimy film on a lawn signals the presence of algae, organisms that thrive in conditions unfavorable to healthy grass growth. This dark layer on the soil surface can range from a greenish scum to a black crust when dry. While algae does not directly kill turf, the resulting dense layer physically blocks the movement of air and water into the soil, which in turn weakens the grass. This article provides steps to eliminate existing lawn algae and implement long-term prevention strategies.
Identifying the Algae and Understanding Its Causes
Distinguishing lawn algae from moss is the first step in proper treatment, as they are visually distinct. Algae presents as a dark green to almost black slimy film directly on the soil surface, especially when the ground is wet. When this layer dries out, it often forms a hard, black crust that can be peeled away. In contrast, moss is a collection of small, fuzzy, green plants that form a dense, spongy mat and are not slimy to the touch.
The underlying cause of algae growth is an imbalance in the turf’s environment, characterized by excessive moisture retention and poor turf density. Algae thrive in waterlogged and compacted soils where the surface remains wet for extended periods. Poor drainage, often caused by soil compaction, prevents water infiltration and air circulation. Low light levels from heavy shade also create a cool, moist environment favorable to algae. Additionally, low soil fertility and an acidic pH level weaken the grass, allowing algae to outcompete the turf for space.
Immediate Steps for Algae Removal
The immediate goal is to physically break up the existing algae crust and apply a treatment to eliminate the organisms. Once the crust has dried, break it up using a light raking or dethatching tool. This mechanical action is necessary because the dried crust acts as a barrier, preventing chemical treatments from penetrating the soil below.
Chemical treatment is then used to kill the remaining algae quickly. Iron sulfate, also known as ferrous sulfate, is a common and effective treatment for controlling algae and moss. For algae control, iron sulfate is typically dissolved in water and sprayed onto the affected area at concentrations intended to blacken and kill the growth. Apply iron sulfate when conditions are cool and wet, rather than in strong sun or high heat, to prevent potential temporary damage to the grass. Alternatively, some commercial fungicides are labeled for algae control. Following any chemical application, minimize watering for a short period to allow the treatment to work and encourage the surface to dry out.
Long-Term Lawn Management for Prevention
Preventing the recurrence of algae requires addressing the environmental factors that allowed it to establish itself. Improving soil drainage is accomplished through aeration. Core aeration involves removing small plugs of soil, which relieves compaction and creates channels for water and air to move into the root zone.
After aeration, topdressing the lawn with a thin layer of sand or organic matter can further improve the soil structure and enhance drainage. If the lawn is in a heavily shaded area, pruning tree limbs or thinning the canopy will increase the amount of sunlight reaching the turf surface. This helps dry the soil and makes the environment less hospitable to algae. A healthy, dense stand of grass is the best defense, as it blocks the light required for algae growth.
Correcting soil pH is another strategy, as algae thrive in acidic conditions. A soil test will determine the current pH, and if it is below the ideal range of 6.0 to 7.0 for most turfgrasses, agricultural limestone should be applied. This process gradually raises the pH, creating a better environment for grass to flourish and outcompete the algae. Proper fertilization and mowing practices, such as maintaining a higher mowing height, encourage dense turf growth, which naturally suppresses algae by shading the soil surface.