Green algae are a diverse group of photosynthetic organisms characterized by the presence of chlorophyll a and b, which gives them their familiar color. While they share a close evolutionary lineage with land plants, their contemporary relationship is complex. In some ecosystems, green algae can be neutral or beneficial. However, when growing in terrestrial settings like gardens or containers, their uncontrolled presence often signals an underlying problem, negatively affecting plant health and cultivation efforts.
The Context Matters: Algae’s Role in Different Environments
The ecological role of green algae differs significantly between aquatic and terrestrial environments. In water bodies, algae are a foundational part of the food chain, serving as a primary food source and producing substantial oxygen through photosynthesis. However, excessive growth, known as an algal bloom, can be harmful. These dense blooms deplete dissolved oxygen when the algae decompose, creating hypoxic conditions deadly to aquatic life.
In terrestrial and horticultural settings, the presence of green algae is an indicator of poor environmental conditions. Algae thrive in environments that are persistently damp, shady, or have poor air circulation. They form a slick film on surfaces like potted plant soil, tree trunks, greenhouse benches, walkways, and patios. The growth itself does not directly parasitize the plant but signifies excessive moisture detrimental to root health.
How Green Algae Harms Terrestrial Plants
The negative impact of green algae on cultivated plants stems from mechanisms that interfere with soil and plant processes. When algae form a thick, cohesive mat on the soil surface, they compete with plant roots for water and essential nutrients. This competition is particularly noticeable for micronutrients like nitrogen, which is readily absorbed by the rapidly growing algal layer. The algae essentially form a biological barrier that intercepts resources before they can leach down to the plant’s root zone.
The physical presence of a dense algal layer over the soil also inhibits proper gas exchange between the soil and the atmosphere. This smothering effect restricts the entry of oxygen into the root zone. The resulting waterlogged, oxygen-depleted, or anaerobic soil conditions damage plant roots. This damage makes roots susceptible to disease and hinders their ability to absorb water and nutrients. For newly sown seeds or small seedlings, a hard, dry algal crust can physically impede emergence.
Indirectly, algae growing on hard surfaces like pots, walkways, or greenhouse floors create a slick film that contributes to a uniformly damp environment. This dampness encourages other plant pathogens, such as fungus gnats and certain fungal diseases. In greenhouses, algae growing on glass or plastic surfaces reduce the amount of light reaching the plants below. This blockage slows photosynthesis and negatively affects the growth and development of light-sensitive crops.
Controlling Algae Growth in Gardens and Landscapes
Controlling green algae begins by addressing the environmental factors that allow it to flourish. Improving soil drainage is a primary preventative measure, often achieved by amending garden soil or using a coarser potting mix in containers. Adjusting irrigation practices helps eliminate the perpetually damp conditions algae require. Water less frequently but more deeply, allowing the soil surface to dry out completely between waterings. Increasing air circulation around plants, through strategic pruning or using fans, also helps surfaces dry more quickly.
For physical removal, small-scale surface algae on potted plants can be gently scraped away or disturbed with a hand tool, breaking the light-dependent growth cycle. A thin layer of dry sand or fine vermiculite can be spread over the soil surface to block sunlight and prevent re-establishment. On hard surfaces like patios, mechanical removal with a stiff brush and a mild solution of baking soda or white vinegar is effective.
In cases of severe infestation on non-plant surfaces, chemical control options are available, though they require careful application. Commercial algaecides or products containing copper sulfate can eliminate the growth on hard surfaces. Their use must be strictly monitored to prevent runoff into soil or water bodies, as they can be toxic to aquatic life. Fungicides are sometimes used, as the compounds that control certain fungal pathogens can also suppress algal growth. When using any treatment, ensure the product is safe for use around edible plants and follow all safety guidelines.